Blowoff valve

Blowoff valves are used to reduce pressure in the intake system as the throttle is closed, thus preventing compressor surge.

[1] A key function of blowoff valves is to prevent compressor surge, a phenomenon that would otherwise occur as the throttle is closed in a turbocharged engine.

[1][3] This rapid cycle of pressurisation and depressurisation repeats until the compressor has slowed sufficiently or the throttle is re-opened.

A vacuum hose connected to the intake manifold downstream of the throttle provides a reference pressure for the blowoff valve.

Effects of this rich running can include fouling the spark plugs, damaging the catalytic converter, stalling and hesitation when the throttle is opened.

A piston-type blowoff valve; this example releases air into the intake manifold (via the side hose on the left, the small hose on the top provides a reference pressure for the blowoff valve from the inlet manifold).