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Unread 05-11-2013, 07:41 PM   #1
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Default What's the difference between horsepower and torque?

Hi guys. I know we have some engineering/physics gurus in here (looking at you speed and tofu and elemein). As long as I've been a gearhead i still have yet to fully grasp what the difference is between horsepower and torque. Obivously I know you need both to go anywhere and neither is more important than the other, but I don't get the difference. How can you tell the difference between the two? As far as I'm concerned there's only one measure for power and that's how hard the wheels are twisted, therefore putting you back in your seat and keeping you there.

Can someone help me out here?
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Unread 05-11-2013, 08:05 PM   #2
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Horsepower is a derived unit of torque. Therefore there is no HP without torque. I would have to dig up my books to tell you the exact eqs involved.
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Unread 05-11-2013, 08:25 PM   #3
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The way I see it, torque is a force and horsepower is, well, a power (power in physics = rate of work). Torque is the twisting force put on the wheels, you're right about that. Because HP is a rate of work, it makes sense that more HP = faster car. I just looked it up and 1 horsepower is the power needed to move 550 lbs one foot in one second.
Dynos actually measure torque, and they can use that to find the horsepower. They don't measure HP directly.

There's a lot of different factors that effect both (most notably how gear ratios affect torque output), but the easiest way to think of it is torque = brute force a vehicle can generate, and horsepower = the rate at which it is generated. I'm sure someone else can explain it better but that's how I understand it.
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Unread 05-11-2013, 08:30 PM   #4
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Quote:
Originally Posted by MetalBeerSolid View Post
torque = brute force a vehicle can generate, and horsepower = the rate at which it is generated
This is really the easiest explanation, well put.
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Unread 05-11-2013, 10:35 PM   #5
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What do you mean the rate at which it is generated?
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Unread 05-12-2013, 12:49 AM   #6
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I have no formulas to offer. just a seat of the pants description.

Torque = the weight lifter, HP = the runner; one moves a lot slowly, the other moves a little quickly.

Torque = the battery cranking the starter, HP = the spark from the coil; one makes serious current, the other can yank your arm.

Torque = a big guy twisting your arm behind your back, HP = a jab in the face.

Torque is felt as a heavy push that is hard to stop, that deep grunt of a big block to move a 2 tom car.

HP is that insane screaming Ferrari that is passing you like you are going backwards, lightening response.

In most vehicles made the torque is often desired as a flat curve so that you can feel the pull, not only off the line but as far to the red line as possible. Generally the torque ends up being a bit higher number than the HP. As you mod a normally aspirated engine the Hp climbs faster than the torque. An engine that made 300 HP & 325 LB/FT of torque may now have 600 HP but only 575 LB/FT of torque. The engine torque peak is lower in the range than it was before compared to the HP. The engine overall have way more power at any RPM than it did before but the engine will now feel stronger up high than it did before and less strong (comparatively) down low than it did before. If the mods are continued until you reach the maximum power attainable the engine may produce 1000 HP but it may have even less torque under 2000 RPM than the stock engine did. If this engine was in a heavy vehicle with tall gears the stock engine would accelerate it quicker, until the modded engine got up to its torque range and then it would overtake the stocker. That's how you would feel the torque.

In a forced induction engine, the added boost behaves a like a larger displacement engine, so you don't loose torque like a cammed up engine.

You can generate torque without HP, but not HP with out torque. As you apply pressure to a wrench on a faster that is not moving, you are generating torque. If you try to pull a locomotive with a piece of string, you cannot generate enough torque to move the weight even if you have unlimited HP.
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Unread 05-12-2013, 01:19 AM   #7
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Quote:
Originally Posted by Mike View Post
What do you mean the rate at which it is generated?
Well horsepower is a rate. It's not just a simple measure of force like torque.

1 horsepower = 550 ft-lb/s. You have an amount of work (550 ft-lb) and a time (1 second). Power is defined as the rate of work over a period of time.

It isn't the easiest concept to explain or understand. Practical examples like what car guy said are easy to relate to but it gets confusing when you get to the physics of it.
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Unread 05-12-2013, 01:23 AM   #8
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I look at it this way. Torque is the raw power turning the wheels. Horsepower determines how fast they can be turned. Interesting, really.

Higher HP = faster speeds
Higher Torque = More grip, better towing and hauling

HP = Hare - fast, but without the strength of the tortoise, it can be easily stopped with resistance
Torque = Tortoise - Strong, and hard to stop, but without the speed of the hare, it's quite slow
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