From Mr. ZF's posts:
Wow, misinformation about the definitions of torque, rpm, and horsepower, and the relationships between them, still run rampant.
I don't know if I have the energy for this again.
Horsepower alone describes the performance capability of an engine, whether it's accelerating or maintaining a speed against a load or whatever. That's a fact, not an opinion. Any engineer worth the paper his degree is written on will tell you. Hell, forget the degree, anyone who's been through Physics 101 and even came close to understanding the material knows the same thing.
Without going into the whole spiel explaining the why's and how's, just consider a couple of things if you doubt me:
1) When you come to a hill and the truck starts slowing down, can you climb the hill faster at your engine's torque peak or by downshifting and putting the engine at it's horsepower peak? Hint: at the engine's horsepower peak, more torque is available at the rear wheels for a given speed even though the engine isn't making it's maximum torque. The additional rpm of the engine allows additional gear reduction which makes more torque at the rear wheels which more than makes up for the engine's torque.
2) On a more basic level, consider this. Horsepower = (torque x rpm) / 5252, right? You can get this formula out of any physics book. Think about it in relation to the rear wheels. If you keep rear wheel rpm constant (i. e. the speed of the truck), doesn't more rear wheel horsepower mean more rear wheel torque? Doesn't the formula say it has to?
That's as far into this as I want to get sucked. There's a whole big multi-page discussion on it if anyone cares to go read it. You gotta let go of preconceived notions though.
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posted 01-22-2003 01:03 AM
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TG, what moves the truck, torque at the engine, or torque at the rear wheels?
Do you know why engine torque and rear wheel torque are different from each other?
Do you understand how an engine with low torque and high rpm can actually put more torque to the rear wheels than a high torque/low rpm engine
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This guy is missing the point. What moves a truck? Torque!! What determines how FAST you can move it? HP!
Sheesh, the GM guys don't know what they think they know! Quote:"Horsepower alone describes the performance capability of an engine, whether it's accelerating or maintaining a speed against a load or whatever. That's a fact, not an opinion"
WRONG!!
HP only deals with CHANGES, since is the time derivative of work! Thus, only acceleration, not maintaining a speed against a load. If you want to maintain 50mph, you want torque. If you want to accelerate from 50-70, you want HP. The relation between hp and torque so close, it's easy to confuse them. By definition, HP is only present when work is done. Also by definition, it is the RATE at which work is being done. Torque is the work done. So, hp is how quickly you can use torque. So a wheel turning with 100-lf-ft or torque doubles it's "hp" if it doubles its rpm, with NO increase in torque.
Furthermore, it's misleading to say that "more HP HAS to mean more torque". While this is technically true, it more conceptually accurate to say that "more torque HAS to mean more HP" Torque is the independent variable. HP is the DEpendent variable, since it depends on both torque and time (or rpm).
At least this stooge had the part right about the gear reduction. Yes, there is more AVAILABLE torque at peak HP, because you can downshift. What you are doing then is trading the time element of HP for the torque element of HP. Torque can be multiplied, but NOT hp. The time will always be reduced by the same amount that the torque is increased.
If I had a membership to their lame site, I would go out-know-it-all the know-it-all.
As for their dmax vs. gasser dispute, the comparison is only valid if you locked both trucks into 1:1 direct and ran up the hill. The higher hp of the gasser should allow it to pull a load up the hill faster. The higher torque of the dmax should allow is to pull a heavier absolulute load. There is a point (on the scale from light weight to heavy weight) where the gasser would be slower up the hill, as it gets to the point where only the tq of the dmax will move the load.
Letting the trannys gear down skew the whole thing in favor of the HP.
HOHN
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