Blow Off Valves

From Nissan 350Z & 370Z Wiki

From Wikipedia

http://en.wikipedia.org/wiki/Blow_off_valve

A blowoff valve (also known as a bypass valve, compressor relief valve, or sometimes hooter valve) is a vacuum operated valve that is located in the intake tract on an internal combustion engine after a turbocharger, 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 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 recirculated into the intake upstream of the compressor inlet; when vented into the atmosphere this can cause erratic engine behavior on motors that use an air flow meter for the electronic fuel injection. Engines using a MAP (manifold absolute pressure) are not affected. Externally-vented blowoff valves have a very distinguished "psshh" sound that is desired by many who own turbocharged sports cars. Some blowoff valves are marketed with trumpet shaped exits that amplify the "psshh" sound.

Blowoff valves are generally not required on automatic transmission vehicles. Automatic transmission vehicles shift without closing the throttle, but are still fitted with blow off valves by many manufacturers so the turbo is able to provide boost sooner if the throttle is only released for a short amount of time.