Res said:blow off valves do nothing but make noise, by releasing the pressure built up by the turbo through a valve to make noise... they also cannot be installed on a car without a turbo
Atmospheric ones make the noise, ones that plumb back in help prevent damaging the turbo and blowing pipes off etc.
Blowoff valves are used to prevent compressor surge, a phenomenon that readily occurs when lifting off the throttle of an unvented, turbocharged engine. When the throttle plate on a turbocharged engine closes, the high pressure air in the intake system is trapped by the throttle and a pressure wave is forced back into the compressor. The compressor wheel slows rapidly and may even stall, and the driver will notice a fluttering air sound. The rapid slowing or stalling stresses the turbo and imparts severe turbo lag if the driver accelerates immediately after the surge event.