A Blow off valve is used to keep a system from overpressurizing.
It is often used incorrectly & refered to instead of using the correct terminology for a "Bypass Valve" when talking about the valve used to vent the boost between the turbo and the throttle plate when the throttle plate closes.
You know. The whoosh sound when shifting. :p That's a BYPASS VALVE, not a blow off valve.
Forced Induction ettiquete police 101
(Below stolen from of all places; my website
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Bypass Valve = is used to stop compressor surge from happening. See compressor surge. The bypass valve in turn allows air to be rerouted and sent back through the turbo inlet between shifts allowing the turbo to keep spinning so spool up time will be quicker. Please understand that the HKS racing bypass valve does not reroute air what it does is allow the wheel to spin freely once the valve relieves all the excess pressure trapped within the intake between shifts. A Bypass valve will also extend the life of the turbo.
A Blow-Off Valve is NOT the same as a Bypass Valve. A Blow-Off valve is a safety device that opens at a set psi level to prevent over boosting the motor
Compressor Surge = when the throttle slams shut and air is caught between the turbo and the throttle body. Air is then forced backwards toward the turbo causing the unneeded stress to occur on the turbo compressor wheel and shaft. This can cause after extended periods of time turbo failure. The use of a Bypass valve will end this event from happening.
