What is a Blow-Off Valve / Compressor By pass Valve?
The Compressor By Pass Valve, also know as a Bow Off Valve (BOV) or Diverter Valve, helps keep the turbo spinning when the driver lets off the throttle.
Blows off Valves are fitted to the inlet side of the manifold. i.e. between the turbo & manifold or to the manifold.
Without a BOV, the throttle plate will close, creating a wall to the pressurised air whooshing out of the turbo. The air would crash into a closed plate, then the air would pile up, and eventually a wall of pressure would travel back to the turbo.
Besides being a shock to the turbo, it would basically cause the turbo to stop.
Then when you wanted to accelerate, you would have to wait for the turbo to spin up all over again. With a BOV, the turbo can free wheel for a while (like when you are shifting gear) because the pressure is relieved.Most factory valves vent this air back into the inlet track, although these days most people simply vent them to the engine bay for extra noise. However this is not always possible as some cars will refuse to idle. It is important to note that the valve must be able to hold more boost than the engine is receiving or else the valve will open unexpectedly.
Rotating the cap in the direction required as shown on the valve makes adjustment of the spring tension. Test-drive the car to ensure that when decelerating or changing gear the engine does not backfire or stall. If backfiring or stalling is noticed then check all connections made during installation, otherwise increase the spring tension
Closed Plate is sometimes referred to as a waste gate