I am surprised not more people has noticed this, Injectors, fuel pumps, spark plug gaps etc don't matter when it comes down to the boost of a supercharger. turbos yes but not superchargers. the more the supercharger spins the more air it displaces and the more boost it makes. for the problem you have is the pulley is to small or the supercharger is to small for the motor. basically you should find at about 3000 rpm at light throttle the boost should be higher than at flat out. This indicates that your motor is sucking more air than the supercharger can provide to maintain higher boost. the next thing is air leaks especially the boost bypass valve used for at idle, if its plumbed in wrong you can loose boost there as it bypasses. Also Has it got a standard cam? Little known fact cams for super charged cars and N/A cars are very different, They change the valve timing and over lap to change your dynamic compression threw out the rev range. The result of the wrong cam shaft will cause a loss of boost because the valve over lap and you sending the pressurized intake manifold pressure straight out the exhaust. Commodore V6s have this problem because they run lots of valve overlap. Its something many people over look when supercharging an engine.
