Blow Off Valve

GlasgowRangers

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Can somebody please explain to me what the hell a blow off valve is and what the benefits are of having one? PLEASE...
 
A Diverter valve is fitted to the current VW/Audi turbo engines.
When you close the throttle the boost is returned to the vacuum side of the turbo, keeping it spinning.

A blow off valve or dump valve will vent this boost to atmosphere, creating a whoosh noise on gear change. These do not work very well on the current VW/Audi turbo engines