Originally Posted by Bill @ BRP,Feb 7 2006, 10:13 PM

Mostly correct, but the last part about the "roots" losing boost at high rpm has not been my experience in helping develop BRP supercharger kits. I have done a lot of boost datalogging on both the Miatas and now the S2000 setups and they all start off at lower boost at low rpm and make more boost at high rpm. However, do not confuse higher boost necessarily meaning a higher boost in power. It is true that with a positive displacement supercharger a fairly constant air volume is moved per rev, however this is not the only thing that determines the boost that is measured in the manifold. The more restrictive the engine is downstream, the higher the boost that is seen in the manifold. The volumetric efficiency of the MP series of superchargers is improved from the last generation of the M series.... particularly at higher supercharger rpm and boost levels. The volumetric efficiency actually increases with rpm. For example, at 10psi the MP62 has about an 82% VE at 10000rpm (supercharger). By 16000rpm at 10psi the MP62 has a 95% VE. At the same time, the engine's VE generally decreases with rpm. This means that with a given pulley ratio for the supercharger, the ratio of air trying to exit the supercharger vs. the amount that the engine can consume shifts. This ratio is part of what determines the pressure ratio in the intake manifold and thus the gauge pressure in the intake manifold. Having more boost is actually not a good thing as it really indicates restriction to flow. Of course you can't always have 0psi and huge amounts of flow , but with a given supercharger pulley ratio seeing less boost at the intake manifold after doing things like headwork or exhaust changes generally reflects itself in higher output on the dyno.

On the dyno measuring boost I thought it was pretty interesting how the boost dropped about 1psi when the engine hit VTEC and torque made a nice increase. Basically the boost started at 5psi at low rpm rose to about 6psi right before VTEC, dropped to 5psi after VTEC and then climbed back up to 6psi at the rev limiter. As can be seen on the dyno graph, the torque remained flat and didn't drop off much at high rpm. I think this is also a testament to the efficient S2000 engine. In contrast, on our Miatas the boost generally rises about 3psi from low rpm to redline. The Miata engine of course doesn't breath as well as the S2000 engine at high rpm .

Bill