Compressor Bypass Valve
We had noticed that when we tried to get boost above 20 psi, the boot level would fluctuate and drop quickly. From doing a little research we learned that the stock EVO VIII compressor bypass valve starts to blow open at around the 20psi point, dumping boost back into the compressor inlet. The solution was to replace the stock valve with one from an EVO IX or get an aftermarket bypass/blow off valve.
Like all mass airflow meter cars, the EVO works better with a recirculating compressor bypass valve instead of an open to the atmosphere blowoff valve. With a MAF equipped motor, dumping air that has passed through the airflow meter will cause the car to momentarily run super rich causing stumbles and stalls. For better driveabilty the bypassed air should be returned to the inlet tract before the compressor but after the MAF.
|The billet Forge bypass valve is a pretty piece; the springs and shims are to adjust its opening pressure for the desired boot pressure run.|
Matrix international came to our rescue with one of their 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 recirculating valve is a well-made, high quality piece. In addition, the Forge valve is completely tunable to the desired boost level by changing the 4 supplied springs.
In addition to the springs the valve can be fine tuned by adjusting the preload with supplied shims. Normally one shim would be used to add the pre-load needed to bridge the range between two of the colors (ex. Yellow to blue) however more than one shim would add enough pre-load to bring a particular spring to the next pressure rating (ex. Green w/ two shims=Yellow) so we do not recommend using more than one shim and have not seen an instance where this would be required. We ended up using the blue spring and one adjusting shim and noticed crisper throttle response and better spool recovery and a more stable boost when we ran above 20 psi.
In our 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” 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 didn’t dyno these parts yet but wait until the next installment when we do some more tuning.
|Vishnu’s race pipe is used in place of the cat when dyno tuning on race gas and for track driving.|