This tip comes from the SSCOR home page. It
was orignally developed by Drake Daum as an improvement for his 1.6 liter
supercharger. Later, Jackson Racing incorporated his idea into its 1.6 and
1.8 kits. If you have a newer kit for the 90-93 and 94-97 miatas, it probably
shipped with this modification. Owners of the older 1.8 kits sold prior
to 1999 like me have tried it and think it's a nice inexpensive upgrade.
If you don't already have this modification, here's how it works.
When the engine is idling, it can draw air in thru the ISC valve located
under the dummy throttle body to adjust the idle for different loads.
Under boost, pressurized air can leak out thru the ISC valve and back
around to the front of the supercharger intake. Drake realized out that
a check valve would stop this leakage and potentially improve the boost.
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| NAPA # 3-29000 | with 1/2" NPT nipple |
I bought an air valve, part #3-29000 from the local NAPA auto parts store chain. The output end was threaded for a 1/2" pipe thread and it needed a short piece of iron nipple so that hoses could be put on both ends. There may be a #2-29000 part which has hose fittings on both ends, but the store I visited didn't have it.
![[ISC Hose]](jpg/isc_hose.jpg)
Locate the ISC hose on your car. ISC stands for Idle Speed Control. If someone else installed your supercharger and you don't know where to find the ISC hose, the above picture shows it just under your crossover tube on the intake manifold. It runs around to the cast aluminum elbow that's by the brake cylinder. Find a convenient place to cut it and splice in the valve. When you install the valve, point it so that it flows toward the intake manifold. You can blow into the valve and figure out which way it flows. You would be able to blow toward the engine. When the intake is pressurized, air cannot flow away from the engine intake.
Here is the valve installed on my car.
Total cost: $ 16.50 for the valve. I had the hose clamps and iron pipe sitting around.
Results:
It increases the boost on my Sebring supercharger between .5 and 1 psi.
That translates to around 5 more HP. I'll take it.
December 31, 1999 -Harry Sue
Update 11-11-2003
Last year, a friend installed a supercharger on his 1997 miata and found it would make a load moaning sound. When we looked at it, someone realized it was resonance coming out of the check valve. Removing the valve got rid of the noise. Now my car has never done this. Perhaps the NAPA part doesn't do this?