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Question: practical understanding torque (and horsepower)

I posted this to rec.autos.driving and didn't get great response, so was hoping you more technical-minded individuals could help me with this. :) Much appreciated.

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First off, sorry to ask about this topic again (it must have been asked over and over since cars were invented). But I've searched the web and the group archives and couldn't get a satisfactory answer. I'm hoping some kind soul can answer my question and explain the reasoning, keeping in mind I may ask other questions as well. :) I would also love if you could EMAIL me your respond, since it's much easier than fishing for replies on google.

So, without further ado, the question (as clearly and concisely as I can ask it):

My car is at a standstill. I rev and hold at #### rpms. I quickly release the clutch and assume ideal, total transfer to the car's motion (no clutch slip, so tire squeal, etc) Which rpm would give me maximum acceleration, max torque or max horsepower (or some other)? And, more importantly, why?

My vague understanding: Torque and horsepower are both related to the car's acceleration, since

a) torque is proportional to force, which is mass x acceleration, or "increasing the velocity of a mass at a certain rate".

b) horsepower (or power) is the rate of change in kinetic energy, which is proportional to velocity squared. So if I'm increasing my "velocity squared" quickly, I must be accelerating quickly...?

Please email me your response, and put one more soul to rest about this confusing topic.

Thanks, Nick

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I've thought about this some more, and have some more specific questions:

1) If an engine delivers say 200 lb-ft of torque at 5000, is that regardless of how open the throttle is? If I'm in neutral and rev to 5000, is it still putting out 200 lb-ft of torque?

2) When I partially apply the clutch, is the engine delivering all it's force to its load (the car)? I'm inclined to believe that if I hold the engine at a certain rpm, apply the clutch JUST to the point before the rpms drop (but no less), I am applying EXACTLY the torque for that rpm to the load. If I release the clutch any more, I'm overloading the engine at that rpm; any less, and I'm not applying the full torque at that rpm.

3) How, numerically and assuming ideals, does the value of engine torque ("200 lb-ft @ 5000rpm") translate to the car's acceleration ("6 miles/hour/sec")? Is there a formula for this? Is it as simple as: Acceleration = torque * drive radius / car weight ? (where drive radius is the factor based on the radius of gears and wheel, essentially converting torque to linear force) This doesn't seem right because it doesn't consider rpm.

Please respond via email. Thank you.

Nick

Reply to
Nick Pan
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Reply to
John Ings

It's wherever your car makes maximum torque. Horsepower is just a number that's based off torque. The equation is torque x RPM / 5252 = horsepower. It's different from car to car. Your question is applied so generally that there is no good answer. Torque determines how fast a car will be going after a certain time. Horsepower is how fast a car will be going after a certain distance.

torque is proportional to force AND the distance from the epicenter of the rotating mass. 250 lbs at 1 ft gives you 250 ft-lbs. 250 lbs at 2 ft gives you 500 ft-lbs. Increasing either gives you more torque.

velocity is measured in distance per second. Acceleration is meters per second per second. Power is not proportional to velocity squared, it's proportional to force and velocity.

No. without any force acting against the engine other than frictional losses, the engine is not making 200 ft-lbs of torque. Since the engine isn't connected to anything, there is nothing to generate a force. It's like punching the air, you might be able to generate a lot of force but if there's nothing to punch, it's ineffective.

No. if the clutch is not fully applied, you have slippage and there is not full transmission from the engine to the transmission. Right before the RPMs drop, for a split second, you are applying torque, I'm not certain if its the torque made at that RPM. Remember, a car that is at a certain RPM does not mean it is making maximum torque. Cruising around, at 3k rpms, your car is only making enough power to keep the car moving.

See equation above for horsepower.

1 hp = 33000lbs moved 1 ft in 1 minute. Recalculate that for whichever units you want. Remember that cars have torque and horsepower curves. This will effect how quickly the car accelerates.

Get a physics book from the library and study up on the formulas therein. You need to understand the fundamentals before you understand how things based on the fundamentals work. Gotta walk before you run.

Good luck,

-Bruce

Reply to
Bruce Chang

Simply put, in layman's language.

Horsepower gets you there -- Torque keeps you there

Reply to
pete selby

Believe you've got that backwards. Torque "gets you there" (accelerates you to a certain speed). Horspower keeps you there (maintains that speed).

DS

Reply to
Daniel J. Stern

Approximately 11/20/03 09:39, pete selby uttered for posterity:

Torque gets you there -- Horsepower keeps you there. ...is why there are gears...

Reply to
Lon Stowell

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