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Date: Fri, 18 Mar 2005 10:21:09 -0500
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From: "Gary Casey" <glcasey@adelphia.net>
Subject:  Re: superchargers
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<<The point is with so many variables, to look to "racing" as an
  endorsement of turbo supercharging vs. supercharging -- or vice versa --
  as an affirmation of either system is of very debatable value. ----->>

I'll certainly go along with that.  The way I see one principle advantage of
the mechanical-drive supercharger is that it leaves the exhaust system
alone, a huge advantage if you have to spend your own money to come up with
a unique exhaust system to feed the turbocharger.  However, the turbocharger
has a convenient way (wastegate) to modulate compressor speed to meet the
requirements, a huge advantage.  Even though the turbo increases the back
pressure a large portion of the required power is truly free as it is able
to harness the blow-down impulse in the exhaust, a big reason to use dual
turbos in a 6-cylinder engine.  It can be argued that the turbo reduces
stress in the engine as the power comes from increased back pressure during
the exhaust stroke, actually lowering the mechanical loading (a bit of a
stretch, I'll admit), where the power has to be put into the crank and then
taken back out in the case of the mechanically-driven supercharger.  The
turbocharger has an inherent advantage in compressor efficiency since the
design rpm can fit the requirements, allowing a smaller diameter rotor and
higher rpm.  because of the reduced leakage paths this is at least
theoretically an advantage.  The superchargers rpm is usually limited by the
drive mechanism.  The lack of this mechanism generally means that a
turbocharger is good beyond the TBO of the engine, another big advantage.
Bottom line is that if I could come up with the exhaust plumbing the turbo
would be the desired choice.  If a mechanically-driven supercharger had a
demonstrated endurance of 2,000 hours at maximum rpm that would certainly
tilt the equation that way.

I ran out of money and time so put neither on mine.

Gary Casey