Measures of engine performance such as torque and horsepower are relatively simple, yet are often badly misunderstood by the public. Most of the car enthusiast always confuse these terms. Enzo Ferrari was once quoted as saying "Horsepower sells cars, torque win races".
Obviously Mr. Ferrari believed that torque was most important of the two provided that both were present in sufficient amounts. With lots of turns, acceleration, and braking, torque wins hands down IF there is sufficient horsepower. Without the horsepower, diesel trucks would win auto races. Without torque, jet-powered cars would win all the races. By the way, I believe a jet powered vehicle holds the land speed record... of course it just went straight and probably didn't handle well in the turns. Lets go over some basics.
Horsepower:<o:p></o:p>
James Watt came up with the concept of horsepower -- which is a measure of, interestingly enough, power. 1 HP is the equivalent of 33,000 ft/lbfs per minute. The reason for the complex unit is that we're accounting for three things with this number: the amount of weight involved, the distance it's being moved, and how long it takes to do it (that last one is important).<o:p></o:p>
Horsepower is how quickly you can apply torque over a given distance. Watt's definition says that 1 hp = 550 ft-lb/s. Now, how do we convert horsepower into something that relates to torque and engine rpm? Well, keep in mind that horsepower is force times distance divided by time, and our distance is going to be in a circle (i.e. in revolutions), so we can do the following <o:p></o:p>
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1 hp = 550 lb * 1 ft / 1 sec<o:p></o:p>
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1 hp = 550 lb-ft * 1 rad / 1 sec<o:p></o:p>
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1 hp = (550 lb-ft * 1 rad / 1 sec) * (1 rev / 2pi rad) * (60 sec / 1 min)<o:p></o:p>
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1 hp = 5252 lb-ft * 1 rev / 1 min<o:p></o:p>
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1 hp = 5252 lb-ft * 1 rpm<o:p></o:p>
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Torque: <o:p></o:p>
Torque is nothing more than a measurement of twisting, or rotational, force. The easiest way to think of this is to imagine a long shaft -- like a car's axle -- and imagine it's in a room suspended in mid air. Hanging on the bottom of one end is a rope with a weight attached -- a very heavy weight.
Now imagine someone trying to, using their hands, twist the shaft so as to lift the weight. Think of them as essentially trying to act like a wench and reel it up. The amount of force they are able to generate to lift the weight in this manner is the torque that they're able to produce. One unit for measurement of this is the foot-pound. A foot-pound is the rotational ?force? generated by hanging a one-pound weight at the end of a 1-foot wrench.<o:p></o:p>
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The Common Mistake<o:p></o:p>
The mistake most people make when engaging in this debate is considering horsepower and torque independently. Almost everyone argues as if they are separate, unrelated values. They aren't.<o:p></o:p>
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Horsepower = (Torque x RPMs) / 5252<o:p></o:p>
This equation is the second most important thing on this page, and it's the reason that anyone telling you that horsepower and torque should be considered equally and separately is significantly off-base. The fact of the matter is that horsepower is the product of torque and another value -- RPMs (divided by 5252). It's not unrelated, separate, or different.<o:p></o:p>
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In fact, there's not a single machine in existence that measures a car's horsepower. It's a man-made number. When a car's performance is tested, its torque is measured using a dynamometer. The measure of an engine's performance is torque. Horsepower is an additional number that's attained by multiplying the torque by the RPMs.<o:p></o:p>
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The Physics of Acceleration
So now for the most important thing on the page. What determines true acceleration for a vehicle isn't really debatable -- its force divided by mass. The formula for acceleration is seen below. <o:p></o:p>
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f = ma which means... a = f/m "The confusion only comes in determining which force we're actually talking about".<o:p></o:p>
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So we are solving for acceleration and we have a constant mass. We've already established that torque is the amount of rotational force being generated at the engine, but we aren't concerned with the force at the engine. What we're interested in is the force at the wheels. The force at the wheels is the f in f = ma (actually, it includes the radius of the wheel as well, but we're simplifying).
But remember, the transmission ultimately gives the force to the wheels, not the engine. And that's the trick to this whole mess. Gearing magnifies torque. The torque at the wheels is the torque at the engine combined with the torque magnification given by the transmission through gearing. So the transmission only sees what's coming off the engine, while the wheels see the resulting force combination of the engine plus the transmission.<o:p></o:p>
That's what horsepower represents. Horsepower is the combination of the benefits of the engine's raw abilities combined with RPMs. And RPMs are what allow us to use gearing effectively, which gives us more torque at the wheels.<o:p></o:p>
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Conclusion:
So yes, the answer to the question is torque, but torque at the wheels, not at the engine. And since we're talking about torque at the wheels and not at the engine, the actual complete answer is horsepower because horsepower encompasses not only the engines torque but the total torque that gets put to the ground to accelerate the vehicle.<o:p></o:p>
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- Gearing is extremely important because it controls RPMs (and therefore horsepower).<o:p></o:p>
- Gears magnify torque -- hence the acceleration available in first gear.<o:p></o:p>
- Racecars have high horsepower due to high RPMs, not due to high torque.
- Below 5252 RPMs any engine's torque will always be higher than its horsepower, and above 5252 RPMs any engine's horsepower will always be higher than its torque. At 5252 RPMs the horsepower and torque will be exactly the same." <o:p></o:p>
- It is better to make torque at high rpm than at low rpm, because you can take advantage of gearing.
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Source: Read multiple articles like from "Daniel Miessler " on this discussion and compiled this piece for reading.</o:p>