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911st 911st is offline
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Join Date: Jan 2004
Location: Sacramento
Posts: 7,269
Originally Posted by staffanbe View Post
Ok, got you about the idle co, thanks.

Sorry for asking so many questions, but this really sorts out many thoughts and ideas I had for years.

What puzzels me about your BOV configuration is how the BOV piston could be open during cruise since the throttle is partly open and there is (in my mind) the same pressure acting on the BOV piston from below and above. In my mind that would lead to that the piston would stay closed due to the larger area above the piston + the spring tension.

At cruse there is actually vacuum in the intake manifold between the throttle plate and the intake valve. This is the section the small hose that connects to the top of the BOV is hooked to. Thus, at curse you have a vacuum trying to pull the larger surface area piston up (valve open) , and you have positive air pressure made in the pressurized / intercooler section pushing up on the end of the small position.

When the throttle plate is opened, air from the intercooler section rushes into the manifold area past the throttle plate and the vacuum starts to go away instantly and depending on the throttle angel, may become equalized to the same level of pressure.


Look at it this way. At cruse, there as about 6 lbs of vacunm force pulling up on the internal BOV piston and say 7lbs of force (.5 bar) pushing up on the piston. The is also a small spring trying to pushing the piston closed.

Under 1 bar boost there will be about 45 lbs of pressure pushing down on the piston (3 times larger surfice area), plus some added pressure from the internal BOV spring, and about 10 lbs of boost pressure pushing up on it (less than 1 sq inch of surface area) .

What else is good about doing it this way is the internal BOV piston is not longer side loaded and no longer has much potental to bind and alow the turbo shaft to snap when the TP snaps closed at 6800rpm between shifts.

Hope this helps some.

Old 03-19-2009, 12:31 PM
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