Why do I need a bypass valve on my car, 
and how does it work?


FROM PES: 

A bypass valve is placed in the boost pipe after the turbo and before the throttle body. A blow-off valve releases pressure between shifts and when acceleration is cut short. When the throttle plate is closed pressurized air back-tracks from the plate and heads toward the compressor wheel in the turbo, this is commonly referred to as compressor surge. When the charge air impacts the blades of the compressor, which are spinning at enormous speed, the compressor can be stalled or even back-spinned. Undue wear of the thrust bearings in the turbo can be experienced. The PES bypass valve avoids this situation by venting the pressurized air prior to impact. The BOV also allows the turbo to continue spinning freely, thus greatly reducing turbo lag between shifts. The plastic factory part with its rubber diaphragm can leak boost at higher than stock boost levels (this would cause a loss of power) and is notorious for failures. The PES bypass valve has been designed for cars running from 10psi to 35psi.