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Wheel h.p. -> crank h.p. conversion???

I have been giving some though to the way many people estimate crank hp
after doing mods and measuring wheel hp on a dyno and must admit that
I'm puzzled.  Usually, they make a baseline run before the mod and look
at the peak figure and compare it to the claimed peak crank h.p. figure
from the manufacturer.  Then the come up with a "correction factor" to
account for drivetrain losses and use it across the board to estimate
new horsepower figures after the mods.

Now here is what I don't understand:  a stock car may lose 20% of the
power to drivetrain losses.  Now a tuner does some modifications and the
wheel hp is up 15 hp.  They then assume that the car is still loosing
20% to drivetrain losses and come up with a claimed power improvement of
19 hp (up from the measured 15 hp number).

Now my question: why would drivetrain losses increase as a function of
engine power?  Aren't drivetrain losses a function of friction more than
anything else?  In that case they should be almost constant and a
measured 15 hp increase at the wheels would be a 15 hp improvement at
the crank (not 19 hp).  Any takers on this one?

Luis Marques
'87 4kcsq