Different superchargers will have different compressor maps and air flow at any given supercharger RPM. The aim is to match the engine RPM to the best supercharger rpm across the entire RPM range. Some companies may choose to lower the engine RPM but increase the supercharger rpm by use of gearing (pulley size). The desired effect will give a different shape of power output with respect to RPM.
Take a Vortech V3Si and match 52000rpm (max recommended limit) to 8300rpm on the E9x M3. You will get a given power output through the rpm band.
Now, reduce the pulley size. You will make more boost (and very most likely more power!) through the rpm range but the supercharger will be over spun if the 8300rpm limit of the engine is maintained. So the engine rpm is lowered and the effect will be more power output through the rpm range but with a lower rpm limit due to increased boost levels earlier.
There are multiple reasons why RPM limits can be changed on an engine. All depends what the designers philosophy is and what he thinks the end user would like.
Some end users demand for more low end torque/HP. A supercharger kit changed in the example above is ideal for them as long as the engine can make use of the increased boost at the lower RPM's. In most cases most engines can including the S65.
On the subject of pushing to the limits. Some superchargers are designed to be run at the maximum engine RPM. The run most efficiently at this point as they go through the most desirable compressor map.
Running a supercharger outside of it's efficiency range by over speeding it is a recipe for disaster however! Not recommended. Then you need a bigger blower!
Back on topic. Nice thread, thanks for the dyno graphs with conditions!
Uncorrected graphs would top if off real nice too.
Last edited by Sal@Evolve; 09-11-2012 at 11:19 AM.