Five Cheap Mods That Add Up to 70 HP to a C4

1)
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.
2) Another 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.
At the same time, either open up the stock air cleaner lid with a pair of
tin snips or purchase the K&N package. 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.
3) 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.
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.
4) 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.
5) 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.