[darkpuppet]

The boost drop is commonly caused by increased engine efficiency.

Remember, boost is all the air sitting in the manifold that the engine CAN'T get into the cylinders.

So (especially) with superchargers, you put in an exhaust, or make the intake more efficient and the boost will go down. This is because superchargers are engine RPM reliant, and often make max boost at max RPM. You create an engine that can take in more air, and the boost will go down -- but the engine still consumes more air than when you had more boost.

Turbos are powered by the exhaust that comes out, not engine RPM, so as long as you have enough exhaust pressure to make x lbs of boost, the car's boost won't fall. (but you'll see more air going in via your MAF if you datalog)...