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: : Lets say a BOV spring/adjustment can hold 20psi of boost before bleeding off. When the throttle body butterfly closes, pressure raises, the air forces the BOV open. BOV closes when the spring force is greater than the force air pushes. I don't know at what point BOV closes, I'd guess if the spring can hold 20psi under boosting, it probably closes just after pressure in the pipes drops below that. I cant imagine the pressure in the pipe is lower than atmosphere pressure to suck in air. : You're correct in thinking that the additional pressure in the intake tract (up to the TB) helps to open the BOV; however, a large majority of the force helping to open the BOV also comes from the vacuum on the other side of the diaphragm. Additionally, the positive pressure from the same vacuum line helps keep the BOV closed while under boost (it helps the spring keep it shut). : So, taken this into consideration, here is the sequence for the opening and closing of the BOV... : 1) Under boost, the pressure on both sides of the diaphragm are both equally positive, but the additional force of the spring, keeps the BOV shut. : 2) TB closes, increases pressure in intake tract up to TBs. This pressure acts to push against the diaphragm and open the BOV. (Just as you stated). : 3) When the TB closes, the other side of the diaphragm goes into vacuum very quickly (since no boost is entering the intake manifold). This vacuum acts to PULL the diaphragm and open the BOV. : 4) The combination of 2 AND 3 open the BOV, as they both contribute to increasing the pressure differential across the diaphragm and act against the force of the spring. : 5) Now, on the spring side of the BOV, the vacuum remains. Thus, the point that the BOV closes is when the force of the spring overcomes the combined force of the vacuum and boost on the other side. : OK, so that said, the point at which the BOV closes depends a lot on how tightly the spring is wound in the BOV (tighter = higher spring constant = more force). Additionally, when it closes, it doesn't close instantaneously, although we would like to think it does. Thus, it is quite possible that the BOV is not completely closed as the diaphragm is returning to its seat... and at that same instant, the boost pressure may have completely been replaced with vacuum... thus it may suck in some air. Again, probably not significant... which is why I sarcastically said 0.01 microseconds. Probably not significant at all, on the other hand, manufacturers DO make filters for BOV's... I wonder why? :)
: Remember this is all done when TB is close. So the motor itself doesn't suck in air.
: That's not entirely correct... even when the TB is closed, the motor is STILL sucking in air... otherwise the motor would die. Air comes through the IAA when the TB is closed... which means there is vacuum in the entire intake tract (which is why there is vacuum at idle).
.jpg) ICQ # 14337317
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