A blow-off valve is a pressure release system found in turbo-charged engines. Its purpose is to guard against compressor surge and reduce wear on the engine. The BOV relieves pressure that builds up in the intake system when the throttle is closed quickly (between shifts, for example). This valve is an extremely important part of any turbo system, and allows quick boost response and greatly increases the life of a turbocharger. Original equipment valves function well, however they tend to be unreliable at higher boost levels.