A blowoff valve is a vacuum operated valve that is located after a turbocharger on an internal combustion engine, but before the throttle body butterfly valve and intake manifold. Its use is to vent extra pressure being developed by the turbocharger when the throttle is closed, such as during a shift. During a shift in a car with a manual transmission, the throttle plate is closed. The pressure from the turbocharger has nowhere to escape to. The pressure pressing against the throttle plate will produce a pressure wave in the opposite direction, causing possible damage to the turbocharger turbines and may also slow or even stop the turbine, thus causing turbo lag when the throttle is pressed again.
A blowoff valve is connected by a vacuum hose to the intake manifold after the throttle plate. When the throttle is closed, a strong vacuum develops in the intake manifold after the throttle plate and "sucks" the blowoff valve open. The excess pressure from the turbocharger is vented into the atmosphere or into the intake. Blowoff valves have a very distinguised "psshh" sound that is desired by many who own turbocharged cars. Some blowoff valves are marketed with trumpet shaped exits that amplify the "psshh" sound. It is also sometimes called a "dump valve", "bypass valve" or "hooter valve".
Blowoff valves are generally not required on automatic transmission vehicles. Automatic transmission vehicles shift without closing the throttle but are still fitted by a lot of manufacturers, this is so the is able to provide boost sooner if if the throttle is only releasd for a second.
