In a car that is equipped with some sort of forced induction system ,such as a turbo or a supercharger, a blow-off valve is usually availible as an aftermarket part to release excess pressure (boost) from the turbine system. Makes a very distinct noise. Like - pssssshhhhhhhhh! Or somtimes a chirping noise depending on the blow-off valve. Usually replaces a stock part usually refered to as a by-pass valve.
1. Gil: Whoah, dude! Did you hear that car go by?
Willy: Yeah, it sounded like it had a blow-off valve.
by Wilburt April 28, 2006

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