as a blow off valve is operated by the neg. pressure drawing open the valve - or more accuratly is allowed to be pushed open by the boost pressure contained before the throttle butterfly. wouldnt it make sense to have the spring in the blow off valve rather low - requiring less vacuum to open it...thusly any time the throttle is closed the valve opens and as soon as pressure is introduced reducing the vacuum it closes and remains closed until it goes again to a state of vacuum. as stated earlier the vacuum i'm getting at idle and really anytime i take my foot off the gas is in the 20 range . because a supercharger produces pressure linearly rather than a turbo which can quickly spike a high pressure and upon shutting the butterfly would have more momentary pressure in the '' before throttle '' plumbing so it could open a rather stiff spring valve mine is only producing the pressure corresponding to the rpm and upon shutting down (even with the vacuum assist) it has a hard time opening the valve - which is causing cavitation between the blower and throttle butterfly - no where to go ! any real downside to lowering the spring tension in my blow off so it opens....after all it will slam shut at the first sight of pressure - even neg. 10 lb