From Wikipedia, the free encyclopedia. A blowoff valve (BOV), dump valve or compressor bypass valve (CBV) is a pressure release system present in most turbocharged engines. Its purpose is to prevent compressor surge, and reduce wear on the turbocharger and engine. blowoff valve is connected by a vacuum hose to the intake manifold after the throttle plate. When the throttle is closed, the relative manifold pressure drops below atmospheric pressure and the resulting pressure differential operates the blowoff valve's piston.