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 blow-off-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.
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.
Only available for car with turbo, those without turbo yet got Pss
sound is either air-charger or they buy the fake sound
equipment.
