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Yargk Yargk is offline
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Join Date: Feb 2000
Location: SF bay area, CA
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Originally Posted by WERK I View Post

P is power, Τ is torque, and ω is rotations per minute. The constant 5252 comes from (33,000 ft·lbf/min)/(2π rad./rev.).

Do either one of you two see how power(P) is calculated? And what is used to calculate P, besides the constant 5252 and RPM?

Horsepower is a measure of Energy like BTU's, Watts. It(HP) is a measure of Force(in this case Torque) over a Time period.
Torque is a measure of force like foot-lbs, Newton-meters. It is the Force of a moment.

Yargk, tell me you can't calculate torque from an HP curve from the formula I've just given.

Yes, I can calculate crank torque from crank hp and crank rpm. Duh.

What I'm saying is that it's WHEEL torque that matters and I get WHEEL torque with just two pieces of information.

1. current crank hp
2. road speed

Just think about that for a moment.

I understand why one would think crank torque matters so much because of f=ma. However, the f is WHEEL torque divided by wheel radius. If you want f from crank torque you NEED to know gearing. Not true with crank hp.


1. we can all agree that wheel torque is what matters

2. if you can calculate wheel torque with only road speed and crank hp, and NOT road speed and crank torque then it is crank hp that gives you a better idea of wheel torque and therefore acceleration.

If you want to compare the acceleration of two cars you want to fix road speed because you want to compare if they BOTH start at 10 mph, or 20 mph, not if one car starts at 20 mph and the other 10 mph. Then with that fixed, it's only current hp and weight....

BTW if our high school physics is failing, then so are our masters degrees in physics ('hint,' 'hint')

Edit: I thought about it more and I think the problem here is maybe you think we are trying to say that crank torque doesn't matter or something strange like that. You can get power from torque and rpm, so of course it matters. It's just that if you consider hp you already have a quantity that gives you the information you want, if you use crank torque, you need to know rpm or other parameters in order to get the information you want, acceleration.
1979 930
2007 WRX wagon

Last edited by Yargk; 03-01-2010 at 03:21 PM..
Old 03-01-2010, 11:50 AM
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