Blowoff Valve

A blowoff valve (BOV) or dump valve 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 valves relieve the damaging effects of compressor "surge loading" by allowing the compressed air to vent to the atmosphere, making a distinct hissing sound, or recirculated into the intake upstream of the compressor inlet.

Read more about Blowoff Valve:  Definitions, Disadvantages, Operation