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difference is rpm...
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Posted by JimBlake [Email] (#141) [Profile/Gallery] (more from JimBlake) on Fri, 7 Mar 2003 13:48:27 Share Post by Email
In Reply to: Re: Torque vs. horse power: what's the difference?, MO, Fri, 7 Mar 2003 10:37:12
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As an engineer, I'll chime in, too.

I knew a muscle-car guy who always said "Torque is what moves the car". He was even an engineer... I told him I could make 850 lbf-ft of torque with my bare hands. I just need a real long wrench. Kinda like the high-school physics question about how much work you do when you push really hard against a stone wall all day long.

Power is what makes anything move. Torque *AT SOME RPM* is what makes power. Lutfisk has the right equation:
HP = Torque * rpm / 5252

But the point most of you aren't saying is that torque & power are each given at their respective maximum values, which don't occur at the same rpm. You usually don't know the full curve.

So my wife's engine (well, the engine in her 9³) makes 194 lbf-ft of torque from 2100rpm to 4200rpm. At 2100rpm that's 77.6hp, at 4200rpm its 155.1hp. At higher revs, even though the torque gets lower, the hp gets bigger, so best power is 185hp @ 5500rpm. That means 177 lbf-ft of torque at 5500rpm.

My Integra GS-R's engine makes 170hp @ 7600rpm. But its a sharper peak, so even when I shift at redline the hp drops more sharply than the Saab's. That's why that car has a close-ratio transmission. It's a screamer, but it acts like a stone if you're afraid to go to redline. Max torque is 128 lbf-ft @6200, which is only 151hp. Guessing at maybe 125 lbf-ft @2000 that's only 48hp. See where this is going?

Best acceleration happens at the moment the tach crosses the point of max power. But since we only have 5 gears, the engine has to spend time over a range of rpm. Integrate the power curve OVER THAT RANGE to get the real answer. Car makers don't list the result of that integration in their brocures. But its a big part of the explanation for why 2 cars with about the same weight, same max power, can have very different acceleration.


posted by 12.28.24...
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