Blow off valve - infor -

[Plex]

What is a blow off valve ?

When your turbocharged car is on boost, the entire intake system is filled with pressurised air; from the turbo compressor, through the throttle body and inlet manifold and into the combustion chambers. When the throttle is closed, this pressured air can no longer enter the engine. The only path for it is to try to flow back the way it came, through the turbo compressor the wrong way. This creates a fluttering noise on the blades of the still-spinning turbo compressor, a noise that is unwanted in a nice new turbo car, though it must be said, one that is actually quite popular amongst modified-car enthusiasts.

It is also claimed that the load placed on the turbocharger from this pressurised air flowing through it the wrong way can cause premature turbo wear or damage.

A blow-off valve (also called a compressor bypass valve or diverter valve) is a valve that is placed between the turbo compressor and the throttle, generally a piston type valve, that bypasses this pressurised air on a closed throttle to prevent both the fluttering noise and any potential damage.The air is either plumbed back into the turbo inlet for silent operation, or to the atmosphere to make the signature blow-off valve whoosh.

How a blow off valve works

A blow-off valve is vacuum/pressure actuated piston-type valve. It uses vacuum/pressure signals to tell the piston when to open and close.

At idle there is engine vacuum on the top of the BOV piston trying to suck it open, and no vacuum or pressure on the bottom of the piston. Since a vent-to-atmosphere BOV needs to be shut at idle to avoid air being drawn in through it, there is a spring inside a BOV with the job of holding the piston closed. The spring preload adjustment is to allow for differences in engine vacuum from car to car, and variations in atmospheric pressure at different elevations.

On airflow metered cars the air drawn in through an open vent-to-atmosphere BOV at idle would confuse the ECU and cause over-fuelling and stalling and in any case, the air drawn in is unfiltered.

Under cruise conditions (off boost) the BOV is experiencing similar conditions to when the car is at idle, but there is less vacuum present on top of the piston because the throttle is partly open. If the BOV spring has been adjusted to keep the piston closed at idle, it will also be closed at cruise.

On boost there is boost pressure on both top and bottom of the BOV, the forces from which counteract each other, so the BOV remains closed.

Immediately after the throttle is closed under boost there is vacuum on the top of the piston and boost pressure on the bottom of the piston, which together, quickly open the BOV to release the pressure. When the pressure has been released, the BOV closes.

More infor see the link below

http://www.gofastbits.com.au/index.php?parentid=faqs&option=faqs&sub_option=faqs_bovs

[solo]

Excellent info PLex. A lot of people think that just adding a BOV to the intake setup is all that's needed, they usually don't know that the vaccum line must be attached for proper operation.
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Ghetto Breed.

[LionTamer]

Also note u cannot put a blow off valve on a air flow metered turbo engine.....what u need is a recirculating valve.