Re: Undersize Pulleys

[Robert T Chafin <rtchafin@xxxxxxxxxxxxx>]

A question was raised as to how undersize pulleys can increase performance,
and how these pullies might do so on the road but perhaps not on the dyno.
The answer is very simple: undersize pulleys allow certain accessories to
be driven more slowly;  when the engine is accelerating, these accessories
are then accelerated more slowly.  (I'm intentionally making this simple so
that certain un-named digest denizens can keep up.)  Now, when the car is
accelerating, while most of the available horsepower is used to increase
the speed of the car, some of the horspower is consumed in increasing the
angular velocity (read "RPM") of the various rotating components.  This is
why lighter flywheels and lighter wheels will increase performance,  and it
is also why slower-accelerating accessories will likewise increase
performance.  This kind of increase is best demonstrated on the "1/4-mile
dyno",  or by the kind of road-testing that the legendary Mark Kibort has
described.  You can expect measurable gains if your car has fast-turning
steel pulleys and an engine-driven fan,  and lesser gains if your pulleys
are already aluminum, and the cooling fan is electrically-driven.  But the
dyno will usually only give steady-state conditions, the accessories are
not accelerating, and so little or not increase in horsepower is seen. And
you're wrong Mark, the alternator does not put out the same current at all
speeds: that's why alternators have performance curves, showing maximum
output vs RPM.  They're much closer to being constant output devices than
generators,  but the output DOES increase with increasing speed.
Hope this has been helpful.
Robert

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