Blow Off Valves
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
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