When in the rpm band do you have boost? Depending on pulley size and such this could be right.

 

4000 and my pulley size is 3.15 inch I might go 2.85

Sent from AutoGuide.com Free App

 

supercharger boost is dependent on throttle position, and in 99.9% of IS300 setups, the s/c is centrifugal and the boost goes up with RPM in a linear fashion. In an LMS setup where it max's at 6.5psi, then you should see around 1psi at WOT 1,000 RPM, and increase in about 1psi per 1000RPM at WOT.

Anytime your feather the throttle, your closing the throttle plate and the perceived boost at the manifold will be lowered, or even diverted completely by a BOV/bypass valve.

If your only getting about 1psi of boost at 4K RPM WOT, you may have the wrong pulley size on, or your blower's health is really bad (bent impeller etc), or I've seen increasing exhaust efficiency lower boost levels as now the car breaths better, and has less resistance to the s/c's fixed flow rate.

 

Check for boost leaks too. Easy to make a tester if you dont have one.

 

I don't have any experience with supercharger setups other than OEM units, but from my understanding it shouldn't have any effect on idle vaccuum. Manifold vaccuum is created when the throttle plate is closed and the downward motion of the pistons sucks all the air volume out of the plenum. The idle air control valve lets just enough air bypass the throttle plate to keep the engine idling. If you have a vaccuum leak this would effect the readings you see on your boost gauge. It would also create a boost leak when your manifold crosses over from vaccuum to positive pressure. Like I said, I don't have a lot of experience with supercharger setups or I might have misunderstood the question, but I don't think it should be effecting your vaccuum.