My engine runs ~ 8.9 lbs of vacuum at idle and a little over ~12 lbs on hard deceleration. If you run a 9 lb spring or less you will likely be sucking in dirty unfiltered air at idle. If you are at sea level, more or less, you should probably run an 11 lb spring so your BOV only opens on quick deceleration to reduce compressor surge and reduce the sudden increase in boost pressure on the throttle body plate and compressor wheel.