BOV Tidbits
This probably should be posted in a different place, but here is some info that I have learned about BOVs.
1) I really need to upgrade mine. I have a Greddy that is "way" too small. With the 2.8" pulley and the T-trim, any time I back-off at over 2800 RPMs my MAF tube gets unequally pressurized and the MAF signal goes bonkers. Anything greater than 4000 RPM, I get compressor surge ... it will even hold open the T-body plate momentarily.
2) There are 2 vacuum ports on a BOV, one above the plunger and one below. The one above is what most of us use. Your vacuum source should be post T-body. If only the top vacuum port is connected, then you can't "blow off" boost because the more boost you make the more the plunger is pressing downward. As soon as the upper port sees less boost than the blower is making (minus the spring), it opens. If you hook up the lower port to a pre T-body source in addition to the upper port, then the spring dictates how much boost you see before it's blow off.
3) [just from conjecture] The exception to (2) is when you are using an intercooler and running really high boost, there could be enough differential between what the upper port sees and what is being produced at the base of the BOV that a weak spring could bleed off boost.