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Most production turbocharged engines are fitted with a BOV, to get rid of excessive turbo boost pressure when the throttle is closed. The valve itself is typically a shiny three- way affair fitted between an intercooler and the air inlet hose to the turbocharger. You may also find a small hose leading from the top of the valve to the inlet plenim on the engine. Inside the valve is a rubber diaphragm and a steel spring.

The BOV has two positions.

When the throttle is open, pressure below the diaphragm is equal to the pressure above that comes through that small hose as mentioned above . As the pressures are equal, the spring holds the diaphragm in place, stopping pressure escaping.

A closed throttle,means reduced pressure to top of the diagragm, moving it away from its seating, allowing the excess pressure to escape. Re-opening the throttle sets the diaphragm back to square one.

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