A compressor bypass valve (CBV), also known as a pressure relief valve or diverter valve, is a manifold vacuum-actuated valve designed to release pressure in the intake system of a turbocharged vehicle when the throttle is lifted or closed. This air pressure is re-circulated back into the non-pressurized end of the intake (before the turbo) but after the mass airflow sensor.
A blowoff valve, (sometimes "hooter valve" or BOV) performs the same task but releases the air into the atmosphere instead of recirculating it. This type of valve is typically an aftermarket modification. The blowoff action produces a range of distinctive hissing sounds, depending on the exit design. Some blowoff valves are sold with a trumpet-shaped exit that intentionally amplifies the sound. Some turbocharged vehicle owners may purchase a blowoff valve solely for the auditory effect even when the function is not required by normal engine operation. Motor sports governed by the FIA have made it illegal to vent unmuffled blowoff valves to the atmosphere.
Blowoff valves are used to prevent compressor surge, a phenomenon that readily occurs when lifting off the throttle of an unvented, turbocharged engine. The sound produced is called turbo flutter ( the slang term ' choo-choo' is sometimes used ). 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 resulting collision of pressure waves creates an effect similar to cavitation producing the unique noise.