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Discussion Starter · #1 ·
I've never understood the idea of using a particular % (15, for example) to "convert" between crank hp and rwhp.

A 400hp car would dyno at 340, using a 15% loss. That means its drivetrain sapped 60hp.

If I mod the car to 500hp, it would dyno at 425hp(using 15%). That means the SAME drivetrain is now sapping 75hp.

At 600hp...........driveline loss is now 90hp!....etc...etc

Does this make sense to everyone? I'd think that a fixed loss per car would make more sense.

Let's discuss!
 

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Well, I'm not sure if this is the right forum, mods can move this
if they deem fit of course.

It actually IS a percent.

The losses from power measured at the crank to power meas.
at the wheels are effectively a combo of two things:

1) Frictional losses

2) Inertial losses

The magnitude of the Inertial losses are generally orders of
magnitude greater.

So we can consider "losses" to mean "inertial losses".

If and when you are talking about a dyno that measures the
horsepower under acceleration, this matters.

Double the power means double the torque means double the
acceleration, this means double the losses thru inertia.

So the inertial losses are proportional to the power output
in a given system. It's a percentage.

Jim
 

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I know that in the ventilation industry you have to figure a certain cfm% loss everytime you turn air. Its pretty much a mechanical function and the % is not affected by the volume. I'm not sure how torque/hp works.

I found this thread . It may help. But most of it is over my head. :-?
 

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13% is the loss that I measured, ASSUMING that my engine produces 405 HP at the flywheel.
 

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For large helical gearboxes we figure 2% per mesh if mounted on anti-friction bearings. It's based on average sliding friction between the gear teeth. Your mileage may vary.
 

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Frictional resistance is proportional to the force with which the two moving objects press together. This means that changes in frictional losses in the driveline are proportional to the changes in the force being translated through the driveline. This means that when you make more HP, you lose more HP. It's one of the cruelest twists of physics.

Kind of like death and taxes. There's no escaping it.
 
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