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Re: Wheel h.p. -> crank h.p. conversion???
Luis Marques wrote:
> 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?
You have two affects going here. One is contant Hp loss (AC, water pump,
PS pump, alt, etc all run at idle, cruise, and WOT), the other is
variable (drivetrain) loss, which is proportional to the drive power
being transmitted. The "constant" loss will remain the same after the
mod, but the proportional part will increase along with the power. If a
gearset is only 95% efficient, then only 95% of the "in" comes "out."
The other 5% is noise, heat, (hopefully not) sparks, etc. So, using 100
Hp, 746 Watts per Hp, and assuming 95% efficiency for the transaxle, the
tranny heats/radiates to the tune of 3.8 kW. Sizzle! That's why _real_
race cars have cooling systems (pumps, rads/heat ex, etc) for the gear
BTW, the chromed diff covers one sees for GM-type rear ends basically
_ruin_ any radiation heat transfer. Shiny generally = bad for radiation.
Radiation can be a _huge_ effect at elevated temps, so it's a good thing
these covers have fins to "help" with convection.