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post #9 of (permalink) Old 02-20-2006
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Originally Posted by fast66
No, there is NO RWHP increase by changing rear gears. This is exactly the same as saying "my car has more HP in first gear than in second gear"
This is NOT the case.
If you put you car on a dyno, and run it in first you will get a certain HP reading. Then run it in second you will get the SAME reading.
Now put your car on the dyno with a 2.8 rear gear and take your HP reading, then swap the rear gear for a 3.75, your HP will be exactly the same and at exactly the same engine revs. It will show max HP at a lower road speed but it will be the same figure.
(there is potentially slightly different friction between different rear gears, but this would be so small as to be ignored)
again, you are correct, the engine hp is the same.

but mathematically, if you measured the power at the wheels, using only torque to compute hp, the mathematical illusion is that there is more hp, as you're deriving hp from measured torque.

The dyno corrects this to measure engine hp. by using the RPM and engine measurements and computing dyno acceleration.

Does this make more sense?

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