[DeTomaso] Check this out

[Daniel C Jones]

> Is that a common chacteristic of the type of supercharger you have?
> I assume it means the cyliders were about equally filled over the
> range of RPM with nearly constant torque?

That's an inherent characteristic of a positive displacement supercharger.
There are various types of superchargers, some are positive displacement
(Roots and Lysholm intermeshing screw) and some are not (axial and
centrifugal flow).  Both types of blowers produce flow as a function of
impellor speed.  The primary difference is how the boost varies with RPM.
A positive displacement supercharger sweeps out a fixed amount of flow
with each revolution of the impellor, so flow varies linearly with impellor
RPM.  Double the speed and you get (roughly) double the flow.  Axial (Latham)
and centrifugal (turbos, Vortech, Paxton, etc.) superchargers produce flow
that is proportional to the square of the impellor speed.  A centrifugal
compressor accelerates a compressible fluid (gas or air) to a high speed
then converts the kinetic energy of the fast moving gas into potential
energy (higher pressure) as it slows it back down in the diffuser.  Kinetic
energy is, of course, proportional to the square of velocity.  Double the
speed and you'll get (roughly) four times the flow.  That may seem like a
good thing (more flow = more boost) but it's not.  The non-linear flow curve
limits the working range of a centrifugal supercharger.  Gear one to produce
meaningful boost at 2000 RPM and you'll over-boost the engine at 6000 RPM,
unless the static compression ratio is very low.  The linear output of
positive displacement blowers allows them to operate over a wider RPM
range without over-boosting.  Turbos can get around this to a certain extent
by bleeding off boost above a certain level but there are still limitations.

Dan Jones