The BOV valve is spring loaded. It opens anytime there is more than a -7.7 psi (15.6 in hg vacuum) difference between the upper side of the diaphgram and the lower (connected via the vent in the base of the BOV to UICP). So at idle when you have 18-20 in hg vacuum in the intake and atm pressure in the intake tract the manifold vacuum pulls against the spring pressure and opens the valve. With boost pressure in the intake tract you don't need much vacuum to open the valve since the boot pressure is pushing against the diaphgram on the lower side against the spring. Under WOT boost the pressure on both sides should be equal and the just spring holds the valve closed.

Steve

 

Is that why I cant build boost by revving? So would getting an aftermarket bov let me build boost by just giving it a rev?

 

Isn't it a vacuum thing? If there's vacuum in the manifold(like at idle) the valve will be open? That's the way I undrstood it. I may be wrong, though.

 

just what i was looking for, thanks guys. I knew it was sposed to be open, but i was looking for the actual mechanics of it.

 

The 1g bov is designed to vent back into the intake. When you are under boost it gets pulled down by the pressure made in the intercooler piping. and when you let off the gas it opens back up to release the air back into the intake pipe. So, thats why it is cracked at idle because there is no force holding is closed.

 

because the turbo is still spinning and the throttle plate is closed. If the cbv/bov is not open you will get boost building up in the ic pipes and will have nowhere to go except backwards through the tubo, thus creating compressor surge.