Who is Dave Emanuel? Read his bio!
Noise Abatement Program
My 1985 Corvette sits quite a bit and when I take it out lately, I notice a raspy/scratchy noise coming from both sides of the engine compartment as the vehicle dips and heaves over uneven surfaces. The noise appears to go away the longer the interval of driving. I have greased all the hood rests and still get this annoying sound. Any suggestions?
Sure, sell your ‘85 and buy a ‘94 or ‘95. For the 1994 model year, Chevrolet made over 200 modifications to reduce Corvette body noise. One of these was the installation of stiffer springs on the hood latches. That might be your problem, but there are a number of other sources. Try pulling gently against the hood release. This will increase the pressure against the latches and if they’re the offenders, the noise should disappear or at least diminish. You also might try releasing the hood, although that will probably cause other noises that will cover up the one you’re concerned about. Also check all the surfaces where the hood seals against the body. Sometimes, a light coating of silicone in these areas will eliminate the noises you’re hearing.
Rock & Roll Fantasy
Recently I had the following items installed on my 1990 Corvette :
1. MSD 6 AL, MSD Capacitor to suppress radio interferences, a new distributor cap and rotor, 8mm suppression wires, Carroll Supercharger System, Lingenfelter manifold & plenum, 1:6:1 ratio roller rocker arms.
When I got my car back from the dealer, my Bose stereo system, cassette player and disc player had popping coming through the speakers. The dealer explained that this popping was caused by the MSD unit, and that I would have to learn to live with the problem if I wanted the extra horsepower. I couldn’t accept his answer, so I took my car to a radio service center. They tried four different types of inline filters to the radio but had negative results. They advised that I trash the MSD 6 AL and go back to the stock ignition system.
I contacted the Carroll Supercharging Co. and they explained that they discovered this problem also on cars with the Bose Stereo Systems and were unable to cure the problem. Next I contacted MSD Tech Service and they suggested that I change the wires, distributor cap, rotor, sprark plugs and add an extra ground wire to the MSD 6 AL from the frame of the car.
Well, I changed everything they requested and this time switched the wires over to Moroso 8 mm suppresser high energy helical wound silicone wires. Needless to say, I still have the popping sound coming into the stereo system. If you can’t help me, nobody can! Therefore, I’m going to wait for your answer before I “trash” this MSD 6 AL.
I appreciate the compliment, but there’s certainly someone in your area who can solve your problem. This just may be one of those problems that needs “hands-on” attention. For openers, determine that the problem is in fact, the MSD unit. Disconnect it and see if the popping is still present when the stock ignition is firing the plugs. If that corrects the problem, you’ve got your answer.
The first step is to make sure everything is properly grounded. That means engine to chassis, stereo to chassis, MSD to engine. Next make sure that the MSD box isn’t mounted near the antenna or the cable that leads to it. You might also try putting some a thin coating of voltage insulating gel on the rotor blade near the point at which it attaches to the rotor body. Sometimes the spark jumps from a mid-way point on the blade (rather than from the tip) and that causes a big arc inside the cap. That arc can translate into radio noise.
If all else fails, add a 30,000 micro farad capacitor to the MSD power cable. Use either MSD part number 8830 or pick up an equivalent at an electronics store. The capacitor must be rated at 25 volts DC. Add the capacitor in parallel with the stereo power cable. That means, just remove the insulation at two points, but DO NOT cut the wire. Wrap the capacitor leads around the wire and then cover the connections with electrical tape.
The Night the Lights Went Out
I own a 1985 Corvette that is giving me a problem. When I bought the car, the under the hood service lights were disconnected. I connected the light and started experiencing a dead battery any time the car would sit for 8 hours or more.
I disconnected the lights again and replaced the battery. Everything was good for approximately four months and now the new battery goes completely dead. I have recharged the battery and checked the vehicle day by day and everything was fine for about three months.
I have checked and made sure that I don’t leave anything on, but this problem reoccurred and the battery is drained again. I cannot find anyone in this area who can help me. One person stated that the computer is draining the battery. I am the second owner of this car The car has 52,000 miles on it and runs like a Corvette. (When it starts) Please help!
Electrical problems can be more frustrating than filling out an income tax return, but at least you don’t have to worry about an audit when it’s all over. First, pour yourself a nice cup of coffee and share the moment with someone– preferably another Corvette owner. Now that your jangled nerves are soothed, attack your problem in a logical manner. It’s doubtful that the computer is the problem. If it was, every car with computerized engine congrols would have he same problem. I’ll assume that you’ve checked the alternator and that it’s charging properly.
When a battery is fully charged and loses its zap after sitting, there’s obviously a short somewhere. Without the proper equipment (and sometimes, even with the proper equipment) the easiest way to find the short is to disconnect the negative cable from the battery and place the leads of a volt meter between the battery terminal and cable. If the meter shows more than a few hundredths of a volt, pull one fuse at a time until the voltage draw drops significantly. That will isolate the problem to a particular circuit and you should be able to trouble shoot it from there.
If you still can’t find the source, fully charge the battery, then disconnect it entirely. If it still loses its charge, either the battery is faulty or corrosion on and around the battery is creating a path for the electrons to escape. If you’re still unable to locate the voltage drain source, start disconnecting one connection at a time until the battery stays charged. A likely culprit could be the alternator or starter.
Feel the Heat
I have owned many Corvettes from 1962 to 1978, but I have a problem with 1974 convertible. When I have the top down, heater off, hot air blows on my feet, both drivers side and passengers side. It is really hot air.What can I do about this problem?
You aren’t related to Orin Nelson, are you? In any event, excessive cabin heat is a common problem in a number of early 70s model years. There are numerous causes, but most commonly, it’s related to lack of insulation beneath the floorboards. I’m not sure which cars have the heat insulators; it used to be standard equipment, but sometime during the 70s it was discontinued. I owned a 1969 coupe and never had a problem with cabin heat (it has the underfloor insulators). On the other hand, the 1973 roadster I used to own would give you a hot foot in a heartbeat and it did not have the insulators.
Unless you can find some new old stock, you’re probably out of luck trying to find original equipment insulators. The part numbers of interest are 20464592 and 20464593) However, several companies offer similar materials, which are routinely used in oval track race cars. Check with BSR Products (704/663-0955) and Thermo-Tec (800/274-8437) for suitable under floor insulating material.
Another possibility is that coolant in the heater core is contributing to the problem. You can verify this by disconnecting the heater hoses from the core (just connect heater hose outlet to the inlet on the engine so you don’t spew coolant all over the ground). You may need to incorporate a valve to shut off coolant flow to the heater core. Another possibility is that the heater air control door is stuck, or not properly positioned when the heat control is moved to the “Cold” position.
Another Hot Vette
I’d like to have information on how to diagnose overheating problems on my ’71 LS5, which is a convertible with factory air. I’ve replaced the fan clutch with a heavy duty model which starts to cut out at abut 60mph. Temperature stays at mid gauge (210 degrees). If I keep my speed at that or higher, the temperature starts to steadily rise until I drop my speed below 55 mph which allows the clutch fan to re-engage.
The thermostat is set for about 180 degrees and the water pump is in good condition. That leaves the radiator. Is this where I go next? If so, what would you recommend if I want to keep the car in original condition?
High coolant temperatures typically result from one of two situations. Either radiator capacity or air flow across the radiator is insufficient. In the latter case, symptoms usually include overheating at idle and during low speed driving. In such instances, the lack of adequate air flow can be the result of a cooling fan that’s too small or not rotating at the required speed (as is the case if the fluid leaks out of the viscous drive). It can also occur because the radiator isn’t sealed properly and air is traveling around it, rather than through it.
At 60 miles an hour and above, there’s usually enough of a ram effect that a cooling fan is not required. For a number of years, I drove a Corvette with no cooling fan whatsoever. The temperature gauge would start to climb rapidly when I was caught in traffic, but it ran right at 160-180 degrees on the highway. Subsequently, I installed an electric fan which helped control temperatures during around-town driving.
However, that isn’t always the case. I’ve driven some Corvettes at highway speeds and heard the fan clutch cycle on and off– usually when the air conditions was in use. From what you describe, your fan clutch isn’t operating in a normal fashion, or you’re incorrectly diagnosing the situation. Fan clutches are either temperature controlled or simply allow the fan to turn at less than pulley speed. I don’t know of any fan clutches that are controlled by vehicle speed.
When overheating at highway speeds is the major concern, it usually means that the radiator just doesn’t have enough capacity. It takes more horsepower to travel 60 miles per hour than it does to move a car 35 miles per hour. Horsepower builds heat and you can figure out the rest of it. However, as speed increases so does the volume of air flowing across the radiator. So unless something is blocking the flow of air to the radiator coolant temperatures should drop as vehicle speed approaches 60-65 miles per hour.
But overheating problems aren’t always cooling system related. If spark timing is retarded, engine temperatures will soar. Make sure that initial timing is set at 8 degrees or more (preferably 10 to 12) and that the mechanical advance adds another 22 to 26 degrees. Total mechanical timing should reach 34 to 36 degrees by 2750 rpm.
I seriously doubt that the radiator is the cause of your problem, unless it has been damaged. A thorough cleaning might help, but I suspect the cause of high temperatures lies elsewhere.
Orin’s Song
Please help me with an air conditioning problem. I purchased a 1975 Vette that had been standing for a while before I got it. The air conditioning worked fine for the first couple of weeks I had the car. Going down the parkway the compressor seized up. Just about everything has been replaced. The first garage had my car for two months. They installed new parts and told me that the unit itself was cold but they couldn’t get the inside of the car cooled off. They never got the temperature under 80 degrees. They told me that they replaced everything they could think of and they blew all the lines out to make sure nothing was stuck in them. For some reason the air coming inside the car was still blowing warm. After numerous attempts at fixing it I took it to another mechanic.
This mechanic said he thought it was a problem with the ductwork. He replaced some parts and said it was fine when he test drove it. But then again, it was November. Now that I’ve used it all summer I know it just isn’t working. The air conditioning is working but it still continues to blow warm air into the car. I would love to have an answer for these guys, they’re really at a loss and I think they’re ready to kill me. I can’t stand to have my car tied up for another four or five months, besides the fact that it’s costing me a small fortune and it’ s not getting any better.
Sounds to me like the real problem is that you haven’t found a competent mechanic yet. I’m continually amazed at how frequently repair shops tell customers, “It was fine when we test drove it”, when it really wasn’t. What’s more, you shouldn’t get hoodwinked into paying the bill. When you pick up your car, take it for a test ride with the mechanic, and let him SHOW you that the problem has been fixed. If it hasn’t, you shouldn’t be required to pay for the repair.
As for some suggestions– the first thing you have to do is determine if the air conditioning system is actually working. Let’s not take anybody’s word for it. When the air is on, one of the lines leading to the condenser should feel cool, and if the air conditioning runs long enough, condensation should form on it. If those conditions exist, that means the cooling portion of the system is operating properly. That being the case, the next thing to verify is that the air flowing into the passenger compartment is being cooled.
Air from either inside or outside the vehicle (depending on whether the system is set to recirculate) should flow through the evaporator core (that’s the large blob that’s attached to the firewall) and into the passenger compartment. If the cooling section of the system is operating properly, there’s no reason for the air not to be cool. Even if there’s a problem with the duct work, there should be a flow of cool air somewhere. If there isn’t, either the evaporator is blocked, or the air is passing through the heater core as well as the evaporator.
The best way to troubleshoot the problem is to work systematically and verify each aspect of operation. It may be necessary to disassemble the under-dash duct work to gain access to certain components. You may be better off trying to solve the problem yourself, or with some help from another Corvette owner. If you decide on that approach, it would pay to invest in a shop manual for your car.
Slow Windows
I have an ’87 Corvette, purchased new, with 94,000 loving miles and have been experiencing a gradual increased slowness in the up/down cycling of the power windows. The mechanic, who is able and trustworthy, has cleaned and sprayed the tracks but the windows are still painfully slow. When he finished that he told me the motors were quite weak and should be replaced fairly soon.
I would like to correct the condition earlier rather than too late since I just know that when the motors fail it will be in the down position, on a rainy day, on a Sunday and when I’m far from home. The mechanic has quoted the local Chevrolet dealer’s price for new motors plus what I think is a reasonable labor cost. I noticed a Corvette parts catalogue had motors that cost quite a bit less than the dealer’s price.
The mechanic says that he will use whatever I tell him to. My question here is, do you think there is any meaningful quality difference between the two sources and should I replace more than the motors in order to avoid having the next weak link in the power window system fail and have to incur labor cost in rectifying that?
We just got done with a new window motor project. After equipping the driver’s side window motor with a supercharger and high lift cam, the window on our test car moved from fully closed to fully open in 1.2 seconds. Still thinking additional performance could be obtained, we added a shot of nitrous. But that was a bit too much because when we closed the window it rose so quickly, it came out of its track, knocked the roof panel off and imbedded itself in the wing of an L-1011 that was passing overhead.
For us, it’s back to the drawing boards; for you, your best bet is to install replacement window motors. It’s not common, but it isn’t all that unusual for window motors to suffer the electric equivalent of tired blood. There is probably no meaningful difference in quality between the motors being offered by your dealer and those available through independent Corvette suppliers. The difference in price is probably due to the fact that you’re paying list (or close to it) at the dealer while you’re getting a discount at the independent parts supplier. Another factor is that many times the distribution chain through which dealers purchase parts results in multiple mark-ups that other distribution systems don’t impose. That’s the reason it’s often possible to purchase AC/Delco parts cheaper from an independent supplier than from a Chevrolet dealer.
There really are no upgrades so that’s not a viable consideration. Since you’ll be installing motors that will function the same as the existing ones when they were new, there shouldn’t be any excessive strain on other components that would require beefing them up. However, if any of the tracks or mechanisms are noticeably worn, it would obviously be wise to replace them while the doors are apart.
Let it Bleed
Twice I have managed to get into the brake system of my ’76 Vette, once to install stainless steel calipers and once just changing the rear pads. I have followed the normal bleeding instructions, but I do not feel the brakes are 100%. The pedal travels too far and is just not solid. Do you have any suggestions?
The rear calipers on C3 models have two bleed screws, one inboard, one outboard.
My suggestions are don’t tail gate and make sure your insurance is paid up. Beyond that, check to make sure the flexible lines leading to each caliper are in good shape. With age, they sometimes become soft and that can cause a spongy feel when you step on the brake pedal. Also, if are using or have used silicone brake fluid, that could be the problem– silicone fluid is somewhat compressible and can lead to a softer than normal pedal.
If you’re convinced that air in the lines is a problem, I’d suggest the following procedure. Open the bleeder on the left front caliper and have an assistant slowly depress the brake pedal. Be sure to close the bleeder before pressure is released from the pedal. Wait approximately 30 seconds and repeat until you’re confident all the air has been removed. Then move to the right front caliper and do the same thing.
Next move to the left rear caliper and bleed from the inboard bleeder, THEN the outboard bleeder. (Remember, the rear calipers have two bleed screws.) Finally, repeat the procedure on the right rear caliper. As a bit of added insurance, try gently rapping each caliper with a rubber mallet (to dislodge any air bubbles) while you’re bleeding it. After installing new pads, it may be necessary to rebleed the system after seating the pads (which usually takes about 100 miles).
Tire Update
I own a 1976 Corvette Coupe and I am in the market for new tires. My current tires are a GR7015 series. I would like to upgrade to a 60 series tire. I have read that people who bought 1978-1982 models had factory P255/60 R15 tires. Will these fit my wheel wells without rubbing? If not, any suggestions would be greatly appreciated. Thank you.
Low profile (50 and 60 series) tires will fit 1968-1982 Corvettes without rubbing.
You’re in luck– the masseuse is on vacation, so there will be no rubbing. I’ve installed P255/60R15 and P265/50R15 tires (front and rear) on 1968-82 Corvettes and never had an interference problem. However, if you have wheels with a bizarre back spacing, you could run into a rub. For your reference, nominal dimensions of the tires in question are as follows:
Size OD Tread Width
GR70 27.3 6.20
P255/60 26.9″ 8.20″
P265/50 25.5 8.45″
Keep in mind that increased tire width may require a change in wheel back spacing to prevent the tire from contacting the chassis when the steering wheel is turned toward full lock. Then again, it may not. So before purchasing tires that are significantly wider than the ones currently installed, cut a piece of cardboard that’s as wide as the tire you intend to purchase. Center it on the existing tire and tape it in place. Then, place a jack beneath the lower A-arm and raise the tire off the ground. Turn the steering wheel to full lock and rotate the tire. If the cardboard checking strip doesn’t encounter any interference your new tires shouldn’t either. However, don’t assume that adequate clearance on one side assures the situation will be the same on the other. Check both sides.
No Traction, No Satisfaction
I have a 1985 Corvette with a 406 package and have no traction in 1st or 2nd gear. I have recently changed to Michelin 315/16s but to no avail. Can you tell me what aftermarket 15″ wheels will fit over the brakes so I can run slicks? As you know, they don’t make 16″ slicks that will fit in the wheel well.
Mickey Thompson 16” diameter ET Street drag tires should solve any C4’s traction problems at the drag strip.
Until someone comes up with a way to put a 16″ peg in a 15″ hole, you won’t be able to install 15″ diameter wheels on your Corvette. However, there are a couple of options. In my opinion, the best choice is the ET Street tire from Mickey Thompson Performance Tires. It’s available in a 10” wide, 26” tall version that fits 16” diameter rims. (That’s the same diameter and tread width as is commonly found with 15” ET Street tires or ET Drag slicks.) I can tell you from personal experience, that ET Streets hook exceptionally well. In some instances, they offer better traction than slicks. Other options are BFGoodrich Drag Radials and Nitto radial race tires.
One other point to be aware of—wheel/caliper clearance can vary significantly according to wheel brand and style.
Squeaking Along
I own a ’79 Corvette with an L48 engine, and an automatic transmission. Approximately a year ago I had the entire brake system rebuilt. The master cylinder, calipers, pads and parking brake were replaced. Immediately after the rebuild it developed a noticeable squeak in the rear brakes without the brakes being applied, especially at low speeds (10 mph). I have purchased a variety of squeaky brake cures and nothing works. I have also checked the rear end to see if something was rubbing, but can’t find anything. So far the only thing that works is to turn up my radio and ignore it. HELP!
Excessive rotor runout can cause a variety of brake problems that are difficult to diagnose.
What’s the big mystery? The noise is being caused by the phantom squeaker who has simply decided to take up residence in your ’79. The phantom always comes to visit when there’s excessive runout in the rear discs. If that’s the case, the squeak you’re hearing is being caused by the rotors moving the pads back and forth on their retaining pins. If applying slight pressure on the brake pedal silences the phantom, you’ve got your answer. Mount a dial indicator so that you can check rear rotor runout. According to Chevrolet specifications, runout should not exceed .002″. But unless you believe in fairy tales, chances are you’ll never see that with a 16 year old brake system. Shoot for a maximum of .004″ and that should solve your problem. A little grease on the retaining pins (little is the operative word) wouldn’t hurt either.
By the way, if you can’t get runout down to the desired specification, you may encounter other problems down the road– the back and forth cycling of the pads can cause the rear portion of the hydraulic system to ingest air, leading to a spongy feel and reduce rear brake effectiveness.
Getting a Brake
I have a 1987 Z52 Corvette Coupe with 24, 000 miles. I am anticipating a brake job soon.
I am confused about brake pads. I am told that I must use factory or genuine GM pads or they will squeak. I can also buy pads anywhere from $28 to near $100 per axle for the car. Is there a difference in pads as far as manufacturer? I know that there is a difference in price for sure!
Can you advise me as to what to do and what to buy? I would like the best value for the dollar as everyone does. There is too much of a difference here and I am confused. It sounds like the tire business.
Semi-metallic brake pads of the non-original equipment persuasion, offer exceptionally good stopping power.
Whoever told you that you must use factory pads is either on drugs, or stands to make a profit if you follow that advice. Just about any name brand, premium quality brake pad will provide more than adequate stopping power. I’ve had excellent results with the pads sold by Vette Brakes and Products (800/237-9991). They offer semi-metallic pads for a very reasonable price and they should be more than adequate for your car. A lot of what you pay for (or should be paying for) in a premium set of pads is fade resistance. Semi-metallic pads offer excellent fade resistance for street driving and autocrossing. Full metallic and other race-oriented compounds offer superior performance and are more expensive, but these types of pads are unnecessary for street driving and some are totally inappropriate because they don’t stop a car particularly well unless they’re REALLY warmed up.
Pull Out All the Stops
I have a daily driver ‘63 roadster. My problem is with the brakes, when I make quick, sudden stops. It pulls to either side, usually the right, so badly that it becomes dangerous. The brakes work better after adjusting, but after a week or two of driving, they get bad again. Is this normal on older Corvettes? I have talked to several other people with ‘64s that have the same problem. I am also considering replacing the drum brakes with disc brakes. Is this feasible?
Use of high quality DOT 4 brake fluid is essential for proper brake operation. It’s also not advisable to mix synthetic and conventional fluids
After all the fluid is drained from a hydraulic system, there’s still some left in the calipers or wheel cylinders. The remains aren’t very appetizing.
Sounds to me like this is a job for BRAKEMAN– defender of asbestos, drum major and champion of oppressed rotors. BRAKEMAN would pull off the wheels and drums and examine them both. He would be trying to determine whether one side was grabbing or the other side was fading. That may seem overly simplistic, but it sounds like something has gone awry between the brake linings and the drums. Fluid on the friction surface may be causing one side to grab, or surface irregularities in the drum may be compromising stopping power. Another possibility is that the wheel cylinders need to be rebuilt. There may be something that’s preventing full actuation of the brake shoes on one side. Excessive play in the steering gear or linkage is another possible cause of your problem. (Reaction to the forces generated by the brakes being applied may actually be “steering” the car to one side or the other.)
BRAKEMAN would also advise you to drain the hydraulic system and refill it with a DOT 4-rated brake fluid. You problem could be caused, or made worse by brake fluid that has been contaminated by air or water. Another possibility is that you’ve mixed synthetic and conventional fluids. The two different types of fluids are supposed to be compatible, but they aren’t. I’ve experienced a number of spooky brake system problems (ranging from spongy brakes to a pedal that went to the floor) that started when I drained a hydraulic system and refilled it with synthetic fluid. Although 95% of the original fluid was removed, some remained in the calipers. The brakes worked properly again after I disassembled the calipers, cleaned out ALL the remaining fluid and refilled the system with DOT 4 fluid. With drum brakes, you may need to disassemble the wheel cylinders to purge all the old fluid.
The situation is definitely not normal, so follow BRAKEMAN’s advice, pull the drums off, check the steering and suspension, drain and refill the hydraulic system and find the cause. As for converting to disc brakes, I don’t have enough room to go into all the gory details, but the easiest approach would be to install the system used on 1965 and later Corvettes. Since the chassis of drum and disc brakes Corvettes are virtually identical, brake changeovers are relatively easy.
Tire Troubles
I have a ’94 Corvette Coupe w/6-speed. It has the original Goodyear GSC tires. The front tires are 255/45ZR-17. The rear tires are 285/40ZR-17. I faithfully keep them pressurized to 30 psig. The initial performance of the tires was great. But, after about 12,000 miles, they began to sound like off-road truck tires. I’ve kept the tires balanced and have had the alignment checked every 5000 miles. The alignment has never required more than a slight adjustment. The tires appear to be wearing evenly. However, as I look at the tread blocks from the side, they have a very slight saw-tooth look. My local Goodyear dealer says the problem is caused by not being able to rotate the tires. I now have 23,000 miles on the car and the tire noise is absolutely ridiculous. I’m thinking about replacing the front wheels with the same size as is on the rear of the car and using 275/40ZR-17 tires on all four wheels. The goal is to be able to rotate the tires. Would this help my problem? Will I notice a substantial change in ride and handling? Would changing to another brand tire reduce the problem? I’m willing to sacrifice a little performance for a quieter ride. What is the motivation for designing the car with smaller front tires? Handling? Speed? Gas Mileage? Ride?
Tire rotation isn’t possible with position specific tires, but that doesn’t usually cause unusual wear problems.
May I suggest ear plugs? This is a rather unusual problem, but I don’t agree that it’s being caused by the fact that you can’t rotate your tires. That would imply that any tire that had been driven 23,000 miles in the same wheel position would become noisy. I don’t think the brand of tire is the cause. Either the alignment is or was off, or there’s a problem in the suspension, or you just got stuck with a bad set of tires.
According to Dave Wilkins of Goodyear, one of the problems associated with tires designed to run in a single wheel position is that vehicle loads are unbalanced from side to side. With the driver and steering gear being on the left, the left front tire is loaded harder than the right front and consequently it’s not unusual for it to develop an unusual wear pattern if a vehicle is driven aggressively around corners. Goodyear knew of potential wear problems when Chevrolet requested unidirectional tires and took steps to minimize them in the design of the GS-C tires. But unbalance within the vehicle can’t be eliminated, so some amount of side-to-side variation in tire wear will exist. But problems such as the one you describe are rare.
Once a wear pattern is established, it gets progressively worse with time. If someone else drove your Corvette, or you began traveling over a poor road surface, that would be enough to initiate an unusual wear pattern. Even after those conditions are eliminated, normal driving will continue wearing the tire at an accelerated rate. However, as a tire accumulates mileage, it becomes less susceptible to wearing erratically. So if you have the tire trued, (wherein the tread is shaved to remove the wear pattern) that should solve the problem. If you’d like additional information about your tire problem, contact the Goodyear Customer Service Department at 800/321-2136.
The concept behind using different size tires and wheels on the front and rear is to optimize handling, ride and traction. Larger front wheels and tires are heavier and place greater loads on suspension and steering components. From my experience, that’s more of an engineering consideration than a real world concern. More rubber on the ground up front will improve handling, but potentially will worsen ride quality. It wouldn’t be a bad swap, installing larger tires on the front, but I’d find the cause of your tire noise before spending money on new tires which may end up being just as noisy.
Time to Rotate
My current Corvette is a 1996 Collector Edition Coupe with the RPO Z51 performance handling package which includes Goodyear GS-C P275/40ZR-17 tires on all four corners. My question is simple. Should I rotate these tires front to back (on the same side, of course) or should I run them in the original positions? I do not do any competition driving with this Corvette.
Tire rotation is always a good idea when it’s possible. With the advent of asymmetrical treads and different sized front and rear tires, rotation is obviously not possible and this has led some people to think it’s unnecessary. Like most things automotive, it’s a matter of trade-offs. Position specific tires potentially offer improved handling at the expense of wear. Front and rear tires wear at different rates, as do left and rights. Rotating the tires is simply a means of evening out the wear patterns, which extends tire life. Some people feel that the time and expense involved in tire rotation isn’t justified because tire life isn’t extended enough to pay for it. Other people prefer to leave tires in their original position so that handling characteristics don’t change. If it were me, I’d rotate the tires if possible. Just make sure that all four wheels are the same size. If the rears are wider than the fronts, tire rotation may not be a viable option.
Tire Worries
Ever since the Firestone fiasco, I’ve really wondered about tire quality and safety. I’ sure that Firestone isn’t the only company with quality control issues, so I guess there aren’t any assurances that tires on a Corvette won’t come apart and cause a loss of control. Do you have any advice for tire shoppers?
The high stresses of autocrossing and similar activities haven’t resulted in an unusually high incidence of Corvette tire failures.
The only advice I can offer is that you’re probably worrying needlessly. Consider all the unusually high stresses placed on Corvette tires (such as autocrossing) and the extremely rare incidence of tire failure. My personal opinion is that the problem that instigated the recall of Firestone Wilderness AT tires was caused by poor manufacturing quality control. In spite of all the claims and counter claims by Ford and Firestone, the fact of the matter is that literally millions of Wilderness AT tires performed without failure. If the tread separations were caused by poor design, or Ford’s recommended inflation pressure being too low, there would have been many more tread separation than there were.
Considering the Corvette’s relatively low center of gravity, and its excellent suspension, a rollover in the event of a tread separation is rather unlikely. And in light of the devastating effects of the Firestone fiasco, you can be sure that all tire companies have stepped up their quality control.
Those considerations aside, the best way to minimize the possibility of tire failure is through regular checks of inflation pressures and inspection of tread surfaces.
A Hot Time
When I autocross my ‘95 coupe, I’ve noticed that some of the other racers use a gauge to check tire tread temperature. I’ve always wondered whether this information was of any real use. These guys don’t seem to run any faster the guys who don’t check tire temperature. Can you really learn anything by checking tread temperature.
Checking tread temperature is an excellent way to gather the information needed to fine tune suspension settings.
The temperatures across the face of a tire’s tread are an indication of the shape of the tire’s contact patch. Higher temperatures indicate more friction, resulting from higher loading. Ideally, temperatures should be nearly identical at the inner, center and outer checking points. In reality, they never are, but adjusting tire pressures and caster/camber settings will minimize differences. As an example, if the outside portion of the tread is significantly hotter than the center or inside, it indicates that the tire is either leaned over (due to camber setting) or rolling over (due to insufficient air pressure). Of course, in order for temperature readings to be truly meaningful, you have to consider the layout of the test course. If right hand turns are significantly tighter or faster than the left-handers, one edge of a tire will always be significantly hotter than the other. Similarly, if some turns are off-camber while others are on-camber, readings will always vary significantly.
My guess is that the guys who checking tire temps and haven’t improved their lap times are not interpreting the data correctly.
Not Good Vibrations
I have a 1978 Corvette with a vibration problem that starts in the manual gear shift. It started at 80 mph now it starts at 65 mph. The tires have been rotated and balanced, the front end has been aligned, the drive shaft has been balanced and the rear bearings have been checked, can you help me out? I don’t know what else to do.
The vibration starts in the transmission, so at the risk of sounding like a smart ass, (which is certainly contrary to my nature) why don’t you look there. It’s possible that one of the bearings inside the trans is worn and allowing one of the shafts to move around. I’d suspect the rear bearing that mounts in the plate between the main case and the tailhousing. I seriously doubt the clutch or input side of the transmission is the culprit because the vibration is related to vehicle speed, not engine speed. The fact that it is now occurring at a lower speed is indicative of a component that’s worn, rather than out of balance.
Some other possibilities are worn bearings inside the differential, bad universal joints or a distorted or damaged transmission yoke– which probably wasn’t checked with the driveshaft.
Gearing For Change
I’ve read about changing the standard rear axle gear on a 1992 LT1 automatic to the 3.45 ratio used in 6-speed cars. I own a 1990 L98 automatic coupe which has the 3.08 rear axle right now. I would like to go to the 3.45:1 ratio. If I do this, will I need to put in a new PROM so the transmission will shift properly or will the stock PROM handle this rear axle change?
With a 700R-4 transmission, a change in rear axle ratio requires different speedometer gears.
The stock PROM will be fine. However, you will need to change the speedometer gears. Torque converter lock-up point is affected by vehicle speed, so the transmission has to “tell” the computer the truth. You’ll need a 15-tooth drive gear (gray color code, part number 8642620) and depending on tire size either a 41-tooth (yellow, part no. 25513048) or 42-tooth (green, part no. 25513049) driven gear. Also required is driven gear sleeve number 25007339.
Lock-Up Lock Out
I own a 1982 Corvette which needed to have its 700R-4 transmission rebuilt at least the second time.
The rebuilder I recently hired told me that whoever rebuilt the transmission prior to him, either cut the wires or removed the connector to engage the lock-up.
He was able to produce the same results as lock-up by hot wiring 2 wires to the cruise control part of the stop light switch. One wire comes from the transmission, the other wire is hot from the fuse “Ign” panel. This leaves me with no cruise control.
Can you tell me where the wires to connect the lock-up feature are located?
Looking for lockup? The clutches are inside the converter, but the connection is on the side of the transmission.
This is starting to sound like a soap opera– or in this case, a trans fluid opera. Where do you find these people? And why do you let them work on your car? The lock-up feature on any 700R-4 transmission is controlled by the computer through a wiring harness that plugs into the left side of the trans case about six inches in front of the governor. Since the lock-up feature is presently working, all the internal wiring must be intact, so I’m at a loss as to what was removed.
Your first step is to disconnect the jury-rig arrangement you have now and get the cruise control working properly again. Then try to find the connector on the wiring harness that plugs into the trans. If that’s the part that’s gone, order a universal lock-up kit from a high performance transmission company. These kits will lock up the converter whenever the transmission is in fourth gear and manifold vacuum is over about 4 in/Hg. Installation is so simple even your former mechanic could do it right. Just two wires and a vacuum line must be connected. One wire connects to a 12 volt source, the other to ground. The best bet with the vacuum line is to connect it to a ported source. That way, the converter will unlock when you get completely off the throttle. When you step on it again, the converter will remain unlocked for a second or two, then lock up again. This type of operation makes for much smoother acceleration at lower speeds.
Transmission Transplant
At present, I’m planning to install a 700R-4 transmission in my 1976 Stingray. Would you please provide me with all the information you might have?
Transplanting a 700R-4 into an earlier model Corvette isn’t particularly difficult.
All the information I have? Not on your life. We don’t have enough room, I don’t have enough time and you don’t have enough money. But you don’t need to know all that much to install a 700R-4. The transmission will bolt directly to your engine with no problem, however some adaptations are necessary. The 700R4 has a 27-spline output shaft; if you’re replacing a TH400, which has a 32-spline shaft, the driveshaft yoke must be changed. (A Turbo 350 has the same 27-spline output shaft as a 700R-4) There’s also a difference in overall transmission length so it will probably be necessary to shorten the driveshaft. 700R4s originally built for Corvettes are about an inch shorter than other models, but they’re longer than either a Turbo Hydro 350 or 400 (some long tailhousing versions excepted) so you’ll need to have the driveshaft shortened. For reference, overall length of a standard 700R-4 is 30-3/4″; the Corvette version is 29-7/8″ in length. Turbo 350s and 400s vary in length from 28-1/4″ to 30-3/4″.
The 700R-4 also employs a different transmission mount than some 350s and 400s, so you’ll have to make sure you have a mount with 3-3/4″ bolt spacing that mates to your crossmember (most TH400 transmissions use a mount with 4-1/4″ bolt spacing). Then you’ll probably have to modify the crossmember bracket to which the mount bolts so that everything lines up. Keep in mind that the 700R4 is a metric transmission and you may need to purchase some nuts and bolts. Once you have all that ironed out, you’ll need to install the appropriate throttle valve cable and torque converter lock-up control which are available from TCI Automotive and other transmission specialists. You’ll also need a dipstick and tube which are available through TCI or a local Chevy dealer.
An appropriate throttle cable mounting bracket for the intake manifold is also necessary. It may be possible to find one at a wrecking yard, but if you can’t, TCI’s bracket, part no. 376700, (for engines with QuadraJet, Carter AFB and Holley carburetors,) or Holley’s part no. 20-45 “TH-350 Kickdown cable bracket” will work.
You’ll also need a universal Throttle Valve cable (TCI part no. 376800) so the transmission can shift automatically and you’ll need to modify the transmission if you desire to retain converter lock-up. In original equipment applications, converter lock-up is controlled by the PCM, so you need a non-computerized replacement.
My apologies for being vague in some instances, but I’ve found that published transmission specifications don’t always match with real life; many times, adaptations that aren’t supposed to be necessary are and those that are aren’t. But once you have all the little wrinkles ironed out, all you have do is connect the lines leading to the radiator and the shifter cable.
Depending on the vehicle, it may be possible to have manual control over all gears with the original shifter. However, it may not have enough range to hold the trans in first. If that’s a problem, an aftermarket shifter can be installed.
Bad Vibrations
I have changed universals (u-joints) for a third time on my 1978 Vette (in 12 months). The right rear outside u-joint lasted about 3,000 miles. Have I done something wrong or could the drive line be bent or could the posi-traction be causing the problem? Wheel bearing is fine.
I felt a vibration between 55 – 65 mph even before the u-joints went out. I notice it more under hard acceleration and I have heard that putting the drive shafts in wrong (flip flop) from one end to other will cause vibrations, is this fact or fiction? I also notice some Positraction noise (clank) on occasion.
It sounds as though your Corvette is suffering from an almost terminal case of rear spring sag. When that occurs, the chassis moves to a lower ride height and unless you realign the rear wheels, excessive stress is placed on the half-shafts, U-joints and the rear itself. If the car has been wrecked and the frame is distorted, that can also be the cause of your problems.
You may need to install adjustable strut rods to properly set rear camber. Start by getting the rear wheels properly aligned. Then install high quality U-joints and you shouldn’t have any problems. There’s no valid reason for a U-joint to last only 3,000 miles. There’s also no need to worry about which end of the half shaft goes where.
Quick Shift Too Quick
I own a 1987 Corvette coupe with 700-R4 automatic transmission. I recently changed the transmission fluid and filter and adjusted the throttle valve cable to specifications. Under normal driving conditions, the transmission shifts correctly and I am pleased with the performance, but with the pedal floored, it’s a different matter. The 1-2 shifts occur too late and slowly for optimum performance. In the past, I have set the TV cable to bring the shift points down to 4500 rpm to improve 0-60 mph times, but normal driving suffered because it shifted too soon.
I am considering installation of a shift kit, but am not sure if any of them will improve shifting under hard acceleration and not affect normal driving. My understanding is that the shift point is determined by hydraulic pressure set by the governor and shift valve and shift firmness by the accumulator. Do these kits only make the shift firmer. The stock transmission when shifted manually isn’t really manual control because shifting still occurs by itself. Is there some way I can bypass the programmed shifting when I manually shift from 1 to 2 and leave the “D” selection unaffected?
The solution to your problems is simple.– it’s called a six- speed manual transmission. What you’re trying to do with the 700-R4 is like driving at night while you’re wearing sun glasses– difficult, but not impossible. The first thing you need to do is work with the governor, which has the biggest influence over shift points. Several high performance transmission specialists offer a kit that includes several springs and weights and allows you to “tune” the governor to achieve a desired shift point. This is strictly a trial and error process because each transmission will react somewhat differently to a specific spring/weight configuration.
To raise shift points, install lighter weights and springs; to lower them, do the opposite. You’ll find that the 1-2 and 2-3 shifts won’t necessarily occur at the same rpm, so you may have to settle for an automatic 1-2 shift and a manually controlled 2-3 shift. Manual control functions properly only if you shift at a higher rpm than the governor does. In effect, the shift lever can be used to hold the transmission in gear longer than the governor will allow, but moving it won’t force an early shift during wide open throttle operation.
Shift improving kits typically affect only shift firmness, but some of them alter internal pressures which in turn affect shift points. If you decide to install a shift-improving kit, do so before you start experimenting with the governor, otherwise you may have to go through the governor recalibration a second time. It may also be necessary to work with TV cable position to arrive at your desired shift points. Generally, you want to move the cable back towards the firewall, (to increase shift firmness) rather than forward. If you pull the cable too far forward, insufficient throttle valve pressure will be the result and that will accelerate transmission wear.
You should be able to tune the transmission so that it shifts at the desired rpm, but you’ll have to spend a bit of time experimenting.
They Often Call Me Speedo
I drive a ’74 coupe with an L82, TH-350 transmission and 3.55 gears all from a ’77 donor, and the car is on 255/60 tires.
This all runs fine except for the speedometer which is also from the ’77 donor. What I’d like to know is what drive and driven gear (no of teeth) is needed, and if there is an adapter needed between the speedometer cable and the driven gear. At the moment the reading is much too high which is very annoying.
I’d look for a part number 8623310 drive gear and a 40-tooth driven gear (black, part no. 1362048) and a part number 1362293 sleeve assembly. If that didn’t solve the problem, I’d try other gears such as part numbers 1359272, (blue, 38 teeth) 1362195, (yellow, 41 teeth)1362049,(green, 42 teeth) or 1362196 (purple, 43 teeth). Many of these gears were also used in 1968-76 Corvettes, so they shouldn’t be too hard to find.
Transmission Mission
I currently own a 1972 Corvette roadster with a 3.70 rear and Muncie four speed. My biggest problem is that on the highway, the engine turns around 3000 rpm at 70 miles per hour. As you know, this is hard work for my engine, quite annoying for passengers inside the car and bad for gas mileage. In town, the car is lots of fun as the rear end ratio allows me to rocket away from stop lights quite quickly. This aspect of the ratio I like, so I’d like to keep the rear gear as it is.
Therefore, can I change the transmission to a 5-speed without much difficulty? I know that Chevy made a 5-speed for Camaros a while back and that hooked up to a small block. Would this transmission fit in my car? If you’ve ever heard of this GM transmission and know of the particulars that would be required to install it in my ‘72 please let me know.
Unlike many of the people in this country, your engine isn’t afraid of hard work, so that’s not much of a problem. On the other hand, cruising at 3000 rpm makes for an annoying and gas consuming ride, so you’re smart to be looking for an overdrive transmission. However, the five-speed that you mention, officially known as the Borg-Warner T-5, isn’t what you’re looking for. It’s too weak to handle the torque of a 350, which is the reason Chevrolet offered it only in 305 Camaros. On the other hand, the T-56 six-speed used in 1993 and later LT1 Camaros, is an excellent transmission. I’ve personally driven one on several low 12-second quarter-mile excursions so I know strength isn’t a problem.
Unfortunately, I don’t think you’ll have much luck installing either of these transmissions in your Corvette. Both incorporate internal rail-type shift mechanisms, so you don’t have any options as to shifter placement. There are two chances that the shifter will line up with the hole in the floor and console– slim and none.
There are a number of other options. The 4+3 transmission is based on the Borg-Warner Super T-10 so fit shouldn’t be too much of a problem. But this transmission has a history of problems, so it’s probably best to avoid it.
The most practical approach is probably to install a Super T-10 with an exceptionally low first gear ratio. The Super T-10 was originally produced by Borg-Warner, and is now manufactured by Richmond Gear. Super T-10s have been produced with a variety of first gear ratios including 2.43, 2.64, 2.88 and 3.42:1. By switching to either the 2.88 (CC ratio) or 3.42 (Z ratio) first gear, you can drop the rear axle ratio to give you the same effect as an overdrive. As an example, assuming that your Muncie has a 2.20 first gear ratio, with the 3.70:1 rear gear, overall ratio in 1st is 8.14:1 (2.20*3.70). To achieve that same overall gearing with a 2.88: first gear you’d need only a 2.82:1 rear axle ratio. On the other hand, with a 3.42:1 first gear, you’d need only a 2.38:1 rear.
For a number of years, I owned a ‘69 Corvette with a Super T-10 that had a 3.42 first gear and 3.08:1 rear axle ratio. That combination provided a 10.5:1 overall gearing in 1st and over 20 miles per gallon on the highway. The only negative to this arrangement is that the 3.42:1 first gear reduces the transmissions torque capacity, and there’s a fairly wide rpm drop between 3rd and 4th gears. (The drop is the same as that found in wide ratio Muncies. Both transmissions have a 1.46:1 3rd gear ratio.)
The Super T-10 is essentially a bolt-in replacement, so installation is easy. All you’ll have to do is change a shift arm and possibly driveshaft yoke. Conceptually, think of the wide ratio Super T-10 as a three speed transmission with overdrive, because that’s the effect you’ll get if you change to a high ratio (low numeric) rear ratio. If a complete new transmission is too pricey, you may be able to find a used gear box and buy the appropriate gears to convert to either the CC or Z ratio gearset. Whip out your calculator and run the ratio numbers to determine which transmission gearset/rear end ratio combination is most appealing.
Slippin’ and Slidin’
I have a 1993 40th Anniversary Edition automatic with Z07. I am curious to know how to tell if the limited slip is working properly. I used to have a 1984 automatic with Z51 and do not recall this being a problem. Here is the issue:
This past winter after a snow storm, I took the Vette out once the streets were clear. When I came back home I attempted to get up the driveway. There is a section of the driveway that is in the shade much of the day. The right side of the drive had about a 3 foot patch of ice. When my right rear tire got on the ice it promptly started spinning. The traction control systems kicked in and reduced power to the point all I could do was sit there and listen to the right rear tire slowly turn on the ice. I turned off the traction control and still nothing. The left rear wheel simply sat there and received NO power. My understanding of Corvette Limited-Slip differential is that the wheel with the most traction gets the power. Not in this case. On dry level ground both wheels clearly get the power. I later tried a similar situation as a test where I had the right side of the car on sandy ground and the left on the street. Same thing, the right tire would spin and the left rear seemed to receive no power. I did the same thing again except with the left wheel on the sand. Same thing, the left wheel would spin in the dirt and the right wheel appeared to receive no power.
All the dealer can tell me is that if I burn rubber with both wheels on dry ground then the limited-slip is working. I don’t recall my 1984 working this way. It has been several years but I seem to recall that both wheels would get power in similar situations. Any help you could provide clearing up this dilemma would be appreciated.
Ah- a dealer to the rescue once again with indisputable words of wisdom. What would we do without them? A limited slip differential is supposed to provide more or less equal traction to both drive tires under virtually all conditions, when left/right side speed differentiation isn’t required. However, in order for this to occur, the limited slip mechanism inside the differential must lock. I have seen instances virtually identical to the ones you describe. A common place to see them is at a drag strip. Frequently when a car pulls into the water to do a burn out, you’ll see one wheel absolutely sitting still while smoke boils off the other. When the same car pulls to the starting line, it may very well spin both tires when it’s launched; the resistance offered by dry pavement under both wheels enables the clutches to lock up.
It sounds like you’re experiencing a similar situation. To double check, try a reasonable hard launch with both wheels on dry pavement immediately before you do a test with one tire on a slick or slippery surface. (Don’t go around any sharp corners between the dry surface and slippery surface tests as that may cause the rear to unlock.) If the limited slip functions properly after the launch, you’ve determined the cause of your problem.
About the only way to cure it is with some GM limited slip additive. If that doesn’t help, you’ll have to convince your dealer to replace the differential.
In the Clutches
I have a ‘95 LT1.. My car is at the dealership now getting the 4th clutch installed. The car has 82,000 miles on it now. The dealer also has installed 3 transmissions, due to excessive gear noises. Have you heard of any particular problems with the clutch in the 92’s? The first clutch lasted only 5K, the second was replaced at 15K and the third at 36k. Now the 4th is being replaced at 82K
Do you think the CAGS system might have an effect on the failure of these clutches. It would seem to me that the shift from 1st to 4th would generate excessive heat on the clutch assembly due to slippage. Is there a way to disconnect the CAGS system?
I wanted GM to purchase the car back at 36K. They offered me a 100K warranty instead.
Look at the positive side of your situation– each clutch seems to be lasting longer. Three or four more replacements, and clutch life should increase to 70,000-75,000 miles.
It sounds as though there have been a couple of gremlins at work. The 46,000 miles that your current clutch survived isn’t anything to complain about. Evidently something changed when it was installed because the first three clutches lived much shorter life.
Depending on your driving style, CAGS could be contributing to clutch wear. In my experience, this is rare because typically, the clutch engages and the engine just lugs along until rpm comes up to a reasonable level. Now if you’re causing the clutch to slip by leaving your foot on the pedal, or purposely depressing it, all bets are off.
CAGS can be disconnected. If you unplug the connector from the solenoid on the transmission, CAGS will not operate. However, the ECM will pick up on the fact that a connector is dangling in the breeze and set a trouble code. Some companies sell CAGS eliminator harnesses which consist of a 15-20 ohm resistor attached to a Metripack connector that plugs into the CAGS connector on the wiring harness. There’s also another connector that plugs the receptacle of the switch on the transmission to prevent moisture from corroding the contacts.
Balky Shifter
I have a small problem (I hope) with my 1980 4-speed shifter. When I attempt to shift into reverse, the transmission always grinds. The only way around this is to shift into a forward gear first, then I can shift into reverse without a problem. When I bring the shifter out of reverse, it doesn’t pop out automatically. I have to slap the shifter.
Your problem probably is small, but it isn’t the shifter. It’s the clutch. Specifically, it isn’t disengaging fully when you depress it, so the transmission input shaft is still spinning when you try to shift into gear. Being the skeptical type, you’re probably wondering why it doesn’t grind when you shift into a forward gear. Synchronizers. If they weren’t there, the transmission would grind every time you shifted into any gear. But when you shift into a forward gear, the synchro ring grabs the speed gear on the main shaft, gets it up to speed and allows the slider to smoothly engage. When it does that, it stops the input shaft from spinning, so if you shift immediately into reverse, there’s no grind. However, you should also notice that it takes considerably more effort to shift into a forward gear than it did formerly, when the clutch was operating properly. That’s because the synchros are being worked a good bit as they try to stop the input shaft from spinning while concurrently attempting to effect a smooth gear engagement.
Assuming this problem occurred with no apparent provocation, you probably can correct it by adjusting clutch pedal free play. You’ll find the linkage adjustment under the hood, at the end of the clutch rod, just above the frame rail on the driver’s side. The clutch needs less free play and when you make that adjustment, the pressure plate will be lifted higher when you push the clutch pedal to the floor. That should result in a clean release of the clutch disc and once that occurs, you should be able to shift into reverse without a grind. Just remember, you’ll have to wait a few seconds for the disc to stop spinning once you depress the clutch. Otherwise it will still grind. I routinely shift into first before shifting into reverse for just that reason– to quickly stop the disc from spinning rather than waiting for inertia to run its course. A clean release of the clutch disc should also allow the shift lever to “pop” out of reverse when you pull on it.
Now, if you haven’t been waving to other Corvette drivers, don’t take baths regularly or engage in any other unbecoming conduct, you may have much more serious problems. The symptoms you describe can also be caused by a warped or damaged clutch disc, broken motor mounts or broken clutch linkage.
It’s the Torque
I have a 1991 coupe that I’m going to start hopping up. I’m pretty well set for engine modifications, but I’m wondering about the transmission. Is it really worth while to change torque converters? I’ve heard pros and cons as to whether they’re worth the money. What’s your opinion?
My opinion is that I’d rather have a car with a clutch pedal, but that’s another story. As for Corvettes with automatic transmissions, a high performance torque converter is definitely worth while. It can make a tremendous difference in launch and acceleration. I’ve seen a modified torque converter improve quarter-mile elapsed times by 3/10 of a second in conjunction with a stock-type engine. However, don’t fall into the trap of thinking more is better when it comes to stall speed. If the converter is too “loose” (that is, stall speed is too high) slippage will be excessive and your Corvette will feel as though it has an old Buick Dynaflow transmission in it. For a street-driven Corvette, all you need is a slight increase in stall speed. A 12” converter from a reputable manufacturer should work ideally.
Looking for Mr. Speedo
I own a 1986 Corvette with a 700 R4 transmission. I recently replaced the stock 3.07 gear set with performance 3.73 set. Could you please tell me the proper drive gear/driven gear combination to correct my speedometer. Currently 67 mph on my dash = 55 actual mph. I am using ’89 rims with 275/40ZR17 Goodyears.
I’d leave it the way it is– it will be almost impossible to get a speeding ticket. But being a Corvette owner, you probably want everything to function correctly, so in answer to your question– you need to start with a gray drive gear which has 15 teeth. Two part numbers are available- 1250760 and 8642620. Next, you’ll have to determine tire diameter so the tooth count of the driven gear can be computed. According to Goodyear specifications, your tires should have an overall diameter of 25.67 inches. That translates to 785.66 revolutions per mile (20168/tire diameter (in inches)= tire revolutions per mile. The formula for determining driven gear tooth count is; #Driven gear teeth=# drive gear teeth*axle ratio*tire revolution per mile divided by 1001. Working through the formula, 15*3.73*785.66=43957.677. That number divided by 1001= 43.91 so you need a 44-tooth gear which is dark blue and a part number of 25513051 without a sleeve and 25522495 with a sleeve. You’ll probably need the sleeve to achieve proper geometry between the two gear. If the sleeve is incorrect, the teeth will be sheered off the driven gear or the two gears won’t mesh. Another option is to modify the driven gear retainer, but that would get me into trouble with Corvette purists.
The Need To Speed
What is speed shifting? My friends and I disagree about whether or not a Corvette or other high performance automobile can be shifted from say, first to second, at full throttle without the using the clutch pedal. Isn’t it a common racing practice? Thanks for a great magazine.
The definition of speed shifting seems to depend on a person’s age, or lack thereof. Traditionally, speed shifting was applied to any rapid gear change, but during such a shift, the gas pedal is released and reapplied after the clutch is re-engaged. Power Shifting was the term used for a gear change that’s made with the gas pedal remaining hammered to the floor while the clutch is released and re-engaged. In recent years, the term “speed shifting” has been applied to gear changes that used to fall under the “power shifting” label. Unless you have King Kong next to you to pull on the shift lever, it’s generally not possible to shift under full power without using the clutch. Pressure generated by the power going through the transmission puts quite a load on the gears making it difficult (but not impossible) to move a slider if the clutch is not used. It can be done, the question is, why would you do it?
I can tell you from personal experience that Corvettes, including C4 and C5 models, can be successfully power shifted with no problems, provided the person behind the wheel has acceptable right hand/left foot coordination. For that reason, Viper owners are generally advised to avoid power shifting.
Rockin’ Rollers
I have a 1994 Corvette LT1 with 6-speed transmission. I am also thinking of changing the stock 3.45 rear end to 3.73. The car is currently running 13.6 at approximately 106 miles per hour in the quarter. About what can I expect from the 3.73 gears?
I wouldn’t bother with the 3.73s; you’d be better off with 4.09s. I know that sounds like a big step, but consider this; in sixth gear, the ratio is .5:1 so your effective overall gear ratio is only half of the actual ring and pinion ratio. In the case of a 4.10:1 gearset, that’s only a 2.05:1 overall ratio, which is a small increase compared to the current 1.73 (3.45* 50%). With the 50% overdrive, engine rpm at 80 miles per hour will increase by only 250-300 rpm. At the track, the 4.09s should improve ET by half a second and three miles per hour. But be advised, you’ll need to install sticky tires for that type of improvement. With the steeper gears and street tires, the car will be very difficult to launch without excessive wheelspin.
From Cross-Fire to Misfire
I have a 1969 Corvette with the 350/300 engine. Recently it began misfiring on cylinders 5 and 8. I have checked spark plugs, wires, cap and rotor. I also checked compression and removed and reinstalled the intake manifold, None of this made a difference. Adjusting the idle mixture screws on the Rochester QuadraJet doesn’t seem to have any effect either. I plan to rebuild the carburetor, but why does it misfire on number 5 and 8 cylinders? Any ideas? I’m going buggy chasing this problem.
Take a few valium and chill. After a little respite, you may start thinking more clearly and gain back those parts of your mind you’ve lost. Then think about your misfire condition. First and most obvious, are you sure the plug wires haven’t been accidentally crossed? If the wires for cylinders 5 and 7 are crossed, that would explain the situation. Chevrolet firing order is 1-8-4-3-6-5-7-2, with the odd-numbered cylinders on the driver’s side. To verify firing order, find the distributor terminal for cylinder number one, then work clockwise around the distributor cap.
Even if the wires follow a route that makes it difficult to trace them, you can still easily find the number one terminal and determine the firing order. The wire connected to the distributor terminal for cylinder 1 will lead over to the driver’s side of the engine; the adjacent terminals, leading to cylinders 2 and 8, will lead to the passenger’s side of the engine. The only other terminal with a similar configuration is number 3, which has the terminals for cylinders 4 and 6 on each side of it. However, if you work through the firing order, (move clockwise around the distributor cap) the two wires attached to the terminals next to number 1 (numbers 8 and 4) lead to the passenger’s side. On the other hand, moving clockwise from terminal number 3, will lead to terminals for cylinders 6 and 5, which are on opposite sides of the engine. That being the case, it’s obvious which terminal leads to cylinder number 1—even if you can’t trace the wire to the spark plug.
Once you have located terminal number 1, you can check the wires for cylinder numbers 5 and 7 to see if they’ve been switched. If you can’t follow a wire along its entire route, you can simply unplug each end and use an ohm meter for a continuity check. If you hook the meter probes to both ends of a wire, it should show relatively low resistance. If, on the other hand, the wires have been crossed, the meter will show an open circuit.
Another possibility is that you’re experiencing inductive crossfire, If the plug wires for cylinders 5 and 7 run parallel to each other and are touching, the current flowing through one wire can induce a current in the other. Over the past few years, inductive crossfire has become relatively rare because most plug wires are built specifically to eliminate that phenomenon. However, if all else fails, try installing new plug wires.
Net Versus Gross
What would the net HP rating be on a ‘71 LS-5, 365 gross HP? Is there a specific formula to use or can you approximate? No air on the car if that makes a difference.
1971 was the first year in which Chevrolet quoted net horsepower figures. And to make the transition a bit more palatable to the horsepower hungry masses, while the attorneys were looking the other way, the marketing folks released both net and gross figures. (I’m sure there must be something about releasing two sets of power figures that a lawyer would find objectionable.) The 1971 LS5 engine was rated at 365 gross horsepower and 285 net. By way of comparison, the LS6 was rated at 425 gross, 325 net and the LT1 had gross and net figures of 330 and 275 respectively.
As you can see, there’s no clear-cut relationship between gross and net. At 275 net, the LT1 produces 83.3% of its gross horsepower; the LS5’s net is only 78% of its gross and the LS6 can muster only a 76.5% translation of gross to net horsepower. The differences are attributable to an engine’s relationship with its intake and exhaust systems as well as its ability to handle accessory load. With newer engines, such as the 1992 and later LT1 and the LS1, system efficiencies have improved so the gap between gross and net isn’t as great as it was in the 70s.
1990 No Go
HELP! I have a 1990 Corvette that will not respond to throttle pressure. The car starts and idles fine; however, when the accelerator is depressed the car will not travel at more than 6 or 7 miles per hour. The problem began 6-10 months ago. When I drove the car on the highway it ran smoothly at constant speed. Then my Corvette began to run very roughly when I stopped at stop signs and traffic signals. I had the battery replaced within the last six months and had a tune-up at the same time. What would cause my Corvette’s LESS than lackluster performance?
Assuming the driver knows how to operate his or her right foot, I’d say the problem is probably related to the throttle position sensor (TPS). The easiest way to verify that is to check for a trouble code. A code 21 or 22 tells you the problem is definitely with the TPS. However, it is possible for the TPS to fail without setting a trouble code. In that case, you’ll have to plug in a scanner or Diacom, turn the ignition on and watch TPS voltage. As you step down on the accelerator pedal, it should change from approximately .54 volts to 4+ volts. If it doesn’t, you know the TPS is toast and is probably the source of your problem. If you don’t have access to diagnostic equipment, you can check the sensor with a digital volt meter. The TPS has three wires connected to it; the black wire connected to terminal “A” is ground, the dark blue wire connected to terminal “B” is sensor output and the gray wire connected to terminal “C” is a 5-volt reference signal. By hooking your meter between terminal “B” and ground, you’ll be able to read sensor output. Other possibilities include a faulty air or coolant temperature sensor, a manifold absolute pressure (MAP) sensor that’s lost its marbles or even a bad ECM. Again, use of diagnostic software like Diacom, or a scanner is the easiest way to locate sensor failures.
What Price Injector Salvation?
I have a 1991 Corvette in which the fuel injectors need to be replaced. My mechanic claimed that the Delco brand by GM were the only ones he could guarantee that would work. These cost $121. apiece. I’ve seen ads in several magazines that list a set of 8 blueprint reconditioned injectors for a cost of $329 or a new set with a lifetime guarantee for $525. I wonder if you could explain the reason for such a large difference in price and whether the Delco brand is the best.
You may want to consider looking for a new mechanic, or at least tell your present one to cut the fairy tales. I’m not aware of any major quality difference between the various brands of injectors. Some people claim that one brand may have a better spray pattern than another, or may be less susceptible to the effects of fuel or foreign materials, but that doesn’t translate into three or four times the life. It might also be of interest to your mechanic that many Corvettes were originally equipped with Bosch injectors.
The “Delco brand” is actually produced by GM’s Rochester division. These injectors are available from MSD Fuel Management at a “General Trade” price of $45.00 each. That’s $360 a set and you can probably find them at a cost lower than that through a discount company. Fuel injection specialist Chuck Leeper of Cody Motorsports in Lawrenceville, GA, states that he’s seen very little difference in the performance of fuel injectors produced by Bosch, Siemens, Accel, Lucas or Rochester. He notes, “Some designs seem to be slightly more resistant the effects of dirt and crud, but I haven’t seen enough of a difference to worry about. As for flow variation, one brand doesn’t seem to have an advantage over another. Within a set of injectors, flow differences are usually—but not always minimal. Regardless of the brand, there’s no guarantee that every injector will match its flow rating unless it’s tested.”
One of the reasons for the large price difference in new injectors is mark-up. Parts are typically much more expensive at a Chevrolet dealer than they are through other supply sources. GM has an unusual marketing arrangement between divisions– Typically, price is marked up every time it passes from one division to another. If an injector passes through several divisions before you purchase it, its price will obviously be higher than if you can cut out a middle-man or two.
Hard Start Blues
I have a 1988 Corvette convertible that starts fine cold, but after you run it a while and shut it off for 5 to 10 minutes, then go to restart it, it cranks over several times before it starts. If I shut it off then restart it right away, it starts just fine.
There are several possible causes for your hard starting blues, but the most likely is that heat is the culprit. A revelation? Yes and no. Engines undergo a phenomenon known as heat soak, once they’re shut down. This condition results from heat in the lower part of the engine and in the exhaust system finding its way into the water jacket and intake tract. If you were to check manifold air temp immediately after shut down and again 5-10 minutes later, you’d find that it had increased. It won’t be until some time later that manifold air temp and water jacket temperature begin to fall. If you’ve spent much time under the hood, you’ve probably found that the engine itself still retains a good deal of heat after the exhaust manifolds are cool enough to touch.
When you restart immediately after shutting the engine off, it hasn’t had time to heat soak, so it starts quickly. It isn’t heat soak per se that’s causing the problem, but the Manifold Air Temp (MAT) sensor’s interpretation of it. A Tuned Port system is actually a huge heat sink and engine heat flows into it like a river emptying into an ocean. Since the sensor is mounted on the underside of the plenum, it “sees” temperatures that are higher than those of the incoming air. This information is transmitted to the ECM which then leans the air/fuel mixture to accommodate an extremely (and artificially) high manifold air temp. As sensors age, their accuracy can deteriorate, which may explain the relative newness of the hard-start problem. Rather than replacing the MAT sensor, I’d advise mounting it remotely. It will probably cure the problem, and also improve performance by supplying more accurate temperature readings. A remote MAT relocation kit is available from Howell Engine Developments, Marine City, MI.
L-82 Versus L-48
I own a 1974 L82 Corvette. I have read magazines and books trying to find out exact different between the L48 and the L82 engines, with little success. I have asked different people, but have always gotten different answers. I have come to the point where you have to ask the experts. Could you please tell me the exact differences in the two engines. I know the L82 engine is the more powerful, but what makes it more powerful of the two?
That’s what you get for reading all those off-brand magazines– unanswered questions and unfulfilled desires. Monique will see you now. But first– the L82 made its debut in 1973 as the throttled down smog-alert successor to the original LT-1. With 9.0:1 compression ratio, 2.02″ intake/1.60″ exhaust valves and hydraulic lifters, it was rated at 250 net horsepower at 5200 rpm and 285 lbs./ft. of torque at 4000 rpm. By comparison, the L-48 had 8.5:1 compression ratio, 1.94″ intake 1.50″ exhaust valves and a milder camshaft, so it’s power rating was 190 at 4400 rpm and 275 lbs./ft. of torque at 2800 rpm. Compared to the L-48, the L-82 has cylinder heads with larger ports and valves, pushrod guideplates, longer duration camshaft, forged steel crankshaft, moly rings, finned aluminum valve covers, higher capacity starter and four-bolt block.
Horsepower ratings of the two engines varied from year to year as Chevrolet attempted to meet changing emissions and fuel economy requirements. L-82 horsepower ratings varied from a high of 250 in 1973-74 to a low of 205 in 1975. In 1980, its last year of production, the L-82 was rated at 230 horsepower. During the same time period, L-48 horsepower ranged from a low of 165 in 1975 to a high of 195 in 1974 and 1979.
Hot and Bothered
We’re writing in regard to a problem that’s been addressed before but we’ve never been satisfied and we hope you can shed some light on it. The enigma seems to plague owners of ’68 through ’82 Vettes. Many of us have expressed our concern at seminars, the Corvette “Hotline” and at fan and centrifugal clutch trouble shooting get-togethers. The result is we have tried lower temperature thermostats, new radiators, flex-fans and even so called “cool-running” liquids in the radiator. To no avail, as soon as the A/C comes on the temperature starts to rise and continues till we have trouble.
Do we have to learn to live with it? About the only thing we haven’t tried is electrical “pusher” or “puller” fans. We could use your help before summer sets in. Thanks for any help you can give us and keep up the fine work.
What you really need is a subscription to the “Orin Nelson Alaska Cooling Journal”, but I’ll try to help you out. Cooling system problems in 1968-82 Corvettes are as common as complaints about me being a smart-ass. It seems as though the radiators are marginally able to keep up with normal engine demands, so when air conditioning or long periods of low speed operation are thrown in during exceptionally hot weather, the system can’t handle it. However, I rarely hear of a Corvette that has serious overheating problems while running along the highway at speeds above 40 miles per hour. (Unless there’s an obvious problem like a piece of cardboard in front of the radiator.) This circuitous route leads to the conclusion that most commonly, air flow across the radiator is insufficient when the car is traveling at low speeds or sitting in traffic.
The key to achieving adequate air flow at low speeds is to ensure that all the air entering in front of the radiator passes through it. That’s most effectively accomplished by thinking in terms of pressure, not flow. The purpose of the fan is to create a low-pressure area behind the radiator. It can’t do this unless the radiator is sealed to the radiator support and shroud. The fan also has to extend well into the shroud to be certain it doesn’t draw air from behind it.
If the radiator is properly sealed and the problem persists, the best bet is to try spinning the water pump faster, (by changing pulley size) increasing the rating of the pressure cap (to about 20 psi) or both. A serpentine flow radiator, as is commonly used in race cars, is another possibility. This design routes coolant through the top half of the radiator, then through the bottom half. (In essence it’s lie having two radiators in one.) By speeding up the flow, cooling capacity is increased. However, these radiators are custom built and tend to be expensive.
Snake Oil and other Magic Potions
I have a 1990 Corvette that I would like to maintain in optimum condition. Beyond scheduled oil changes can you recommend or offer an opinion on the following products:
1) A “Preluber” to pressurize oil system prior to ignition thus sparing the engine wear and tear; I cannot find anybody who uses the system for comment or complaint. Besides from the $600 cost there is the danger that critical oil supply might be interrupted by flying debris or a leak in the hoses that connect to the externally mounted oil pump. In your opinion, is the risk worth the reward?
2) Amsoil Co., maker of synthetic motor oil also markets a bypass oil filter that claims to filter down to less than 1 micron. As my eyes are not that fine, have you heard of any independent confirmation of this claim? This product taps oil at the pressure sensor and filters oil slowly (4 gallons every 5 minutes at 45 mph) but surely. Is there a net gain due to fewer abrasive particles circulating in the engine or a net loss due to a drop in either pressure or oil volume? Would the use of Slick 50 or its teflon derivatives tend to clog such a fine filter comprising its benefits?
3) Marvel Mystery oil claims to be an engine top lubricator. Would the use of such a product harm fuel injectors or oxygen sensor? Will piston ring wear be retarded as label indicates?
4) In the past 2 years all the major oil companies have brought in their own synthetic oil products. Is there any hard evidence to suggest which products are superior in terms of consistence, friction, film-strength, lower wear and thermal stability?
Before I answer your questions, I want to state that no matter what I say will be wrong. People seem to have very strong opinions regarding lubrication products– in spite of the facts. Now here are the facts. I’ve driven vehicles with engines that have gone over 200,000 miles with nothing more than regular oil and filter changes. How critical can the need be for special additives and accessories? Some time ago, attended a seminar put on by Valvoline Oil and the amount of testing they do is unbelievable. Motor oils have to pass rigid tests before they are found acceptable by the auto manufacturers, so the major oil companies have all developed top quality products that can more than handle the job of adequately lubricating your Corvette’s engine.
My recommendation is to use a name brand synthetic oil with the most current quality rating (SH, SJ) and change it and the filter at recommended intervals. Personally, I’ve had excellent results with Valvoline, Mobil 1, Red Line and Lankow Purple. That isn’t to say other brands aren’t any good– I just don’t have enough experience with them to voice and opinion. If all that other stuff was cost effective, all the NASCAR teams would be using it.
Ach! Das Is Goot
I’m stationed in Germany and would like to ask your advice in a few different areas. Presently I own an ’89 Vette with @ 41,000 miles runs great but would like to raise red-line to 5500 to 6000. I’m running a TPI Specialties FastPak with Dynomax mufflers, PROM & a modified MAF. The future modifications I would like to make include a Mini-Ram (TPIS), Fuel rail (TPIS), cam, lifters, pushrods, roller rockers, springs and mildly port the heads along with headers. With the right combination of these various components is the previously mentioned 5500-6000 possible? I wanted to leave the bottom stock and concentrate on the valve-train and intake components. The car is driven 8 months out of the year and only on dry roads. Please give as much information and as many tips as possible regarding increasing rpm’s as well as top-end speed. Will I need any other components to achieve the above mentioned? With increased HP, will 6th gear pull closer to red-line?
Ach du liber, vhat fill das Porsche drifers say fen das Corvette machs schnell und blows dere doors offen? Your proposed changes should make your Corvette Autobahn ready. My suggestion is to use the components you mention, but also install a TPI Specialties ZZ-9 camshaft, larger diameter valves, (2.0″ intake, 1.60″ exhaust can be used without changing valve seats) and a set of 1-3/4″ tube headers and low restriction cat-back system. A high flow catalytic converter, such as the ones produced by Random Technology, would also be worthwhile. With all these changes, you’ll need a custom PROM and yes, your car will get closer to red line in 6th gear– if you’re person enough (can’t say “man enough” anymore, now that we’re in the 21st Century)to keep your foot down.
Solid Lifters and other Lies
I own a ’65 Vette with an “HH” code 365 HP engine. The prior owner of the car had it professionally restored. He didn’t want the bother of adjusting solid lifters, however, and had the engine redone with a 327/350-hp cam and hydraulic lifters. I often think about putting in the correct cam and solid lifters. I hesitate because; A) the engine runs beautifully and is very strong, and B) the engine compartment is detailed, and I hate the thought of getting it messed up due to frequent adjustment of solid lifters.
I was wondering if you could enlighten me regarding the world of solid lifters since I don’t know too much about them. Specifically:
1) Do they get knocked out of adjustment if you “redline” the engine occasionally?
2) Must they be adjusted frequently?
3) What happens to the engine if the lifters go out of adjustment and are left alone?
4) Are the cam and lifters available from Chevy?
5) Does an aftermarket manufacturer make a solid lifter which is less apt to require frequent adjustment?
6) Does an aftermarket manufacturer make an adjustment locking nut which works better than OEM equipment?
Basically, what exactly goes out of adjustment thereby requiring attention?
Personally, I’d stick with a hydraulic lifter cam, but if you’re on a guilt trip because your Corvette isn’t all original go ahead and abuse yourself and install a mechanical cam and lifters. In answer to your questions:
1- Solid lifters generally don’t get “knocked” out of adjustment. Lash changes primarily because of wear and/or the adjusting nuts backing off.
2- Define frequently– the interval is usually about every 7,500 to 10,000 miles, maybe more, maybe less.
3- If lash becomes excessive and no adjustment is made, performance will drop off and the valve train will wear more quickly because it’s hammering itself to pieces.
4& 5- If memory serves, the original cam for your engine carries part number 3972178. You may be able to find one somewhere in the Chevrolet parts system because it’s the same cam that was use in 1970-72 LT1 engines. Duplicates are available from a number of aftermarket manufacturers but they won’t be any less likely to need periodic adjustment than a cam that comes in a GM box.
6- All major cam manufacturers offer positive locking adjusting nuts (also called poly locks) which incorporate an Allen set screw to lock the nut to the rocker stud. These are preferable to the original equipment type rocker nuts.
Underdriven, Overwrought
I am writing to ask about long term usage of the reduced size performance pulleys.
I purchased the three pulley set for my 1990 L98 about a year ago. This set included the crank, alternator and water pump pulleys. In the past year, both my alternator and just recently, my water pump have failed.
My Corvette only has about 36,500 miles on it. It is a daily driver to work, and other than eating up a 5.0 Mustang or two from stop light to stop light, it is not formally raced. I do take it out and run it hard on some back country roads here in Virginia, but I also religiously maintain it.
Have you heard of other failures like this from performance pulley owners? Supposedly, the alternator and water pump should last longer since they are not turning as fast. The pulleys seem to be balanced. I have not noticed excessive vibration from them with the engine idling. I would appreciate any information about this that you might have.
Underdrive pulleys do slow down the rotational speed of the water pump and alternator, so you’re correct– these parts should last longer than they would with standard pulleys. If you’re alternator and water pump failures are related to the pulleys (as opposed to some other cause) it could be the result of misalignment. Another possible water pump and alternator killer could be the combination of the wrong belt and an overly enthusiastic mechanic. If a belt that was too short was forced over the pulleys, the resultant load on the bearings could be the culprit.
Cool Enough For Alaska
I drive a 1985 Corvette 4-speed automatic with “Heavy-Duty Cooling System” and “Z51 Performance Package”.
1) Which thermostat is original for my car? 180 F or 195 F thermostat?
2) Is the coolant fan controlled by the electronic chip or by the engine coolant temperature sensor?
3) With the original chip and the 195F thermostat the coolant fan started at 225 F and ran until temperature dropped to 197 F. I put in a power chip and a 160 F thermostat. The coolant fan starts now at 178 F down to 167 F. Is that normal or is the engine running too cool? Should I perhaps put in a 180 F thermostat? Average temperature here in Switzerland during summer is between 77 F and 95 F.
Your Corvette was originally equipped with a 195-degree thermostat. The fan is controlled by the PROM (chip) and the performance of a high performance chip is consistent with its design. Extremely high coolant temperatures are part of the original emissions/fuel economy calibration, but one of the advantages of an aftermarket PROM is it turns the fan or fans on at a lower temperature to keep the engine cooler. A 160-degree thermostat serves the same purpose and the lower coolant temperatures have a positive effect on horsepower. There’s no need for a 180-degree thermostat– the engine is running within an acceptable temperature range and is consistent with the changes you made. You’ll also find that coolant temperature rarely, if ever stays at 160-degrees. Unless the ambient air temperature is extremely low, normal coolant temperature will be about 10 degrees above the thermostat rating.
Cam & Injection To Go
I have driven a 1967 Corvette since 1968. It has a 327 c.i. engine 300 h.p. engine, four-speed manual transmission and 3.08 rearend. The car has approximately 210,000 miles on it. It does not consume inordinate gas or burn oil, cylinder compression measures between 137 lbs. and 155 lbs. and it is smooth and quiet. It has lost some performance probably due to cam lobes that are worn.
I am considering rebuilding the engine, I have researched different cams and I am seeking your counsel on a replacement cam that will be fairly powerful but smooth. With pump gas octane around 93 I am told that the original GM cam and a 10.0 compression ratio are not suitable. One of the new computer generated cams might better serve my needs. I am also considering replacing the Holley 1850 carburetor I have adapted to the car with the new Holley Projection TBI unit.
A compression ratio of 10.0:1 is marginal with short duration cams, but if pump octane is a legitimate 93, you should be able to get by with a cam having a duration at .050″ lift of between 205 and 215 degrees. Just about any of the major cam manufacturers can supply an appropriate camshaft. When you rebuild the engine, have it machined for zero deck clearance. In addition to increasing power, a zero deck reduces octane sensitivity.
You might check with Grimes Automotive Machine (770/475-5272) before you get into the engine. Grimes has built several low cost, high performance engines including a smooth-idling 355 cubic inch small block that produce 340 horsepower and just over 400 lbs./ft. of torque. These engines use a mild cam which is also available separately.
I’ve had good experience with Holley’s digital Pro-Jection system, but another option is a Howell HP/TBI system which uses a GM computer in conjunction with either a two- or four-barrel Pro-Jection throttle body. This system includes all the standard sensors and controllers, so you get all the efficiency of a late model computer-controlled fuel injection system. The system is also considered to be a factory replacement in many installations, so it’s emissions legal on some late model vehicles. Another advantage is that it will control torque converter lock-up if you decide to install a 700R-4 transmission.
Missing Heads
I’m planning to build an engine using cylinder heads with casting number 3998993. I can’t find any information on these heads and wonder if there’s some reason I shouldn’t use them.
The 993 castings, which have 76 cc combustion chambers, are not generally considered a high performance head. Also, they’re a lightweight casting and prone to cracking. If you do use them, make sure they’re Magnaflux inspected for cracks before you spend any money on machine work.
Torque of the Town
I have a question about torque. If it takes 200 ft/lbs to spin the rear wheels, why do I want an engine with more than that? I know it takes the most torque to get a car moving. Since it takes less to keep a car moving or to accelerate it, when do I start trading torque for horsepower?
I want a stoplight rocket and a freeway screamer. RPM does not scare me. Besides, it sounds cool. Thank you.
On the off chance that this is a serious question and that you have reached puberty, I’ll supply an answer. I have no idea why YOU want an engine with more than 200 lbs/ft. of torque. However most Corvette owners want an engine that exceeds that level so their cars accelerate rapidly. Even if it only takes 100 lbs./ft. to get the tires spinning, at some point they will recover and hook up. At some point, you’ll also shift into a higher gear with less mechanical torque multiplication. Then what? You’ll be driving a Chevette. An engine will have to put out 300 to 400 lbs./ft. or more to keep a 3,000 to 3, 500-pound car accelerating briskly in high gear.
You don’t ever trade torque for horsepower. Horsepower is simply a means of rating an engine’s ability to produce torque at various rpm levels. It’s computed using the equation HP= Torque x RPM/5252. Horsepower is typically mentioned in reference to high rpm operation because of the influence of engine speed. If two engines produce the same amount of peak torque, but one reaches that peak at 3,000 rpm and the other reaches it at 4,000 rpm, the latter engine will produce more horsepower. Whip out your calculator and run the numbers to see for yourself.
Trouble in Paradise
I have a 1987 Roadster that came with a racing engine when I bought it. The car runs beautifully and is very fast. The problem I have is that it takes a great deal of force applying the brakes to stop the car. The car will stop but it kind of gets me scared when the road is wet and you’re trying to stop suddenly. Also, the engine RPM will stay high while applying brakes to come to a complete stop. Engine idle then goes back to normal. This has been bothering me for a long time now and I can’t think of anything else other than a vacuum problem. It could be something that wasn’t connected right while re-installing this engine or it could be the ’87 models just didn’t have a good braking system from what I’ve read about this model year. I hope you can help me out on this one.
You’ve just won a free weekend under your hood. The symptoms you describe almost certainly are the result of a vacuum leak. Chances are, the leak is somewhere in the brake vacuum system, which explains both the need for excessive pedal pressure and the increase in engine speed when you step on the brake pedal. To verify that, disconnect the power brake vacuum line from the intake manifold and plug the opening. Then step on the brakes and the problem should be gone. (If you drive the car when you conduct this test, remember you’ll have to plan ahead because you’ll have no brake power assist.
Keep in mind that with computerized engine controls, the engine will run a lot better than you’d expect when a problem like a vacuum leak is present. The computer is hip to what’s going on and takes corrective action, so what would be a major problem with an old fashioned carburetor turns into a minor annoyance. And you thought computers weren’t your friends.
Getting the Points
I own a 1974 coupe with L-82 engine and automatic transmission. The problem I am having is every two or three months, I have to replace the points and condenser. The contacts in the points just completely burn up. What could be causing this, and how do I go about solving this problem. Also I understand it’s possible to eliminate an overheating starter problem by replacing the starter solenoid with a remote starter switch. How will this solve the problem since you still have to have a solenoid with a remote starter switch and the solenoid still has to pull the Bendix drive in? How much rewiring will there have to be done to install this remote switch?
The problem is, you’re 20 years behind the times. Can you spell e-l-e-c-t-r-o-n-i-c-s? Why mess with points when you can replace them with an electronic trigger. A number of companies, including Stinger, Pertronix and Mallory offer conversion kits that fit inside the original distributor. If you insist on running points, first, join the Flat Earth Society, then check the resistance in the wire that brings 12 volts to the coil. Chances are there is none (or very little). If you add a ballast resistor your problems should be solved.
The solution to the hot start problem is to install a remote solenoid, not a remote switch. By removing the solenoid from a high heat area, it is able to activate the starter with no problem. Installation involves running the positive battery cable to the remote solenoid and then running another cable to the original solenoid. It’s also necessary to reroute the wires from the original solenoid’s “S” and “R” terminals to the remote solenoid and jumper the original “S” terminal to large 12-volt terminal. This arrangement retains the original solenoid only as a junction block for connections and the remote solenoid handles starter activation. However, an even better solution is to replace the starter with a gear reduction model such as the ones used on late model vehicles. Even though these starters still have the solenoid mounted to them, the extra torque developed by the gear reduction unit eliminates hot start problems. And you get a new starter to boot.
Back to 1970
I’m about to deliver my original 1971, 270-hp base engine to my machine shop for a rebuild. I’d like to have it restored to 1970 power levels of 300 horsepower and 10.0:1 compression ratio. I know the low compression dished pistons have to be replaced, but what else? I’m sure there were other modifications made when the shift to regular gas was mandated.
Surprisingly, there weren’t all that many changes made to accommodate the petroleum distillate that was graciously called gasoline in the early 70s. The biggest difference was in combustion chamber volume, which grew from 64 to 76 cc’s. The easiest way to achieve a 10:1 compression ratio is to have the block cut for zero deck clearance, install flat top pistons with minimal valve reliefs and have the heads milled so that combustion chamber volume is 70-71 cc’s. While you’re at it, treat the heads to a three-angle valve job and have the valve pocket area cleaned up. Then spring for a mild performance camshaft (with a duration at .050” lift of 212 to 218 degrees) and you’ll be the proud owner of a 310-330 horsepower engine.
Another Hot Corvette
I have a 1978 L82 Corvette with a 3.90 gear and 350 Turbo transmission. I would get 8 to 11 mpg. After talking to all my Corvette friends it was decided that the best way for me to go would put be to put a 700 R-4.
I took the plunge and we had to put an extension on the motor mount because the drive shaft was so close it wouldn’t fit. The job proved to be a little frustrating and far from easy.
As far as the transmission itself it sure rides a lot smoother. I feel like I’m driving a Cadillac!! The first tank was 13 mpg, which wasn’t good so I made sure everything was in tip-top condition. (From proper inflation of the tires, re-adjusting the carburetor, changing the oil, filter, plugs and keeping my foot light.) On my next tank I got 18 mpg with A/C!
Now can someone tell me how to get my temperature below 225 degrees in town (will hold at 200 on the interstate). I’ve rebuilt the engine, radiator, and replaced the fan clutch and installed a 180-degree thermostat. I keep coolant in the overflow but it always goes out. We have pressure checked the radiator and no leaks show. What else can I do??
I’m not sure I’d brag that my Corvette runs like a Cadillac, but that’s another subject for another time. As for your cooling problems, take a look at the big picture. If an engine runs hot, it’s either because it’s generating too much heat for the cooling system, or because the cooling system isn’t transferring heat to the air efficiently– or because the gauge isn’t operating properly. As a general rule, if coolant temperatures rise when the car is driven slowly, or allowed to idle, air flow across the radiator is insufficient (assuming the radiator is in good condition). If temperature problems arise during highway cruise, chances are the engine is generating excessive heat.
Make sure the radiator is properly sealed to the support and that the proper fan shroud is installed. Also make sure you haven’t installed a late model water pump designed for serpentine drive (its impeller will turn in the wrong direction if that’s the case).
When you rebuilt the engine, did you make sure the proper head gaskets were used and that they didn’t block any of the coolant passages in the heads?
Another common cause of overheating is retarded ignition timing. Set initial lead to 10-12 degrees, combine that with a centrifugal advance of 22-24 degrees for 36-38 degrees of total advance (that doesn’t include vacuum advance which is vital to decent fuel economy). I’d also suggest a blend of no more than 50% antifreeze– water is the best coolant and since you’re in the south, you can probably get by with no more than 25% antifreeze. I’d also pour in a bottle of Water Wetter from Red Line Oil.
Cam Specs
Could you please supply me with the specifications for a 327/350-horsepower and 327/300 horsepower cam. I want a little extra power from my 195-hp 350.
A little more power, a little more power. Let’s be honest, you want all the power you can get. With that settled, the 350-horsepower cam is officially known as part number 3863151 and is a symmetrical design– both intake and exhaust lobes have the same duration and lift figures. Those are 222 degrees at .050” lift and .447” lift. The 300-hp cam was actually used in both 327 and 350 engines (including the L48) from 1967 to 1981. It’s a dual pattern cam with 194 degrees of intake and 202 degrees of exhaust duration (at .050” lift). Intake lift is .390 and exhaust lift is .410.
The 300-hp cam is probably identical to the one that’s currently installed in your engine, so it doesn’t hold much potential for increased performance. On the other hand, the 350-hp cam is a little too radical for an otherwise stock 195-hp engine. It was designed for a powerplant with 11:1 compression ratio and large port cylinder heads with 2.02”/1.60” valves.
If it were me, I’d look for something a little tamer. Competition Cams offers a couple of suitable dual pattern cams; one has intake/exhaust durations of 203/212 degrees with lift of .421” and .451”. The other has intake/exhaust durations of 211/221 and .442”/.465” lift. Both cams are available only as part of a cam and lifter kit and the part numbers are CL12-207-2 and CL12-208-2 with the first number pertaining to the milder grind.
Tune-Up Time
Six months ago, I inherited a 1973 Corvette. It has been running just fine until recently, but now it’s hard starting and the gas mileage has gone way down. When I say running fine, I mean that it has fairly good pickup, but is nothing to write home about.
In talking with friends, they said it probably just needs a good tune-up. As I was going through the papers that came with the car, I found a sheet revealing that the engine was rebuilt in Nobember, 1986. The list of parts installed was extensive.
The main questions I need answered are, What jet changes should I make to the Rochester carburetor; what spark plug brand, part number and gap should I use; What RPM should timing be set with distributor disconnected; What should timing be set at; Which octane gas should I use?
I can’t answer some of your questions, because you didn’t supply enough information. I can’t tell you what jet changes to make because you didn’t state the current jet size nor did you mention whether the engine was running too rich or too lean. As for spark plugs, I prefer Champion. For this engine, a J10-Y gapped at .035” should be ideal. Don’t disconnect the distributor when you set the timing, the engine won’t run worth a damn. You may have to disconnect and plug the vacuum advance, but that won’t even be necessary if it’s connected to a ported vacuum source. To find out, connect a timing light and with the engine idling, pull off the vacuum advance line. If the timing moves then obviously vacuum advance is active and the line will have to be disconnected while timing is set. If there’s no change, then it’s just as obvious that vacuum advance is cut of at idle, so there’s no need to disconnect anything.
Set the initial timing to 10 degrees BTDC and look for a total mechanical advance of 34-36 degrees with maximum advance being reached at 2750 to 3000 rpm. If you don’t have much experience, you may have to remove the distributor and take it to a shop to have it “recurved”. The idle at which to set initial advance is determined by the engine– find a speed at which it’s relatively happy. In your case, 700 to 750 rpm should be adequate.
As for fuel octane, run 89, 92 or 93. Let the engine decide. If it doesn’t knock or ping on 89 then stick with it. Higher octane fuel doesn’t necessarily increase power, it simply resists detonation. If an engine doesn’t detonate on a fuel of a given octane, there’s no need to run fill the tank with anything higher. The only exception is with 87 octane– some of this stuff is from the bottom of the barrel and isn’t of consistent quality. One tank may run fine and the next have your Corvette sounding like a marble grinder.
No Oil, No Engine
I have a 1989 Corvette and the crank, rods and bearings have been destroyed due to lack of oil. I would like to replace the complete engine with an LT1. Will my injectors, computer system and other parts work with this engine? If not, is there a complete motor that can be ordered from Chevrolet that will work? I have heard conflicting stories from the Chevrolet hot line, the dealer and local “experts”.
The original engine in your car is an L98 which is of the first generation persuasion. An LT1 is a second generation powerplant and the heads, block and intake manifold are not interchangeable. The only way to install an LT1 in an older model Corvette is to install a complete engine and have a custom wiring harness built. You could run the engine with your existing harness except for the distributor. It requires completely different wiring and a completely different computer. If your goal is to have your car run like an LT1, consider ordering a High Output 350 (also known as “ZZ4” engine, part no. 24502609) and equipping it with a Big Mouth manifold base, large tube runners, ported plenum and 52mm throttle body. All of these parts are available from TPI Specialties in Chaska, MN
Pump Up the Headers
In the shop manual for my 1981 Corvette, it shows that a smog pump is required along with the correct manifold and necessary hook-ups. Can I get away with the smog pump and put regular headers on it or will that cause trouble passing an emissions test?
As it turns out, this is as much a semantics as a technical question. By “get away with” do you mean eliminating or using the smog pump? If you mean eliminate, the answer is no unless you like the possibility of a $2,500 fine. According to EPA regulations, it’s against the law to remove or render inoperable any emissions control device. State emissions inspectors may not be aware of the fact that a particular vehicle was originally equipped with a smog pump, so enforcement is an iffy matter. I’m mentioning that because many people know someone who has a car that successfully passed an emissions test without all the proper equipment being in place. They lucked out, you may not.
On the other hand, if you’re asking whether the stock smog pump can be used with headers, the answer is yes. Any emissions-legal header must have provision for connecting the stock smog pump, which will work fine.
Oil’s Well that Ends Well
I recently purchased a 1985 L98 Corvette with 57,000 miles. The original owner faithfully changed the engine oil with regular 10W-30; however, I would like to switch to synthetic motor oil. Many people have advised me not to do this without providing me with a valid reason. The only warning that has been suggested is that changing to synthetics may cause seals to shrink resulting in engine damage. I would appreciate a definitive answer whether it’s safe to switch to a synthetic in an older vehicle.
Go back to the people who have advised you not to change to synthetic oil and ask them whether the earth is flat or round. I’ll bet most of them are members of the Flat Earth Society. There’s no valid reason not to change to a synthetic motor oil. Keep in mind that synthetics are derived from the same petrochemical family as regular mineral-based motor oils. The difference is that synthetics are “massaged” into a uniform molecular structure. That structure along with the specific additive package provides synthetics with superior lubricating qualities.
With a good synthetic oil, the size of the molecules is uniform, so there are no “light” compounds to burn off easily, and no “heavy” ones to contribute to sludge build-up. Synthetics also tend to reduce friction by their very nature and because you can typically run a lighter weight synthetic than mineral oil. Some Winston Cup race teams are even running 0W-10 oils in an attempt to squeeze out maximum power and fuel economy. I can’t vouch for the suitability of using a 0W-10 oil in a street car, (which would be hard to do because they’re typically blended on a limited basis for racing only) but a 5W-30 oil is entirely applicable.
There’s no guarantee that a synthetic won’t have an adverse effect on the seals, but that’s also the case with a mineral oil. Typically, it isn’t the oil, but one of the components in the additive package that affects the seals. Consequently, the risk is no greater with synthetics than with mineral oil. Also keep in mind that there are only two external seals in an engine– the front and rear crankshaft seals. If either one of these shrink, the oil on your garage floor will alert you to the problem. Similarly, if the oil has an effect on the valve seals, the trail of smoke behind your car will serve notice that Murphy’s Law has been enforced. In either case, if you’re not comatose when you drive, you’ll be well aware of any problem before engine damage results.
Frankly, I’ve never heard of any legitimate oil-induced seal problems and with the technology available to the oil companies, it’s hard to believe that any exist. At least on a round earth. I run synthetic oil in all my vehicles and haven’t ever experienced the heartbreak of seal shrink.
Idle Up, Idle Down
Help!! My girlfriend has a 1989 Corvette convertible with only 25,000 miles. The car has an intermittent miss at idle. Otherwise it runs perfectly, does not hesitate at high or low speeds, but when sitting at a light the idle dips, it’s very annoying.
Nobody can figure it out. It has been tuned up and also it has been put on a scope. All cylinders are even and they say its not fuel. It’s very frustrating. Can you help me? The manifold has been tested for leaks. They say maybe an EGR valve but the cost would be high to take a chance of fixing and there’s no guarantee it is the problem, a computer analyzer was also plugged into the system, and the car was driven for 10 minutes.
Who has been checking this? The automotively challenged? It should be a simple matter to plug in a scanner or hook the car up to a Diacom program and see exactly what’s going on. Since the ace mechanics who have looked at it have ruled out ignition or fuel, it might make sense to look at the idle air controller (IAC). This device is controlled by the ECM and adjusts the amount of air admitted to the intake manifold exclusive of what passes by the throttle plates. Sometimes, IAC’s can become balky and not respond properly. Or, the idle stop in the throttle body may be misadjusted and the IAC may be near the end of its travel. Under certain conditions, it may need to open or close more than it can, which would result in your idle speed problem. This would be immediately obvious to someone using a scanner or Diacom, provided they kney what to look for.
Another potential problem could be the oxygen sensor. It may be suffering from an accumulation of carbon that’s skewing its readings. Try removing the sensor and cleaning the shell with a rag. Then blow a bit of compressed air into the opening. Don’t use high pressure, just enough to move things around a little.
A Smokin’ Corvette
I own a 1977 Corvette with 60,000 miles. It has an L-48 with automatic and it starts and runs super. But after driving it for a little while and shutting it off for about 1/2-hour or more, it puffs a bit of oil– sometimes quite a bit. If I start it within a half hour of shutting it off, it doesn’t smoke. Could it be the valve guides?
The problem you describe is typically associated with valve guides and seals. But it can also be caused by a leak between the intake manifold and cylinder head. If this seal isn’t adequate, engine vacuum can draw oil out of the lifter valley into the intake ports. Another possibility is that the PCV valve is the culprit. However, that’s only a consideration with aftermarket valve covers that aren’t properly baffled.
The other question is whether the puff of smoke is a result of oil or gas. In the right amounts, a fuel leak can cause smoke on start-up. However, fuel-related smoke is whitish if the leak isn’t severe and black if it’s a gusher. Oil smoke has a bluish tint and usually hangs in the air longer.
LT1 Versus LT1
What are the exact mechanical differences between the LT1 engine as installed in the Corvette versus the Z28 versus the Impala SS?
Horsepower. The Corvette version is rated at 300, the Z28 at 275 for 1993-1995 model years and 285 for 1996-1997. The Impala SS/Caprice version is rated at 260 horsepower. The Corvette and Z28 engines are virtually identical– both have the same aluminum cylinder heads and camshaft, but Corvette blocks have four-bolt mains whereas blocks originally installed in Camaros are of the 2-bolt persuasion. For the 1995 model year, Camaro engines were supposedly switched to four-bolt mains, but some 2-bolt versions have turned up.
Impala/Caprice LT1s have cast iron heads, 2-bolt blocks and a milder camshaft, which is largely responsible for the lower power rating. The difference in horsepower between Corvette and Camaro engines is largely a result of restrictions in the intake and exhaust systems. For 1996, the optional Camaro Z28 SS package features a ram air system that put horsepower on a par with that of Corvettes.
Rich as a ….
I have a 1986 Corvette with 85,000 miles and I’m having a performance problem. The car does not idle very well and power at takeoff and at top end is very sluggish. But it isn’t always like that. Sometimes I get in the car and it runs good and the next time it doesn’t. The check engine light has never come on and I checked the ECM and there were no trouble codes stored. I did a tune-up and when I took the plugs out, they were all wet and had black splashes on them. This has happened before and I changed plugs because they were all wet. I checked the compression on all cylinders and they were all 145 psi. The engine has never used any oil and does not smoke at all. I have checked everything I know how to check. What’s causing the spark plugs to get wet? Could it be the electronic spark control?
Pour yourself a cup of coffee. Sit down and relax and let’s think about this. An excessively rich fuel mixture is a common cause of sluggish performance. If a mixture is too rich, the engine is either receiving too much fuel or too little air. Since the car runs well outside of its problem areas, chances are air flow is adequate, so let’s assume the problem is too much fuel.
Your engine is electronically controlled, so the ECM does everything it can to keep the air/fuel ratio at 14.7:1. Fuel flow is regulated by exhaust gas oxygen content when the system is in “Closed loop”– that is, after the engine is up to temperature and the oxygen sensor is calling the shots. When it’s in open loop fuel control is handled by reference tables programmed in the chip.
There’s a common thread between a sluggish idle and wide open throttle operation. When you first start the engine, the oxygen sensor isn’t active yet so the system’s in open loop. At wide open throttle, the system stays in closed loop, but doesn’t process the oxygen sensor readings; it relies on tables in the chip.
It sounds as though you’re having a problem under conditions when the oxygen sensor isn’t in charge. This can be caused by a number of factors ranging from one or more faulty injectors to an improperly programmed chip to a bad ECM.
Pulling the injectors on a Tuned Port engine requires removal of the plenum and runners, so leave that as a last resort. The first thing you need to do is to find someone with a Diacom program or a scanner. See if the problem is in fact related to open loop operation. Also take a look at the “Block Learn” number. When in “Closed Loop” mode, it indicates whether the system is increasing or decreasing fuel flow in order to maintain 14.7:1 air/fuel ratio. A number higher than 128 indicates fuel flow is being increased compared to the base calibration; a number less than 128 indicates fuel flow is being decreased (compared to base calibrations).
If an injector is leaking, the block learn should be well below 128. If it isn’t, chances are there’s something amiss with one of the sensors. In any event, be sure to monitor oxygen sensor output; it should cycle between about 900 millivolts and 100 millivolts during steady state closed loop operation.
Now, if I’ve missed the mark altogether, and the problem exists only in closed loop, I’d suspect the oxygen sensor or an air leak in the exhaust system, the intake system or both.
Seeing the Light
I have a 1982 Corvette and have experienced problems getting it through New Jersey emissions inspection because of excessive hydrocarbon and oxides of nitrogen emissions. I noticed a difference in the engine performance at idle when the warning light would go on as rpm would increase. I replaced the oxygen sensor which improved the performance at idle and passed the emissions test. However, now I notice the engine warning light comes back on and stays on. I have been told the catalytic converter could be plugged causing this condition. Could this be the case or do I need to go to a local Chevrolet dealership and pay $150. to $200 for a diagnostic check?
The local dealership is probably the last place you need to go. Try to find a reputable Corvette specialist in the area if it turns out you need “professional” help. Keep in mind that the “Check Engine” light is your friend, not your enemy. When it illuminates, it’s telling you that something’s wrong. More than that, it will also give you an indication of where the problem lies. “Check Engine” lights come on because the car’s diagnostic system has found a problem and has stored a trouble code in the ECM (Electronic Control Module). By plugging in a scanner or Diacom system, you can not only read the trouble code, you can also look at all the sensor outputs and see what’s happening in real time. If neither of those are available, you can read the trouble code or codes by placing a jumper between the “A” and “B” terminals of the ALDL (Assembly Line Diagnostic Link) that’s located beneath the dash.
When these two terminals are jumpered together, the “Check Engine” or “Service Engine Soon” light will flash a “12” code (A “12” code is one long blink followed by two shorter ones.) The light will flash out a code “12” three times, then will flash any other codes stored in the ECM. I’d advise you to pick up a service manual for your car so that you can decipher the trouble codes.
A clogged catalytic converter could be causing some of your problems. If it is, back pressure in the exhaust system will be high. To check this, you’ll have to drill a small hole in the exhaust pipe in front of the converter and connect a pressure gauge. Checks are typically made at wide open throttle and back pressure should not exceed 6 psi at 5000 rpm. Another option is top pull the converter off and examine it. If you do need a new converter, I’d suggest a Random Technology high flow model. It will not only out-perform a stock converter, it’s also lower priced than a factory replacement.
300-hp Confusion
I’m having a new Chevrolet 350, 300-hp engine installed in my 1967 Corvette roadster. I’m a little confused about torsional dampers, flywheels and flexplates. The engine part number is 12355345 and the description does not mention internal or external balancing. Will damper no. 10051170 (7-1/4” diameter) work with this engine and will it fit in my Corvette?
I have an M21 four-speed with a new clutch and pressure plate. Since I probably have to purchase a new flywheel and flexplate for the one-piece rear seal, will my clutch and pressure plate work? Two flexplates are recommended, but are nowhere to be found in the GM Performance Parts catalog. Also, the new products bulletin lists a .035” plug gap, the catalog shows .060”. Which one is correct?
You sound confused and probably for good reason. All small block Chevy engines are the same but they’re all different. It used to be that all 350 cubic inch and smaller small blocks were internally balanced, but that changed a few years ago. Since you’re installing a new-style engine, you need a flywheel which has a counterweight on it. GM Performance Parts lists three flywheel part numbers for engines with one-piece rear main seals. Part number 10105832 is 14” in diameter and has a 168-tooth ring gear; part no. 14088650 measures 12-3/4” in diameter and has a 153-tooth ring gear. Also available is a special lightweight flywheel (part no. 14088646) which weighs 16-pounds. If you have a standard 10-1/2” clutch, it should bolt right up to either of the 153-tooth flywheels. If you have an 11-inch clutch, it will mate to either the lightweight or the 168-tooth flywheel.
A flexplate is the automatic transmission equivalent of a flywheel. It bolts to the crankshaft and the torque converter bolts to it. Since you have a manual transmission, you don’t need a flexplate. The two recommended flexplates are probably not in the Performance Parts catalog because they’re stock items.
Regarding vibration dampers, all 350-cubic inch and smaller conventional small blocks are internally balanced; only the 400 small block is externally balanced. As a general rule, smaller vibration dampers are desirable because they’re lighter. However, they don’t damp crankshaft torsional vibrations as well as larger, heavier dampers. For mild street engines, a stock vibration damper will provide acceptable performance. But for the longest crankshaft and bearing life, use of a high performance damper is advisable.
Wide Open or Not?
I have a 1991 L98 that puts out 4.2 volts at the Throttle Position Sensor (TPS) at wide open throttle. Is this enough voltage to provide the correct signal to the ECM for maximum engine output? The 1991 TPS is non-adjustable and a parts store states that a new TPS could be the same. The Corvette manual calls for 5 volts at wide open throttle.
Sit down, relax and don’t worry about your TPS. It’s undoubtedly in fine shape. I’ve never seen a TPS voltage at wide open that more than 4.3 volts or so. As far as the TPS being non-adjustable, that’s of no consequence with speed density systems because the ECM determines the voltage that constitutes closed throttle. When you start your car, the ECM assumes that the throttle is closed and equates whatever TPS is at that point with closed throttle. If you happen to start the car with your foot on the gas pedal, and then remove it, the ECM will recompute closed throttle position based on the new lower voltage.
The ECM uses TPS voltage to determine percentage of throttle opening and if you connect a scanner or Diacom to your car, push the gas pedal all the way to the floor and look at “Percent Throttle Opening”, you’ll see 100. As long as you’ve got that reading, the ECM knows you’re at wide open throttle. Pretty smart, these semiconductors.
The Charge is Super
Last summer I was able to purchase my first Corvette which is a 1986 automatic, L98 Z51 with 45,000 miles. The car is quite fast, but I would like more power.
I’m considering putting on a supercharger. There is one particular supercharger that sounds too good to be true. The Procharger says it could get my L98 down the 1/4 mile in 12.1 seconds at 116.2 mph. Also, I’m really interested in how much top speed is increased with the addition of a supercharger.
There’s an old adage that has been used and abused ad nauseum, but it still rings true–There’s no substitute for cubic inches. Some people contend that supercharging IS a viable substitute. That’s true too, because supercharging is really just another way of increasing engine displacement. As a general rule of thumb, with normal atmospheric pressure being 14.7 psi, (also known as one atmosphere) a supercharger that builds 15 psi of boost effectively doubles an engine’s displacement. Another way to look at this is that if a 350 engine can process 350 cubic inches of air at one atmosphere of pressure, it can process 700 cubic inches of air at two atmospheres, all other things being equal. Obviously, there are other factors that influence the equation, but you get the basic idea.
Up to a point, power increases are directly related to the amount of boost. Beyond that point, either the increases tail off or the engine grenades itself– there are limits as to the maximum amount of boost an essentially stock engine will tolerate. Consequently, most centrifugal superchargers such as the Procharger, produce between six and 12 pounds of boost when installed on an L98 or LT1 engine. At these boost levels, power increases will range from about 70 to 150 horsepower, depending on camshaft, cylinder heads, intake and exhaust system. Higher boost levels are obtainable by changing the drive pulleys.
One of the negative aspects of supercharging is an increase in inlet air temperature, (resulting from compression of the incoming air) which reduces the amount of power increase. Intercoolers were created to bring inlet air temperatures back down and in turn bring power increases up.
I’ve taken the long way around answering your question so you’ll have some basis for understanding that low 12 second quarter-mile times are certainly possible. Keep in mind that many an L98 Corvette has run 12.30s with a modified, but naturally aspirated engine. There’s plenty of documentation to prove that when boost is applied to an engine the power curve makes a sharp turn upward. There’s also plenty of documentation that proper tuning and an adequate fuel supply system are required if a supercharged engine is to be prevented from launching internal parts into lunar orbit.
More Power
I own a 1980 L48 Corvette that puts out around 190 horsepower. What can be done to improve output to 300 horsepower or better. Can it be done with the existing engine and what sort of costs am I looking at?
A 350 small block is a 350 small block so the fact that the original engine is rather anemic is of little consequence. There’s no need to switch engines for horsepower considerations, but if you’re concerned about down time, building an engine before it’s installed in the car will speed up your journey on the road to horsepower.
In addition to engine modifications, you’ll also need to ensure that the exhaust system is adequate. As a minimum, a high flow catalytic converter (assuming you want to keep the car emissions legal.) and low restriction mufflers are a necessity. I’d advise a Random Technology Super High Flow catalytic converter and either Dynomax, Borla, Flowmaster or Random Tech mufflers. Headers will also make an important contribution to air handling capability and Hedman or Hooker should be able to supply what you need.
As for the engine itself, have the heads ported and the combustion chambers modified to provide a compression ratio of approximately 9.5:1. If you feel so inclined, you can have 2.02” intake and 1.60” exhaust valves installed, but the standard 1.94”/1.50” combination is more than adequate for a 300-350-horsepower effort. However, rather than stock valves, spring for some high performance types made of stainless steel.
That leaves us scratching our collective heads regarding camshaft selection. Should we go for a hydraulic roller or stick with a flat tapped design? If cost isn’t a major consideration, my recommendation would be a ZZ9 cam from TPI Specialties Although this cam was designed for fuel injected Tuned Port engines, it also works extremely well when a carburetor is controlling the air/fuel mixture. If the cost of a roller conflicts with budgetary guidelines, you might try the TPI Specialties “Torque Secrets” flat tappet hydraulic. Of course, there are a host of other grinds available from Comp Cams, Reed, Lunati, Erson and other companies.
All of these changes can be made without removing the engine. If you decide a complete rebuild is in order, take the block to a performance-oriented machine shop. In addition to the normal boring, honing and align honing, have the block machined for “zero deck” (which means at top dead center, the pistons will be flush with the deck surface, not below it. Install Federal-Mogul hypereutectic pistons and Sealed Power piston rings and your engine will not only produce the desired amount of power, it will also have excellent durability.
Overall costs can run the gamut from about $1,000 to $5,000 depending on the extent of the work you have done. Shop around and you’ll find that component prices vary dramatically. Some companies offer complete engines for under $2,000, others will charge that much for a set of ported heads and a camshaft. But if you’re dealing with reputable suppliers, you’ll get what you pay for.
In the Chips
I have a 1995 Vette and am aware of several chips/modules to improve performance such as Hypertech Power Programmer, JET inline module and ADS inline module. Are there any real life comparisons as to which solution would give best results for an
The Hypertech Power Programmer, JET and ADS inline modules are simply different methods of solving the same problem– coaxing more power out of the engine. I don’t think one is inherently better than the other– it’s the programming inside that makes the difference. Keep in mind that all off-the-shelf high performance PROMs or ECM/PCM recalibrations alter air/fuel ratios and spark timing only at or near wide open throttle. While these products will generally improve maximum performance levels, they don’t address low speed and mid-range drivability or idle quality.
Hard Start Blues
I have a 1988 Corvette convertible that starts fine cold but after you run it awhile and shut it off for 5 to 10 minutes, and go to start it again, it cranks over several times before it starts. If I just shut it off then start it right away it starts just fine, its only after it sits awhile that it’s hard starting. Any information you can give would be very helpful. It has been given a complete tune up.
Assuming that all relative components were checked out when it was tuned up, and also assuming that this problem has just recently reared its ugly head, I’d look for a leak in fuel delivery system. Hard starting (when the engine spins over, but doesn’t fire, as opposed to not spinning over) is frequently a result of too much or too little fuel. It may be that when you shut the engine off for 5 or 10 minutes, the pressure in the fuel rail bleeds off and it takes a few seconds to build it back up. It might be that the leak is through the injectors, in which case excessive fuel in intake ports is causing the hard start problem. If you connect a fuel pressure gauge to the fuel rail, you can readily determine if a system fuel leak is the culprit.
Other possible causes include a defective mass air sensor or EGR valve, a sticking throttle position sensor or Idle Air Controller, bad ignition coil or improper electrical system ground. You may have to find someone with Diacom software or a scanner to help pin the problem down.
North, South, East or West?
I was wondering if you could point me in the right direction to get some info on GM cams. I need all details, lobe separation centerline angle, opening and closing points for an L82 camshaft. Any help is appreciated.
Directions: Proceed south to the bottom of this column. Turn east. Travel one pica, then turn north and proceed to the top of the next column. Turn south and continue until you find the information you’re seeking.
L82 Camshaft Specifications
Part no. 3896964
Duration @ .020 (I/E) 250/250
Duration @ .050 224/224
Lift: .450/.460
Lobe Separation: 114
Timing: Intake Open 52BTC
Intake Close 115 ABC
Exhaust Open 98 BBC
Exhaust Close 63 ATC
Free Breathing Restored
I’ve heard of people removing the screen in the stock MAF to increase airflow and performance. Does this work, why, and what detrimental side effects are there?
The screens found inside a Mass Air Flow sensor are designed to prevent trash that enters the intake system from damaging the sensor. With an air cleaner element in place, and a sealed intake system, there isn’t much chance of any debris making it all the way to the MAF sensor. Removal of the screen is worth a few horsepower because air flow capacity is increased. These screens are also called diffusers and in theory, they should smooth the air flow through the sensor and reduce turbulence so the sensor can operate more accurately. In practice, I’ve never seen any practical benefits from the screens and I’ m not aware of any detrimental side effects if the screens are removed, provided you keep the air cleaner in place.
TPI Mods
I am still somewhat confused about the desirable sequence of bolt-on improvements, particularly engine breathing, for L98 engines. I have managed to obtain locally a 3.73 ring and pinion and K&N Air Filer (which are both currently installed in the car), a 52 mm throttle body, and headers. I seek your advice on appropriate add-ons to install at the same time as the throttle body and headers. I will have to get any further parts sent from the USA, and I don’t want to do much more at this stage than put in the parts I have, and any immediately desirable complementary ones. I have presumed from various material that runners would be appropriate, and would appreciate your views. I have also seen much literature which suggests that reduced running temperature will result in more power. Some publications recommend lower temperature thermostats as a first step modification. If it is better to operate at a lower temperature, why did the Corvette engineers design the thermostat to open at the temperature it does, and why is the fan set to come on at such a high temp?
You’ve probably heard that an engine is an air pump, but I prefer to call it an air processor. It draws air in through its intake system where it is mixed with fuel. Then it compresses the air/fuel mixture, ignites it and eliminates it through its exhaust system. Power output is directly dependent upon air processing efficiency. If sufficient quantities of air can’t be drawn in, or pushed out, power output is compromised.
The quality of air flow is just as important as quantity. If air doesn’t flow in or out at reasonable velocities, power output will be reduced. An example is an engine with ported heads that runs poorly. In such an instance, enhancement of air flow capacity has reduced velocity through the intake ports to the point that the cylinders aren’t adequately filled. On the flip side of that consideration is an engine with stock cylinder heads that doesn’t produce the desired power because the ports are too small (which increases velocity at the expense of total air flow capacity.
The tradeoffs between velocity and capacity doesn’t so much kill horsepower as they alter the shape of the power curve. Cylinder heads with ports that are “too large” are in fact simply unsuited to the application in which they’re being used. If such heads were installed on an engine that had the proper cam, intake and exhaust systems, they would produce excellent power, but at relatively high rpm.
The intent of this somewhat rambling explanation is to bring the coordination of components into perspective. Stock engines are typically “restriction rich” on both the intake and exhaust sides of the air processing equation. Tuned Port engines are especially restricted on the intake side because the manifold, runners and plenum were really designed for a 305 cubic inch engine. It has been well documented that the long intake runners pretty well limit the horsepower range with maximum power typically being recorded at 4700 to 4800 rpm. That being the case, significant power increases require a performance manifold, large tube runners, ported plenum and 52mm throttle body. A more aggressive camshaft and ported heads are also beneficial, but that’s outside the realm of intake and exhaust system modifications.
Returning to air processing considerations, temperature is part of the quality picture. Cooler air is denser and consequently of higher quality because it increases power output. A Tuned Port intake system is like a large heat sink on top of the engine and that must be addressed. Installation of a 160-degree thermostat and eliminating coolant flow through the throttle body do help lower intake system temperatures and that has a positive influence on power output. The Corvette engineers keep coolant temperatures high because they’re addressing emissions, fuel economy and drivability issues. Federal regulations specify that certain standards be met and higher coolant temperatures are instrumental in meeting those standards.
H.O To Go
I am the proud German owner of a ‘63 convertible Corvette. I have a GM350 HO engine in it. In Germany, it is quite difficult to find specialists in American cars or engines and I am not too well in technical stuff. My question to you is how can I improve the performance of my engine? Thank you very much. Please excuse my bad English.
Hartmut Salzmann
Germany
First of all, there’s no need to apologize for your English– it’s better than that of many Americans, and as an added bonus, your letter wasn’t written in crayon. As for improving the performance of your engine, that’s relatively easy. Just keep in mind that frequently there are trade-offs, so you have to determine what you’re willing to give up for the sake of more horsepower. Also keep in mind that the engine is only part of the power equation. To take full advantage of its capabilities, you have to make sure it can breathe. For that you need a set of headers and a free-flowing exhaust system. You also need to ensure that the carburetor can grab all the air the engine demands. The carb should be topped off with a 14” diameter air filter that’s at least 3” thick, and a low restriction element wouldn’t be a bad addition either.
As for additional modifications, unless you can find some competent help, your options are limited. One thing you can do yourself is recurve the distributor. This will involve some trial and error testing, and that usually involves a lot of errors, don’t get discouraged. Your goal is to have full centrifugal advance by the time engine speed reaches 2750-3000 rpm. Set initial timing at 12 degrees (with the line to the vacuum advance canister disconnected) then experiment with springs and weights until you convince the distributor to deliver another 24 degrees of advance before the tachometer needle hits 3000 rpm. You can check this by just revving the engine and watching the timing marks on the vibration damper with a timing light. Several companies offer adhesive-backed timing tapes that can be applied to the vibration damper so you can check timing at higher advance rates. Advance kits containing springs and weights are also available.
Beyond that, you’re looking at cylinder head and camshaft modifications. That’s where most of the trade-offs come in. A longer duration camshaft will increase horsepower at and near the upper end of your engine’s rpm range, but you’ll lose some low speed torque. If cam duration is altered very much, you’ll also have to take steps to increase compression ratio. We’re starting to get into an area that requires a bit of expertise and considering the lack of help you have available, I’d stick with the ignition and exhaust modifications until you can find a competent mechanic, or gain enough experience to feel comfortable enough to tackle the job.
Position is Everything
I recently purchased a 1988 Corvette with 45,000 miles on it. I want to tune it to the max and I’ve read that adjusting the voltage on the throttle position sensor (TPS) improves throttle response. What should the voltage setting be for maximum performance?
Put your tools away for a few minutes and think about throttle position and the sensor that records it. Throttle position is one of the components the ECM uses to control fuel flow. Typically as the throttle is opened, an engine requires more fuel, which is the reason the TPS is there in the first place. By biasing the closed throttle voltage, you’re in essence lying to the ECM, and since it has blind faith in sensor input, (so long as it’s within range) it responds by increasing injector pulse width, which in turn richens the mixture. Each engine will respond somewhat differently to changes in TPS setting, so there’s no hard and fast recommendation for an optimal setting. The factory specification is for a TPS voltage of .5 to .54 at closed throttle. If you’re convinced you want to use TPS output as a tuning device, start by setting it at .54 volts, then vary it in increments of 5 millivolts (.05 volts) until throttle response is optimized. But keep in mind that the ECM has the capacity to learn. It continually monitors O2 sensor input and adjusts air/fuel ratio as necessary to maintain 14.7:1. (While in closed loop operation, which is active during part throttle conditions after operating temperature has been reached.) So within a relatively short period of time, much of the effect of moving TPS position will be neutralized. However, you should still see a slight difference during rapid throttle movement.
Tuning for maximum power at wide open throttle is most easily done by adjusting fuel pressure. Ideally, oxygen (O2) sensor voltage should be between 825 and 850 millivolts during wide open throttle operation. As an engine’s air consumption capabilities are increased, it’s fairly typical for O2 voltage to drop because the engine is processing more air than anticipated when air/fuel calibrations were established. Since the ECM doesn’t reference the O2 sensor during wide open throttle operation, the mixture can slide to the lean side of the scale. Custom PROM calibrations are one way to rectify the situation, adjusting fuel pressure is another (within a discrete range). You’ll need a scanner or Diacom software to monitor O2 sensor voltage.
Out of Line
I own a 1980 Corvette, and we are in need of some knowledge. The rear alignment has been a constant expense. Please explain how to do a “rear alignment” on a 1980 Corvette so that we can do this here at home. I’m tired of paying to have the alignment done over and over again.
The best bet is to find a reputable alignment shop with computerized equipment. Have the alignment set by the shop, then take your Corvette home and use your own equipment to establish baseline settings. Assuming the shop sets rear alignment properly, you’ll be able to refer to your baseline readings to reestablish those settings.
Camber, which is the vertical inclination of the tire/wheel assembly, is easy enough to set. All you need are a couple of wrenches. Loosen the nut on each strut rod cam bolt, turn the bolt as required to establish a reading of 0 degrees (plus or minus 1/2o) and tighten the nut. Obviously, a camber gauge is required, but you can substitute a bubble level held against the wheel. To accurately set camber with a bubble level, make sure the rim isn’t bent and that the level contacts the rim only and doesn’t contact the tire sidewall. You might also check with some of the companies that supply equipment for road racing. Several of them sell relatively inexpensive camber gauges. If camber keeps changing, your spring is sagging or the bushings in the strut rods are history.
Toe is set by adding or removing shims between the control arm and frame. Depending on the type of shims used on your particular car, (some have holes, others have slots) it may be necessary to completely remove the control arm bolt to add or remove shims. All you need to check toe is a tape measure. Mark the center of each rear tire with chalk front and rear, and measure the distance from mark to mark. If the dimension across the front is smaller than across the rear, a toe-in condition exists. You can figure out the other scenario. The problem with this and similar home methods (you can also place a board adjacent to each tire and measure the distance between boards in front and in back of each tire) is that it doesn’t reference the wheels to the frame. So if both wheels are cocked, toe will measure zero, but one will be toed-in and the other toed-out. As with camber, the best bet is to have an alignment shop set toe properly, then make your own measurements so you have a baseline. Check them the same way every time, and you’ll be able to get a good feel for any changes.
Old Fashioned Power Steering
I found the final solution to power steering leaks in my ’79 by removing the pump and ram. I don’t miss the assist, but I would like to know if I am stressing any parts by providing my own power. What part changes, if any, are required for a conversion to manual steering?
The only parts you’re stressing are your arms. The steering box doesn’t really care whether you or a power booster supplies the power. But if you’d like to make life a little easier, crawl underneath your car and take a good look at the position of the tie rod end in the steering knuckle. You’ll notice that there’s another hole behind the one in which it’s installed. If you move the tie rod end on each side to the rear hole, you won’t have to use as much effort turning the steering wheel. However, you’ll have to turn the wheel a bit further when making a turn since moving the tie rod ends affects steering ratio. See, there’s no free lunch.
The Wanderer
Despite extensive stock restoration of my mid-year’s suspension (springs, bushing, etc.), and switching to good low-profile radials, my car still exhibits the “constant, nagging (steering) corrections” and “unsophisticated handling” , which is apparently typical of mid-year Corvettes.
I am considering a suspension package such as the Grand Touring Suspension Kit as offered by Vette Brakes & Products, and disabling the power steering in an attempt to correct this I am not looking for 1g lateral acceleration; I’m looking for some semblance of steering precision and response.
1) Will this approach address the problem given that the inner pivots aren’t changed?
2) Why do most after-market suspension packages attempt to decrease the front to rear roll stiffness difference when GM apparently improved the handling by increasing this difference?
3) How necessary is the steering damper on manual steering cars? Since I would like to maintain the fast-ratio tie rod locations, will the amount of kick-back through the steering wheel warrant my finding a manual steering relay rod to
I don’t think the power steering is the problem. When a car wanders, it’s usually due to improper wheel alignment or loose or damaged steering components. Check the front end over thoroughly and have it aligned. Shoot for 1/2-degree of negative camber and as much positive caster as you can get. Toe-in should be set as close to zero as possible. Also have the rear alignment checked. Camber and toe should be as close to zero as possible.
The Grand Touring Suspension Kit should improve all around handling. Like most aftermarket suspension kits, it increases rear roll stiffness as a means of improving handling. GM didn’t improve handling by decreasing the front/rear roll ratio, they just made the car easier for idiots to drive and not get into trouble. When rear roll stiffness is increased, a car tends to exhibit oversteer tendencies, which means the rear end will be prone to swap ends with the front, when provoked by an over-enthusiastic driving style. Rear-engined vehicles, which have a high percentage of weight behind the rear wheels also demonstrate this characteristic. (Note the relatively large number of Porsches that are crashed backwards.) As a means of minimizing oversteer tendencies, most vehicle manufacturers specify considerably lower rear roll stiffness (compared to front roll stiffness). Purveyors of high performance suspension equipment offer the hopefully discerning and capable driver the means to increase rear roll stiffness for crisper handling. However, if you’re right foot-hand coordination isn’t very good, you’d be well advised to avoid changing the rear suspension. Otherwise you may find yourself at the body shop most frequented by Porsche owners.
Corner Cutter
I occasionally race my ‘86 Corvette coupe at a road course. I find that my Corvette is not competitive in the corners when compared to NSX’s, RX-7s and some newer Corvettes. I would appreciate your comments as to the best solution to make my Corvette corner better. One of the newer Corvettes that I race with is equipped with a roll bar which the owner stated reduced the flex in the car and helped it corner better. Would you recommend this option and can you let me know where I can have a bar installed or who sells one I can install myself? What about monoleaf fiberglass springs?
Some other questions– my oil temperature is extremely high during racing. Would the installation of an oil cooler that attaches to the oil filter mount restrict the flow of oil? Since I am required to run DOT approved tires, what is your recommendation as to the stickiest tire for the road?. Can I change the brake bias on my Corvette to help prevent nose dive in braking for corners?
Is it the car or the driver that’s responsible for the differences in cornering speeds? I don’t mean to bruise your ego, but some of these cars may have semi-professional drivers behind their respective steering wheels. It might be advisable to have a professional or semi-professional driver take your car out for a few laps and give you a critique.
Assuming that the cornering capability is in fact inadequate, start with a suspension upgrade. The stock suspension on your car makes a number of compromises to keep the wusses, accountants and models who buy Corvettes happy.
Not knowing the level of cornering power you’re looking for, or your budget, I’ll start off with some inexpensive modifications. About the cheapest thing you can do is to lower your car which will drop the center of gravity and thereby improve handling. Vette Products offers a lowering kit for ‘84-’94 Corvettes that drops your Corvette about an inch and is reasonably priced. That probably won’t give you the cornering power you’re looking for, so the next step is to install stiffer anti-sway bars. These will increase roll stiffness without significantly impacting ride quality.
The next step would be to upgrade the suspension with front and rear monoleaf springs, adjustable shocks, adjustable strut rods and polyurethane bushings. I’ve found that fiberglass monoleafs are superior to steel springs for a number of reasons. They’re lighter, so they reduce unsprung weight (which improves handling) and they also tend to be smoother and more compliant. That translates to better tire adhesion and less tendency for the car to be come unsettled when traversing surface irregularities.
Stiffer anti-sway bars won’t necessarily reduce flex, but they will keep the tire more upright in a turn. That increases tire contact patch size which in turn improves cornering power. Just don’t fall victim to the “if some is good, more is better and too much is just enough” philosophy. If you do, you’ll find that you’ll have your hands full when you take to a road course. Like most things automotive, it’s the overall combination, not the maximum capacity that provides the best results.
After all the new equipment is installed, experiment with different caster and camber settings. Most racers set their cars up differently for competition than they do for street driving. Although differences in alignment won’t make a major change in race track performance, they will make a measurable improvement.
An oil cooler shouldn’t restrict oil flow. Just make sure you run a sufficient amount of oil to keep the system full. The stickiest DOT tires I know of are the DOT race tires offered by companies like BFGoodrich Goodyear, Toyo, Yokohama and Nitto. All are designed for road racing and are plenty sticky. Which one is best? That’s a matter of personal opinion, so no matter what I tell you, I’ll be wrong.
I wouldn’t worry about changing the brake bias just yet. Nose dive is as much a function of the suspension as it is the brake system. You may find that stiffer front spring will solve the problem.
Autocross Action
I currently autocross a 1978 L-48 Corvette on 245X50X15BFG R-1 tires in “A” stock and NCCC 1-D. The suspension is new and stock except for a 340 lb. glass spring – what caster/camber/toe do you suggest and what tire pressures do you suggest I start with?
Trying to get me in trouble are you? Making hard and fast recommendations about suspension settings is an invitation to disaster. Considering the potential differences in driving styles, chassis set-up and modifications, you’ll undoubtedly need to make a few tweaks to optimize your particular vehicle. Now that I’ve made my obligatory disclaimer, I’ll stick my foot in my mouth. As a general rule, the best place to start is with about 1-degree of negative camber (front and rear) and as much positive front end caster as you can get. Typically on 1963 to 1982 Corvettes, the maximum is about 2 or 3 degrees. C4 and C5 suspensions were designed for higher caster settings, so you can usually dial in considerably more.***check recommendations*** Toe should be from zero to 1/8” in both front and rear.
You’ll find many so-called experts that will disagree with those recommendations, but keep in mind that there’s no magic involved in suspension settings. All you’re trying to do is keep the tires flat on the ground when you’re cornering. The ‘63-’82 Corvette chassis has a surprising amount of front end flex, and suspension settings have to compensate for that. Obviously, an aggressive driver combined with super sticky tires will induce more chassis flex than Johnny Milquetoast on street tires; suspension settings have to be adjusted accordingly.
One of the best tuning aids is a video camera. Have an assistant shoot close-ups of the tires while you’re in competition to see what’s happening. Remember, the outside tire in a turn takes the highest loads, so that’s the one you should concentrate on. If the tire is leaning past vertical, towards the outside of the turn, you need more negative camber; if it’s leaning towards the inside less negative caster is required. Also look for tire squirm. If the sidewalls appear to be rolling over, increase air pressure; if the front end tends to skate, reduce pressure.
Positive caster also helps to keep a tire properly positioned during a turn and improves steering control. But like anything else, too much of a good thing is still too much. I doubt “too much” is obtainable in the case of a ‘63-’82, unless the suspension has been modified specifically to allow higher caster settings. (By way of comparison, C4 and C5 Corvettes run about 6 degrees of caster.
As far as tire pressures are concerned, I’d start with 30 psi and experiment. Altering the front-to-rear pressure bias is an easy way to tune in more oversteer or understeer. In many instances, two to four more pounds in the rear tires delivers a very neutral feel, but again, this is driver and vehicle dependent. Also keep in mind that tire pressure will usually increase after a few hard laps, so you may—or may not—want to compensate for that with your initial setting.
Corvette a la Porsche
I have owned two roadsters, a ’61 and ’65 and I really enjoyed them for what they were. For the past few years, I have owned a ’92 LT1 six speed coupe and it is about this Corvette I am asking for help. I have been building engines and modifying suspensions for 30 years and I know the rules about understeer and oversteer. I have the Z07 package with FE7 heavy duty suspension and yes it is a little harsh but I have always hated mushy road feel. However, I now believe that the FE7 option was designed to be run only on a smooth street with new pavement unless you only drive in a normal, legal manner. As soon as you push it in a corner, with neutral throttle, it can tell you what oversteer really is. Those new GSC’s really stick on the front end, I can never make them slide. The problem is with the rear end. Any slight road irregularity, rise or dip and look out, you’re going to start sliding sideways. I can feel the rear tires break loose and sometimes hook up again but usually you have to do some good recovery maneuvers or you may spin out. I don’t mean at warp speed either. I am talking about speeds of only 30 mph and you may be doing a 180 with a visit to the ditch. I know, I have done it without even trying. Plenty scary and expensive too. For the record, my tires are new, 32K miles on the car and it has had a dealership alignment by one of the top Chevrolet mechanics. Lately I have had visions of selling it.
I believe that the monster rear anti-sway bar is far too large a diameter and the rear spring rate may also be excessive for street use. I am afraid that the FX3 ride control system may perhaps be a joke, the ride is the same at any setting and does not feel the least bit speed sensitive. I know that Chevrolet improved this in newer models. So what do you suggest? Should I install the standard rear bar or spring or do coil over springs work better than the leafs? I realize that no vehicle is perfect and life is full of compromises but I know a car with 50/50 weight distribution can do better than this. What do you think? Am I crazy? Have others had the same experience?
I thought it was only Porsches that liked to surprise their drivers, swap ends and head down the road rear end first. Maybe I’m confused and it’s only Porsches that are DESIGNED to do that. And of course, you know the difference between a Porsche and a porcupine.
It definitely sounds as if something is askew with your rear suspension. The ’92 rear suspensions were softened considerably compared to those in some of the earlier models. The fact that adjusting the suspension has no effect points to the fact that it is not functioning properly. I’ve driven FX3 cars and played with the suspension settings; there definitely is a difference. Since you purchased the car used, there’s no telling what the previous owner did or had done. There are some real geniuses out there. You may even find that the FX3 control has been purposely disconnected or rendered inoperative. Don’t assume– check it out.
Once you get the suspension control functioning properly, if the problem persists, disconnect the shock absorbers and push down firmly on the rear end. If the car moves up and down smoothly and easily, you can pretty well rest assured that the spring isn’t too stiff. Another approach is to put the car on jack stands and place a jack beneath one wheel. Jack it up until it’s at normal ride height position and while you’re doing it, make sure that the suspension doesn’t bind at some point. Then try the same experiment on the other side. You might also try disconnecting one side of the rear anti-sway bar and going for a test drive. With the bar disconnected, you should notice definite understeer. But if the car corners without sashaying into the next lane, you just might be on to something. The car may in fact need a smaller rear anti-sway bar, but frankly, the problem sounds more like a mechanical malfunction than a design error.
Whole ‘Lotta Shakin Goin On
My ’63 Roadster has a problem we have not been able to find: the vehicle has a significant front-end shimmy at speeds between 55 mph and 60 mph. I had new tires and new knock-off wheels installed last spring. This made no difference. My mechanic has done test drives and visual inspections of the under-carriage and can see nothing obvious. He has checked the front end for worn or loose parts and again, nothing was found. Can you give us an idea of a list of things we should do, in their order of which is more likely to be the problem? Any advice would be greatly appreciated.
Maybe your ’63 just has rhythm. Try tuning in a classical music station and see if that makes any difference. Beyond that, assuming your mechanic is competent, you’ve got a head scratcher on your hands. A shimmy has to be the result of looseness or imbalance. Assuming the wheels are properly balanced and the shock absorbers are in good shape, the most common causes are worn A-arm bushings, worn wheel bearings or loose steering components. Many of these are difficult to diagnose by simply looking at them. The springs put quite a load on suspension attachments and this can easily hide a worn component. Keep in mind that for a wheel to shimmy, either the springs and shocks aren’t providing adequate control or the suspension is flopping around in response to forces generated by the wheel and tire assembly as it rolls on the pavement. Worn ball joints and tie rod ends are the most common causes of front end problems, but worn bushings are also fairly common.
First some seemingly obvious questions- were the new wheels and tires properly balanced prior to installation? Was the knock-off hardware checked for runout? Has alignment been checked recently? Are you sure the tires aren’t out of round? If the answers to those questions are all yes, it’s time to start digging deeper. Make sure the steering gear is securely mounted to the frame and that the tie rods are all in good shape. If everything checks out, remove the front springs and shocks and see if you can find if anything is loose. Unless it’s been done recently, chances are a front-end rebuild will uncover and solve your problem.
