Brian, you cannot have it both ways. It either has low torque or massive torque. Understanding that torque is torque at the wheels of course. That is the ony one that matters.

JL

I was speaking of torque from the engine.

Torque at the wheels is precisely what drives the vehicle. The point of high horsepower and a good set of gears is what allows you to have maximum torque at the wheels.

BTW, nobody quotes torque "at the wheels". It's always the engine output torque and horsepower that are discussed. The torque at the wheels is subject to all the powertrain losses and would need to be measured on a chassis dyno as Duke explained.

Of course you are right. Nobody will disagree with that. I thought the point of disagreement was wether torque or power determines acceleration. In that sense, as you very well pointed out, the only torque that matters is the one at the wheels, that is why I used it. And I do not believe you need high power to get high torque (at the wheels). Just for high speed.

JL

My understanding is also we were discussing whether torque or horsepower determine's acceleration. Leaving out gearing, frictional losses, etc in the drivetrain seemed to me sensible as long as one knows there is no acceleration without driveshafts and wheels. Torque is a measure of twisting power at the crank that is transferred to the flywheel. We can limit the discusiion to end at the flywheel with no loss in discussing the basic questions. Given Watt's equation, we are told that if we know either torque or horsepower we can determine the other. Equations are equalities Watt's equation, in my opinion, is a simple transformation. Given that, I believe we see that torque and horsepower are not so much separate metrics, but are shifting one's gaze to either horsepower or torque, one totally dependent on the other. One of my points is that we are basicaly looking at one spinning crank or flywheel and measuring it's torque or horsepower. But, whichever we measure, we automatically know the other. So, they're not such separate entitie's even though we can design and build engines which generate differet hp & torque trajectorie's as a function of rpm.

It's drive thrust at the wheels that determines acceleration.

If the tire rolling radius is 12" 500 pounds of thrust at the wheels is 500 lb-ft of torque at the axle. Let's establish the final drive gear ratio at 2.5:1 and assume no loss in the axle, so axle input at the drive shaft is 500/2.5 = 200 lb-ft.

The relationship P=FV still holds, but we can rewrite it as:

F=P/V

or

F = TxN/5252V (we're still talking about torque and power at the rear wheels)

Now substitute an engine that makes half the torque, 250 lb-ft, but at twice the speed, and we shorten the axle ratio from 2.5:1 to 5:1, so that peak torque occurs at the same road speed.

At this road speed which setup provides the greatest drive thrust? Which one provides the greatest RWHP?

Duke

Sure thing, in fact engines are taylored to meet certain hp and torque requirements. It is a matter of changing rotating masses, crank lengths, pistons etc. (mostly for torque) and displacement, rpms, engine breathing, etc. (mostly for hp). And now we have those hybrids with those electric motors that give you gobs of torque regardles of rpms. In my opinion though, for a particular engine, horsepower depends on torque, but torque does not depend on horsepower.

JL

If I understand your post correctly the first one , as you state in the post, provides 500 lbf of thrust. The second one, by my calculations 250 lb-ft x 1ft = 250 lbf, unless I misunderstood.

JL

PS I just reread your post and now I am really confused. I am going to bed and let's continue tomorrow. Maybe then I can think straight.

JL

Well, the fact that you consider this an opinion and not "fact" means there is hope.

HP = TxN/5252, or using algebra to rearrange

T = 5252xHP/N

My 2.6L M103 is rated at 162 lb-ft of torque at 4800 RPM and 158 HP at 5800. How much HP does it produce at 4800? How much torque does it produce at 5800?

At 5800 is torque a unique function of power? If not what determines torque at this engine speed? How about at any other engine speed?

You're answer to the previous question was not correct. Give it another try with a freshly rested mind.

Here's another brain exercise. You're checking wheel bolt torque. You apply 80 pounds of force on your torque wrench, which has a lever arm of one foot, but the nut does not move. How much work have you done? How much power did you apply?

Hint: Review the definitions of work and power in previous posts.

Duke

This is mucho mucho fun!
 

Well let's see. I went to my Physics 101 book, I cleaned the dust off and got out my big manifying glasses and read the answer:
0 (Zero)
0 (Zero)

Am I right prof?

Now, here is another brain exercise:

Contestant A climbs in his car with tremendous 1000 HP engine and a puny 200 lb-ft of torque.

Contestant B climbs in his car with puny 200 HP engine and a tremendous 1000 lb-ft of torque.

Both cars have the same weight, gearing etc. and similar torque, HP vs rpm curves.

They both release the clutch at, lets say, 2000 rpm.

Which one will have a higher initial acceleration?

Hint: Review the all the definitions you can find on your Physics 101 book. (Then, burn the book).

Is that 1000 hp at 0 mph or 100mph?

P.S. please post pics of the cars...

Those are good guesses. I'll have to figure out the mph for the torque too. I will borrow a digital camera and send pics later.

JL

It's a fruitless exercise because the two vehicles would have vastly different gearing. Furthermore, they cannot have similar torque, HP vs. rpm curves.

With proper gearing, the 1000 HP engine would blow the doors off the 200 HP engine. To think otherwise is ridiculous.

Yes it is a fruitless and ridiculous exercise, but the physics are correct -ridiculous does not prevent it from being so. The fact is that we are talking about this engine, that engine, my engine; they may all have different characterstics and seem to have different behaviors, but the underlying physics are the same (obviosuly the interpretation of the physics is not). Duke's questions about power and work done were very astute and reveal a clear understanding of the issues. You cannot have power or work without motion, but you can have torque. Similarly you can have acceleration despite the fact that your velocity is zero (otherwise we would never get going).
It was not my intent to further confuse the issue, but I seem to have succeded at it. Sorry.

JL

I had no idea I was essentially creating another oil thread...:D

Amazing, isn't it? This is one of the things that makes this forum so great!

JL