[Neutrino]

Proper answer

Originally Posted by TatII

other turbo guys, allow me to do the honors.

the blow off valve or a compressor bypass valve is basically a pressure relieve valve. the turbo is a free wheeling device. when your boosting and you all of hte sudden close the throttle plate. the extra built up boost has no where else to go. the built up boost causes turbulance at the turbo compressor. this will cause a fluttering sound coming from the turbo called "compressor surge" when a turbo compressor surges, its not go for the turbo casue the turbo can spin up to 200,000 rpm. and all of the sudden you have backed up boost goin back tryin to stop the compressor wheel. so what you do is put a pressure relieve valve inbetween the turbo and the throttle body.

the blow off valve itself is basically a piston or a poppet valve. the piston type is a piston with a cup on the either end with a spring. the spring i used to keep the valve closed. on the top end of the valve there is a hole in it where you hook a vacuum line up to it. what the vacuum does is that it helps assist the opening of hte blow off valve cusae hte vacuum is pulling on the piston. but once you hit boost, the engine vacuum turns into engine boost, and the boost assist in keeping the piston sealed. ( lets say your boosting 10 psi, your blow off valve will have 10psi pressing down on the piston along with a stiff spring to keep it sealed )

the other style of valves is called a poppet valve. which are the greddy type S. and Apex i duel chamber. with those you basically replace the piston with a intake or exhuast valve looking thing, hooked up to a rubber diagraphm. and the way it works is identical to the piston.

the best type in my opinion is a compressor bypass valve. if your car runs on a MAF ( mass air flow meter ) when you vent off the boost. the MAF reads lets say 1000cfm ( cubic feet per minute ) of air go to into the engine, but you just vented it out like 800 cfm. the engine will run rich becuase the that 800 cfm never made it to the engine and the MAF already told the ECU to inject fuel to burn 1000cfm. this will cause poor idleing problems, and stalling problems at high boost. so what the compressoy bypass valve does is that it routes the vented boost back into the intake tract between the MAF and the turbo.

so overall, the BOV and CPV are there to improve the reliablilty of a the turbo, and to prevent turbo lag. plus it sounds really cool.

Lazy answer

Originally Posted by Neutrino

wrong forum but i'll answer quikly....

its to relieve the pressure caused by the turbo against a closed throtle body...like when you shift for example