Air Bypass Valve: Description and Operation

Turbo By-Pass Valve:

The by-pass valve, mounted to front of cylinder head, senses the intake manifold pressure and prevents surging when the throttle valve suddenly closes at high engine RPM (high turbo pressure).
When the throttle valve closes, the manifold pressure drops and the by-pass valve opens. This directs charge air from the intake manifold back to the turbo charger inlet. As a result, it dampens the pressure shock waves which would otherwise allow the air mass meter to register an oscillating intake air flow. This would cause the engine to surge.