21 in. Hg, not psi, vacuum is good for idling. means your car is running strong, no leaks. mine sits around 14, but my car runs like crap and i beat the living mess out of it.

vacuum is supposed to happen whenever youre not boosting. it shows how much the car is pulling into the engine. on an n/a car youd actually see the vacuum go up when you got harder on the throttle.