A blowoff valve (also known as a dump valve, bypass valve or sometimes hooter 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 produced by the turbocharger has nowhere to escape to. This excess pressure could potentially cause damage to the turbocharger's impeller and may also slow or even stop it, thus causing turbo lag when the throttle is pressed again.

a device that provides an escape for high-pressure intake air to vent back to the atmosphere when boost is not needed