|GREEN ||7-14PSI |
|YELLOW ||15-19PSI |
|BLUE ||20-23PSI |
|RED ||24-28PSI |
Compressor Bypass Valve
We had noticed that when we tried to get boost above 20 psi, the boost pressure would fluctuate and drop quickly. After doing a little research we learned the stock compressor bypass valve starts to blow open at around the 20psi point, dumping boost back into the compressor inlet.
Like all mass airflow (MAF) meter engines, the EVO's 4G63 works better with a re-circulating compressor bypass valve instead of an open-to-the-atmosphere blow-off valve. With a MAF-equipped engine, dumping air that has passed through the airflow meter will cause the car to momentarily run super rich resulting in stumbles and stalls. For better performance the bypassed air should be returned to the inlet track before the compressor but after the MAF.
Matrix International came to our rescue with one of its EVO drop-in Forge compressor bypass valves. Designed as a direct replacement for the factory bypass valve and manufactured from billet aluminum, the Forge EVO15 re-circulating valve is a well-made, high quality piece. In addition, the Forge valve is completely tunable to the desired boost level by swapping in one of the four supplied springs.
In addition to the springs the valve can be fine tuned by adjusting the preload with shims. Normally one shim would be used to add the pre-load needed to bridge the range between two of the colors (e.g. Yellow to Blue) however more than one shim would add enough pre-load to bring a particular spring to the next pressure rating (e.g. Green w/ two shims = Yellow). We ended up using the blue spring and one adjusting shim and noticed crisper throttle response, better spool recovery and a more stable boost when we ran above 20 psi.
In anticipation of another tuning session running real leaded race gas, we installed Vishnu's race-only pipe to bypass the cat for tuning and when running on the track. The test pipe is a well-made 3-inch diameter stainless piece. Between the test pipe and the Forge bypass valve we noticed a surprising gain in throttle response and seat of the pants power. We are waiting to dyno these parts until the next installment when we do some tuning.