genrex

As we're having dinner tonight, my daughter says, "What is the difference between torque and horsepower?" I thought about it, and lamely said that torque is good for lower-rpm acceleration, and horsepower is good for higher-rpm acceleration. Of course, that got me a blank look - she wanted a better answer than that.

She knows that torque and horsepower both provide "Work" to accelerate a car, and she wants to know "How much work do they do?" If any of you physics nuts/perverts in the audience happen to know the Conversion Formulas for converting torque (ft-lbs) into horsepower - and vice versa - it would really help me out! Geez, I hate it when I can't answer an automotive question for my daughter... that is so embarrassing. Thanks, and cheers!

Flobb

Horsepower is an energy potential. Or an ability to do work.
Torque is a force that is related with creating rotation. A product of force and radius.

So horsepower is more an ability of the motor to do work.
Torque is the force that gets the wheels turning and moves the car.

rxseven

Proper definations are as follows;
The force exerted by the engine multiplied by the displacement the force produces is defined as the work done by the engine. the rate at which this work is done is defined as power, measured in HP in english units or watts in metric. It can also be defined as power = force x velocity.

Torque is defined as the product of force which produces a rotation and the perpendicular distance from the line of action of the force to the point of application.

Ok, I guess I am a nerd.

genrex

Man, for some reason I'm getting sleepy..
okay... what was the middle part again?

There are Conversion Formulas to convert from one to the other.. giving the "horsepower" equivalent of a certain torque, and vice versa... I saw the equations in a physics textbook.

REVHED

What sort of textbook you been reading? Horsepower is simply a product of torque and rpm. Here's the formula for calulating horsepower.

hp = torque x rpm / 5252.1

The number at the end is just a conversion factor to give you the right units.

genrex

I had a feeling Oz would help... thank you!

I've been looking at some websites from doing a "horsepower
torque conversion" search, and you other guys are right, too.

You've all been a big help, actually.. thanks!

peejay

The above descriptions are excellent... but there was a demo on a Usenet newsgroup a few years ago that put it all in practical sense.

Let's say you have a 1' long wrench and you're putting 50 pounds of force on the end of it. The force on the nut is 50lb-ft of torque. Of course, this is one of those bastard nuts like the kind that hold on the flywheel, and only 50lb-ft isn't going to budge that sucker, so you're just standing there forcing the wrench and not moving. With no motion, there is no work being done.

Now let's put 400lb-ft of torque on the nut. (Big lever, strong buddy, whatever makes it 400lb-ft) NOW it moves. Now you're doing work, and we can calculate this work as horsepower. (Ain't gonna be much BTW) The faster you do the work, the more power you require. Likewise, the less torque you need to impart, the less power you require. (Like when it finally cracks loose and you can take it off with your fingers)

The way the car accelerates in a given gear is DIRECTLY related to the torque curve. If torque peaks at 4000rpm, the car will accelerate hardest at 4k for a given gear. So why do we care about horsepower?

Horsepower is work combined with time. Let's say peak power is at 6000rpm. That's less than peak torque will be (probably around 92lb-ft by quick guesstimation), BUT... for a given speed, acceleration will be hardest at peak HP. That's because when you downshift (to change engine speed from 4k to 6k), you gain mechanical advantage = more torque goes to the driveshaft, despite the fact that the engine itself produces less torque than peak.

So what does it mean? You make HP by making more torque, or by raising the RPM where you make it. If you raise the RPM, then you use lower gears to take care of mechanical advantage. Now you know why I've been drooling over Kia 4.778:1 rearend gears lately

genrex

Peejay, what you said is excellent.. thanks! So what happens between 4000 and 6000 rpm, as torque is decreasing and hp is increasing?

The car accelerates at about the same rate until 6000 rpm.. my guess is that the total "power output" (in whatever units) of the engine would be nearly constant between 4000 and 6000... that seems intuitive, but..

rockhead

Torque yields acceleration, horsepower yields sales to losers who can't smell the difference.

Ever notice that a Turbo-Sprint is hauling *** away from you with his two-hundred horsepower? Not likely ;-) that's because his torque is not high throughout the rpm range (powerband). Car makers love horsepower because it doesn't discuss usable torque it gives the brain-dead suv jockey a big number to talk about at starbucks.
If your engine produces 20ftlbs of torque everywhere but has a sweet spot 100 rpms wide where the torque climbs to 40ftlbs you can claim a whopping 22 horsepower at 3000 rpm.
If your engine produced a flat 30ftlbs of torque without the big peak you'd have a paltry 15 horsepower at 3000 rpm, but you'd pull away from the previous engine because he's only got a narrow powerband and outside that you are generating 1.5 times the torque he is.

peejay

The car accelerates harder until you get to peak torque, and then acceleration lessens as you come down off the peak. The trick is that even though you make less torque past peak, HP will still increase because engine speed compensates for it. (IE you're better off staying in gear instead of upshifting and losing mechanical advantage) Peak HP is always made on the downslope of the torque curve because of this.

When engine speed can no longer compensate for torque loss, that's basically where peak HP is. After that, torque falls off too rapidly and you're just making lots of noise. (Like running a purely stock 12A past 6500... it sounds cool but that's about it)

Then you get into things like the shape of the torque curve, which directly relates to how wide the powerband is. (13B 6-port = wide and flat, my ported 12A = pointy like the Eiffel Tower )

REVHED

That example may by correct in theory but we're talking about REAL cars here. To put it simply so you can understand. It's better to make high rpm torque, ie. horsepower, than it is to make low rpm torque because it allows you to take advantage of gearing. I'm not gonna say anything else here because peejay already explained it pretty much spot on.

Jeff20B

iTrader: (13)

Where is the URL for that Corvette page?

peejay

Come to think of it, it was a Corvette guy who posted it to the Usenet newsgroup... (either r.a.driving or r.a.tech) Darn, now I'll have to fire up the ol' 286 where I keep all those old files....

RX-7-8593

I think this is the one that you are looking for with the vetts. It explains in an easy to follow way the difference between torque and horsepower.
http://www.zhome.com/ZCMnL/tech/torqueHP.htm