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What is s Horsepower? What is Torque? What's The Difference?

Ideal Power And Torque Graph
Ideal Power And Torque Graph

If you watch car and pickup truck commercials, you will hear all manner of numbers thrown around; among these are miles per gallon, horsepower and torque. Numbers sound good and if "bigger is better," then bigger numbers must be better, right? Well, maybe. But before you buy a car or truck because one number sounds "better" than "the others," it helps to know what these things mean.

One Pound-Foot Of Torque
Drawings By The Author

The first question is, "What Is Horsepower?" The short answer is, it is a variant of "work," as defined by a physicist. In physics, "Work" is the amount of weight lifted and the distance it is moved. The basic unit, in the English system, is the "Pound-Foot," which means you've lifted one pound a distance of one vertical foot in one second. Or it could mean any of an infinite number of variations, since a "Pound-Foot" is basically a ratio. We'll discuss how mathematics varies in a minute.

The phrase, "Horsepower," entered the language during the Industrial Revolution, when steam engines were starting to replace ponies in coal mines and horses in just about everything else. Since horses, like people, vary greatly in size and strength, mine and transportation company owners wanted an idea how many beasts they could replace. After some fiddling and experimenting, a standard was agreed-upon: One horsepower equals 550 Pound-Feet of work. This number may or may not be accurate, but like feet, inches, ounces and pounds, it became a standard and we've had it ever since. The metric system has a rough equivalent called the "Watt" -- which is 4/3 of a horsepower. Or more simply, one horsepower is 3/4ths of a Watt.

As I said above, calculations of work, and therefore horsepower, are not etched in stone. They are the result of mathematics, and while the result of the calculations may be constant, there are an infinite number of ways of getting the answer. For example, one Pound-Foot of work can be one pound lifted in one second. Simple enough, right. But it could also be half a pound lifted two feet, or one ounce lifted sixteen feet, or 12 pounds lifted a single inch. Such are the ways of mathematical formulas--any two numbers can play, so long as they give you the same product.

This is going to get worse.

Next question: What Is Torque? In one two-word phrase, torque is 'Twisting Force." When you open a door by turning a knob, open a jar by unscrewing the lid, reel in a fish or crank a winch, you are applying Torque because you are applying force on something by turning or twisting it. The basic unit of Torque is the "Foot-Pound," which is a force of one pound applied at the end of a leaver one foot long. The drawing at the beginning of this article shows a wrench applying one Foot-Pound of Torque.

Important Note: Please do not confuse a "Foot-Pound" of Torque with a "Pound-Foot" of work. Torque is a basic unit of force which is independent of time, where the "pound-foot" is a unit of work, which is force applied over time.

Now back to the mathematical confusion. As with work, a "Foot-Pound" of torque is a single answer with countless variations. Assuming the wrench in question is weightless, you can have one pound of force  pushing on a one foot leaver. Or two pounds of force on a six inch leaver, or an ounce pushing on a sixteen-foot leaver or...well, you get the idea. You can use any two numbers for length of leaver arm and force, so long as they multiply out to the same thing.

So how, you ask, are horsepower and torque related in an engine, and which one is "more important."

We'll take the easier question first.

As already noted, torque is a basic unit of force; that is, it has only two variables, the length of the leaver arm and the force applied. Horsepower, on the other hand, is torque applied over time. Within the limits of the engine's ability to take in fuel-air mixture and push out spent exhaust gas, the amount of horsepower an engine produces varies according to how fast the engine is turning. It also depends on the amount of torque available at that number of Revolutions Per Minute -- commonly abbreviated RPM -- of the crankshaft.

When an engine is tested on a dynamometer, called a "dyno," for short, the readings the operator records are the torque the engine produces at various RPM. Click ---> HERE for a somewhat technical Wikipedia entry on how a dynamometer works. Once the engine has been set to a specific number of crankshaft rpm, the operator records the Torque it produces. Then he calculates the Horsepower at that RPM level using the following formula:

For imperial units,

HP = T x RPM / 5252


HP is the power in horsepower
T is the torque in pound-feet 
RPM is the speed of the crankshaft in revolutions per minute.

If you remember high school algebra, you know that formula can be manipulated and redistributed to solve for any of the variables. So if you want to solve for Torque, you re-write the equation this way:

T = HP x 5252 / RPM

As  you can see, there are a lot of ways of getting a certain number of horsepower.

Now back to the question of whether Torque or Horsepower is more important. We'll do this by comparing a couple of different engines, each of which produces about 800 BHP.

First, the NASCAR V-8, which is familiar to most of us. It's a big thing, displacing six liters (the amount of air-fuel mixture used when each of the cylinders has its share). Now let's say peak horsepower occurs at 8,000 rpm, we have:

T = 800 HP x 5252 / 8,000 RPM

or Torque = 525.2 Pounds-Feet, which is a very healthy engine, indeed

Now let's look at a 2.4 liter Formula One engine. Assuming it hits its peak power at 17,000 RPM, we have...

T = 800 HP x 5252 / 17,000 RPM

Its Torque is 247.2 Pounds-Feet, which is about what a street-legal V-6 may produce. It is also considerably less than the NASCAR engine, even though both are V-8s producing 800 bhp.

See the difference? Same power, different ways of getting it.

The NASCAR engine turns relatively slowly, but it has large pistons and a long stroke, probably over three inches. It makes its power with a lot of torque. The Formula One engine, on the other hand, is only about 40% the size of its NASCAR cousin. It makes its power not through torque, but from an awful lot of smaller bangs per minute.

So how would each engine "feel" if installed in a car? In simple terms, the NASCAR engine is a brute -- all that torque would either spin the tires or give you a solid slam in the back. The Formula One engine, on the other hand, would not accelerate as rapidly, but it would be smoother and push the car forward with the same amount of power. It would not give you the same slam in the back, but it would feel somewhat smoother.

As for which is "more important," that depends on what you're trying to do. If you want to go fast, the answer is horsepower. Like speed, it varies with time, so the more power you have, the faster you can go. On the other hand, if you want acceleration or pure pushing force, you want Torque, because that is how you leaver a weight into motion, up a hill, etc.

Where a Formula One car is optimized for power, the Diesel engine in a truck optimized for Torque. Diesels operate at low RPM, so they don't produce a lot of Horsepower, but they do produce prodidgous amounts of torque. Let's go back to the formula for figuring out Torque again, only this time with a 250 BHP truck engine running at 2,000 RPM.

T = HP x 5252 / RPM  or in this case, T= 250 HP x 5252 / 2,000 RPM

In this case, the engine is not very powerful -- the Formula One engine may make almost that much power at idle -- but it produces 656.5 Pounds-Feet of Torque, which is enormous. If our theoretical Formula One engine produced that much torque at is 17,000 RPM Horsepower peak, it would be making 2,125 horsepower. And it would accelerate a 1200 pound car like a rocket.

Now for a quick look at the slide show.

The first slide shows the "theoretically ideal" Power and Torque curves. Here, Torque remains dead-flat constant, so Horsepower is equally linear through out the engine's power band.

The second slide shows what you might expect in a good, well-developed street engine. Here, the torque is not quite constant because of the way the engine takes in fuel-air mixture and pushes out exhaust. At low RPM, there is not much vacuum, so the mixture flows slowly. At  high RPM, the valves can't open wide enough to fully fill the cylinders, so power drops off. But this would still be considered a "strong," "flexible" engine with a broad power band.

Slide three shows the opposite. Here, Torque varies -- it is considerably higher at higher RPM and there is a dip in the curve about half way along. Worse still, most of the Torque is produced at high RPM, with much less in the lower range. This is more the kind of power curve a Formula One engine would produce. If you were driving a car with an engine like this, its power would be "Peaky." That is, it would feel "gutless" at low speed and hard to get moving. Then it would explode as it neared its torque peak. The car would be fast enough, once you revved the daylights out of the engine, but it would not be "flexible" because it only has power in one narrow band. Not the sort of thing for idling through stop-and-go traffic on I-93 at rush hour just south of Salem. If you were driving a truck with that kind of power band through that traffic'd be prone to stalling the engine. You would also be using the kind of language mothers don't allow their children to hear.

For street driving, if you're given a choice between an engine with a lot of Horsepower and one with a lot of Torque...take the Torque. You'll feel better and you can tow a trailer.


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