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.

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.

Handling...

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.