C4 Performance Enhancement

One of the more confusing questions about owning a C4 Corvette is: "How do I obtain additional performance?"

The claims made by the various aftermarket suppliers are one reason for the confusion since all of them promise performance gains.  

Further down the page, you will find some specific suggestions on how to improve your C4 Corvette's performance  but first, David Rodabaugh, Mechanical Engineer, has provides us with an in depth treatment of this subject.  www.c4vettes.com is indebted to David for sharing his knowledge.

Power, Velocity and Drag.  What's it all mean, anyway?

Every car enthusiast wants more power. Just as addictive as so many illegal substances, top speed and acceleration provide a nice “hit”, but soon, the driver just wants more. And more. And still more. And if that enthusiast is the driver of America’s premier sports car, the craving can result in obsessive tinkering.

For every want, there is an appropriate power mod. Some cost big bucks. Speed costs money; how much money you got? Still, money is no substitute for appropriate design. Corvette owners with deeper pockets can simply pay somebody to do the thinking for them. But for the average owner, performance improvements are the result of skinned knuckles, grease under the fingernails, and most importantly, proper mod selection. Some categorize an effective mod as “neck snapping acceleration”; others think that an increase in top speed would be good. Confusion can easily arise with the (usually exaggerated) performance claims made by competing manufacturer.

Proper mod selection starts first by answering “How do I wish to change my Corvette’s performance?” There are two answers to this question: accelerate faster, and higher top speed.

How To Accelerate Faster?  Answer: increase torque at the rear wheels at all times. This can be done by increasing the output of the engine, or by changing the gear ratio. If the output of the engine is to be changed, the mods must be chosen so that they are harmonious with the car’s gear ratios and rear end. In general, modifications which substantially increase the horsepower figure of the engine will also tend to narrow the power band. It is important that this power band not be so narrow that the simple act of shifting gears moves the engine to an RPM point where it does not immediately produce power.

How To Increase Top Speed?  Answer: a massive increase engine power. What’s massive? 100 HP, 200 HP, or more. That’s the simple explanation. For those who want more detail, but don’t want the math, here’s the skinny. The engine power needs to be increased such that the available torque at the drive wheels can provide more drive thrust to overcome aerodynamic drag.

What is aerodynamic drag?  It is a force which opposes the car’s forward motion, created as the car must move through the air around it. How big is aerodynamic drag? At low speeds, it is so small that the limiting factor of performance is the vehicle’s mass and the torque at the drive wheels (acceleration!). But it builds quickly.

How quickly? Assuming that nothing is done to affect the aerodynamics of the car, the aerodynamic drag force increases with the square of the vehicle’s speed. That means that every time the vehicle speed doubles, the drag force increases by four times.

Example: If the aerodynamic drag force is 40 pounds at 30 MPH, then at 60 MPH, the drag force is 160 pounds, and at 120 MPH, the drag force is a whopping 640 pounds.

How does this affect power? Well, power is actually force multiplied by velocity. Thus, the power required to overcome the aerodynamic drag force increases with the cube of the vehicle’s speed. Wow! That means that every time the vehicle speed doubles, the power required to overcome the drag force increases by not four times, but EIGHT TIMES!!!

Example: If the horsepower required to overcome the drag force at 30 MPH is 10 HP, then the horsepower required to overcome the drag force at 60 MPH is 80 HP, and at 120 MPH, the horsepower required is a monumental 640 HP! Of course, these numbers are not accurate, but they illustrate just how tough it is to increase top speed. They also explain why a 140 HP vehicle that only uses 15 HP to go 60 MPH tops out somewhere near 125 MPH; it truly runs out of power!

Of course, there will always be skeptics. Some people just won’t believe that adding 100 HP to their Corvette will only get them another few MPH on the top end. Those skeptics are encouraged to visit Appendix A below. There, a full mathematical analysis will be provided, complete with charts to guide somebody wishing to make mods to their Corvette.

Appendix A

Aerodynamic Drag Force, Horsepower, and Corvette Top Speed

The following is a detailed technical explanation correlating aerodynamic drag force, horsepower, and Corvette top speed. It is offered for the 5% of you who want the nitty gritty.

Aerodynamic Drag Force

Aerodynamic drag force is a force which opposes the vehicle’s forward motion, created because the vehicle must displace air as it moves forward. The only time when the drag force is zero is when the vehicle is stopped, or there is enough of a tailwind that the Corvette sees no headwind.

Aerodynamic drag force may be defined as follows:

Fd = (1/2) Cd r A V^2 (1)


Fd = aerodynamic drag force
Cd = coefficient of drag
r = density of air
A = frontal area of vehicle, into the direction of motion
V = velocity of vehicle, into the direction of motion

This equation may be found in any fluid mechanics text. In fact, the equation is quickly recognizable as the first integral of a basic momentum equation, which makes sense, since the aerodynamic force is created by the momentum change of the air as the vehicle moves it out of the way.

This equation shows that aerodynamic drag force increases with the square of the vehicle velocity. One way to state this is to say that a doubling of the vehicle velocity increases the drag force four times. Or, a 41.4% increase in vehicle speed doubles the drag force. Of course, astute mathematicians will immediately note that this makes sense, as the square root of two is 1.414.


Aerodynamic drag force is one thing; the power required to overcome it is something entirely different. If drag force goes up quickly as vehicle velocity builds, then the power to overcome it grows even more ferociously. Specifically, it increases with the cube of the vehicle’s velocity.

Power is widely defined as follows:

P = F x V (2)

where P = power,  F = force and  V = velocity

The force in question here is aerodynamic drag force, of Fd.

Substituting Fd into (2) yields:

P = Fd x V (3)


P = power,  Fd = aerodynamic drag force,  V = velocity

Substituting (1) into (3) yields:

P = ((1/2) Cd r A V^2 ) * V  or  (1/2) Cd r A V^3 (4)

This shows, mathematically, that the power required to overcome aerodynamic drag force really does increase with the cube of the vehicle’s velocity. Every doubling of the vehicle velocity requires an eightfold increase in horsepower. Or, a doubling of horsepower only results in a velocity increase of 26%. As before, an astute mathematician will recognize that 1.259 is the cubic root of two.

Corvette Top Speed

So how does this affect Corvette top speed? Or more importantly, what are the real world guidelines which result from the math shown above? A Corvette is a slippery and powerful vehicle. It has a small frontal area, a low coefficient of drag, and a throbbing engine eager to dump power to the pavement. It can hurtle forward at frighteningly high speeds, but some wish to go faster still. They make power mods to their Corvette, maybe getting another 100 HP or more, and then are terribly disappointed when they see a single digit gain in top speed. Is this realistic? Is the problem with their modification, or with their expectation? It is the latter.

Here are some sample numbers. They are not pertinent to the Corvette necessarily, but they are typical.

In general, 60 MPH requires about 15 HP. Table 1 shows the power rise that must accompany an increase in speed.

Speed (MPH) Change (%) Horsepower Change
60.00 -- 15 --
75.60 25.99% 30 100%
95.24 25.99% 60 100%
120.00 25.99% 120 100%
151.19 25.99% 240 100%
190.49 25.99% 480 100%

Table 1 - Speed/Horsepower Correlation for a Typical Passenger Car

Whoops! Look at the HUGE jump in required horsepower between 151 and 190 MPH. No wonder a 345 HP Corvette cannot move much past 175 MPH; the power required simply increases too quickly.

Suppose that the Corvette modder succeeds in adding 100 HP to the Corvette (a difficult feat all by itself; non-engineering modders universally overstate the power gains they get from any particular mod, a vice that not even some engineers escape). What kind of increase in top speed can he expect?

If 345 HP yields a top speed of 175 MPH, then 445 HP would get close to 190 MPH, all other things being equal. This makes sense. A 100 HP increase is a 28.99% increase in horsepower, which can provide a measly 8.85% gain in top speed.

If anybody doubts John Lingenfelter’s genius, behold that his 200 MPH C5 fits this math perfectly. If 345 HP gets a Corvette 175 MPH, how much is required to go 200 MPH? 514. It would seem that Mr. Lingenfelter is a pretty smart guy! His 200 MPH C5 produced 500 HP (and a whopping 500 ft-lbs of torque; gotta love turbocharging)! He always decides what he wants the car to do first, and then designs his mods to meet his goal. The novice Corvette modder would be well served to follow his example!

Contributed by David Rodabough

Fast is not always where it's at.....

Unless you race your car or live in Germany where the Autobahn is part of your daily commute, increasing top speed is not going to do you much good.

When is the last time you took your pride and joy up to 150 mph?

Not recently I bet.

So let's concentrate on increasing the seat of the pants acceleration at the stop light Grand Prix and by far the best way to do that is with a rear end gear ratio change.

In order to get the miles per gallon up, miserly - fuel saving ratios are the norm with General Motors...even on the Corvette.

The C4 rear axle ratio RPO numbers decode like this:

RPO Rear End Gear Ratio
GH0 3.54
GM1 2.59
GU2 2.73
GW4 3.31
G44 3.07

3.07 "Performance Rear Axle"

If you want to return to the days of your misspent youth (assuming you are a refugee from the 1960s) or if you want to experience the muscle car acceleration your daddy spoke of, change the ring and pinion gear in the rear end to 3.73 for the automatic transmission C4 and to 4.09 for the manual version.

Changing just the two gears is not a cheap thing to do plus you would be a fool not to replace the bearings, etc. while you have the rear end opened up.

But if you want to return to the days of yesteryear without increasing horsepower or doing extensive engine mods, this is how to do it.

(By the way, you will have to change the speedometer gear on the transmission or buy a ratio calibrator from one of the catalog houses to put things right in that department after the change).

Finally, as always there is no free lunch:  Whereas you probably experienced RPMs in the 1800-2000 range at 65 MPH, now you are going to see 2500-2700 at the same speed.

Obviously the engine requires more gasoline to run faster for the same load so your mileage is going to go down but if you want to trade performance for more frequent stops for fuel, this is the answer.

I don't have that kind of cash....

If you don't have a thousand plus dollars for a do it yourself rear end rebuild (or $1500 to have someone do it for you), there are a couple of relatively easy things to do that will yield incremental horsepower increases.

First, relocate the the intake air temperature sensor to the air clearer duct.   The heat load where it is located on a stock C4 gives the ECM (Electronic Control Module) a false reading and the ECM responds by cutting back on the fuel/air mixture to a leaner value than is actually required.

Mid-America sells a kit for this project for around $45 dollars for 85- through 91 models (L98 in other words) which includes a new sensor.  The LT1 version costs approximately $35 dollars and you reuse the old sensor.

This is a plug in modification after you drill a 7/8 inch hole in the air cleaner shroud and does slightly increase horsepower.

There is a reported 3-5 horsepower increase but there are no published dyno runs to back this up.

Another suggested change involves the air filter.  The stock filter and air cleaner lid restrict air flow.  Discard the stock filter and replace it with a K&N air filter (LPE, 219-724-2552. $42.95 for 85-89 models, $58.95 for 90-96 models).    At the same time, either open up the stock air cleaner lid with a pair of tin snips or purchase the K&N package from Mid America Designs, 1-800-500-8388, ($109.00, 85-89, $119.00, 90-96) which includes both the filter and the unrestricted lid.

The claimed horsepower increase is 3-5 for the filter and another 3-5 for the opened up air filter cover. The change is relatively inexpensive and very easy to do.

Another source of air flow restriction are the two screens in the Mass Air Flow Sensor on the 85-89 models. Just removing the screens will increase the airflow by approximately 125-150 CFM and that works out to around another 10 horsepower gain.  (This was a common practice in the days of the Corvette Challenge races.  Everyone did it).

This mod costs you nothing except your time. It is easily the cheapest, easiest way to get an increase in performance you can feel  (NOTE: Make absolutely sure you clean the MAF device bore of all metal fragments from your screen removal activities but don't touch the wires on the inside of MAF sensor or you will ruin it).

While you have the MAF off, take some carb cleaner and a shop rag and clean out the throttle body bore of any residue. clean both sides of the butterfly.  It will improve the idle.

Also, replace the MAF power and burn off relays with new ones. It only will cost you around $15.00 for both and will help hard starting plus keep the wires on the MAF clean of residue.

The air foil for the TPI throttle body is marginally effective.  It's claimed horse power increase is around 12 and a claimed 12.8 ft/pounds of torque.

The problem with it is you aren't going to see much difference in horsepower until around 4400 RPM and that is about where the restrictions in the intake runners start to limit performance on the L98. (It helps more on the LT-1 and LT-4 since they peak around 5000 RPM).

Still, it is easy to install, costs less than half a "C" note and does also improve idle some.

Back on the semi expensive kick, buy a $180-200 Streetrunner Hypertech chip and replace your stock ECM PROM with it. The advertised increases are 15-20 HP and 20 ft/pounds of torque. (Incidentally, forego the 160 degree thermostat that is sometimes paired with the Streetrunner chip by some sources. The only time you see 160 degrees is during the winter (when the car is cold) or when you first start it up and it goes past 160 enroute to 180+) and if you do succeed in keeping it at 160 degrees, your car's computer will stay in the open loop mode which will limit, not raise performance..

Finally, if you can stand the noise you can also spring for performance mufflers and obtain a 30-50 HP increase plus a torque increase you can feel. 

If you do all of the things listed above, you will bump your C4's HP 70 to 85 ponies and your torque 30- 50 foot pounds.

For perspective, a 1999 C5 is rated at 345 HP and 350 ft/pounds torque.  I did everything except the mufflers on my '89 C4 and calculate that the net HP is now between 290 and 300 net (the stock engine is 245 net) and the torque is now around 370 ft/pounds versus a stock 340 ft/pounds.

This means my car has between 45 and 55 less HP than the C5 but has 20 ft/pounds more torque.  If I add the mufflers, the engine performance will be pretty close.

Not bad for a Saturday afternoon's work.

Here are some things that won't really do you much good...

Throttle Body Changes

Unless you are going to replace the TPI runners with larger diameter versions, there is no point in buying a 52 MM or 58 MM throttle body on an L98 engine. (It will help on an LT-1).

The problem is the L98's runner restrictions.  By doing the things listed above that improve your intake air flow, you have maximized the flow as much as you can realistically expect to increase it without major (read expensive) changes. Any increase in performance from the 52 MM throttle body will be very small and not worth the expense.

Also, increasing the runner diameter or installing shorter runners will cost you low end torque but only slightly increase your high end horse power.  As mentioned above, unless you can run your car at very high speed, you should stick with the stock runners and enjoy the torque boost they provide. 

Adjustable Fuel Pressure Regulator

A high pressure fuel regulator is a waste of time unless you are putting in a custom PROM in the ECM that allows for the increased pressure. (The stock PROM will sense the increased pressure and immediately react to it so you can't get a result you can feel from your efforts). 

Basically, the O2 sensor will sense the richer mixture,  the ECM will read this and shorten the on time for the injectors.  A custom PROM chip would react differently but the stock chip and most mass market performance chips don't address this issue.

You should however check the pressure that your stock regulator is producing.   Make sure it is at or above 37 pounds.  (A gauge to check this is available from virtually all after market suppliers for around $40.00).

It is also true that under some conditions, the higher pressure will cause the fuel mixture particles to be more evenly dispersed  in the cylinder and a more uniform mixture will result in a more efficient burn.  There are many factors involved with this however and it may or may not work for you.

Finally, increasing the fuel pressure will sometimes cause lumpy idle overall plus poor initial acceleration response can be present as the mixture goes rich for an instant before the computer dials it back down.

Higher Output Injectors

The same logic applies to using higher output injectors.  Unless you are going to change the PROM to take advantage of the increased amount of fuel, it is not very productive to pump more fuel into the engine. (You may get a better dispersion of the fuel and thererfore a better burn with some aftermarket injectors that is not a valid reason to buy non-stock injectors unless you are changing the pressure regulator and ECM/PCM PROM as well (or reprogramming the EEPROM in the later model C4s).

Cold Air Induction Systems

Lastly, cold air induction systems are not going to accomplish a great deal in an L98 engine although it does help slightly in an LT1 or LT4. The theory behind these cold air induction add ons is you get a 1% increase in horse power for each 10 degrees you lower the temperature of the incoming air. (That is why you sometimes see drag racers packing the top of their intakes with ice before a run.  They are trying to cool the air and make it handling a bigger fuel/air charge because it is denser).

A cold air induction system grabs the air from a point outside (or nearly outside) the area immediately in front of the radiator.

The problem is, in an L98 engine, coolant is passed through the intake manifold after it has passed through the block and  in varying degrees the intake manifold is going to heat the cylinder's input air up regardless of what you do to the air before it gets to the intake.  (This is not true in the LT1 or LT4.  It goes through the intake manifold first).

Also, the throttle body has coolant passed through it to guard against the throttle body blades becoming iced up during certain temperature/moisture combinations (30 - 70 degrees F and dew points at or above 50 degrees F).   You could bypass the coolant but shouldn't unless you are never going to drive it when it could ice up. An iced up throttle body blade can stick and you do not want that to happen to you.

Bottom line is: all of this heat is not going to be overcome by lowering the intake's air temperature 10-15 degrees F.  Even if you do lower it 10 degrees, 1% of a LT-1's 300 HP is only 3 HP which can be measured on a dyno but is in the noise as far as actually feeling it.

If you are a drag racer, ice down the intake manifold before each run and you will make a difference.  For running in the stoplight Grand Prix, the cold air induction system costs a great deal of money for what it provides in the way of increased horsepower.

Judgement Calls:

Roller Rockers were introduced in the LT4 engine in 1996.   This is something that hot rodders have known about for years.  Roller Rockers allow better control of the valve train, and do not generate as much friction as stock rockers.

They are expensive however and the HP/Torque gain is measurable on a dyno but probably not something you can feel by itself.

If you change the cam to a taller variety and go to 1.6 rollers, you will both feel and measure the difference but you are talking serious money now particularly since you will also want to blueprint, port, polish and stroke the engine whilst you have it torn apart.

Going to a stroked, blue printed engine is something that will definitely turn you L98/LT1 into a screamer but you will need a fat wallet to accomplish the changes.


It has nothing to do with engine performance but you can have your favorite alignment shop set up your chassis to give you superior handling.

Tell them to set it up thusly:

Setting Front Rear
Toe 0 1/8 inch toe-in
Camber .25 degrees negative .50 degrees negative
Caster 6 degrees positive N/A


Advice offered on this website is for educational purposes only and is acted on strictly at your own risk.  If you do any or all of the things suggested here and something happens that you don't like, you were warned. 

The owner of c4vettes.com is not responsible for damages or consequences of any action you take to implement any of the information listed above.