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-   -   Torque vs Horsepower (http://www.f150online.com/forums/general-discussion/35687-torque-vs-horsepower.html)

J-C 01-04-2000 02:37 PM

Torque gets it moving.
Power keeps it moving.

More of each is good.
Too much is not enough! http://www.f150online.com/f150board/wink.gif

Tiron 01-04-2000 04:33 PM

Here's a link with a fairly good explanation of the relationship of torque and horsepower. Keep in mind it's on a corvette page and written from a "go fast" point of view and not a Ford Truck "pull 15000 lbs" point of view.

J-C 01-04-2000 05:40 PM

Interesting read, Tiron.

fast46triton 01-04-2000 08:35 PM

The Definition
The term horsepower was invented by the engineer James Watt. Watt lived from 1736 to 1819 and is most famous for his work on improving the performance of steam engines. We are also reminded of him every day when we talk about 60 watt light bulbs and kilowatts.

The story goes that Watt was working with ponies lifting coal at a coal mine, and he wanted a way to talk about the power available from one of these animals. He found that, on average, a mine pony could do 22,000 foot-pounds of work in a minute. He then increased that number by 50% and pegged the measurement of horsepower at 33,000 foot-pounds of work in one minute. It is that strange, arbitrary unit of measure that has made its way down through the centuries and now appears on your car, your lawn mower, your chain saw and even in some cases your vacuum cleaner!

What horsepower means is this. In Watt's judgement, one horse can do 33,000 foot-pounds of work every minute. So imagine a horse raising coal out of a coal mine as shown above. A horse exerting one horsepower can raise 330 pounds of coal 100 feet in a minute, or 33 pounds of coal 1000 feet in one minute, or 1,000 pounds 33 feet in one minute, etc. You can make up whatever combination of feet and pounds you like - as long as the product is 33,000 in one minute and you have a horsepower. You can probably imagine that you would not want to load 33,000 pounds of coal in the bucket and ask the horse to move it one foot in a minute because the horse couldn't budge that big a load. You can probably also imagine that you would not want to put one pound of coal in the bucket and ask the horse to run 33,000 feet in one minute, since that translates into 375 miles per hour and most horses can't run that fast. However, if you have read the HSW article on the block and tackle, you know that with a block and tackle you can easily trade perceived weight for distance using an arrangement of pulleys. So you could create a block and tackle system that puts a comfortable amount of weight on the horse at a comfortable speed no matter how much weight is actually in the bucket.

Horsepower can be converted into other units. For example, one horsepower is equivalent to 746 watts or 2,545 BTUs (British Thermal Units) per hour. So if you took a one-horsepower horse and put it on a treadmill, it could operate a generator producing a continuous 746 watts. If you took that 746 watts and ran it through an electric heater, it would produce 2,545 BTUs in an hour (where a BTU is the amount of energy needed to raise the temperature of one pound of water one degree F). One BTU is equal to 1,055 joules, or 252 gram-calories, or 0.252 food Calories. Presumably the horse would burn 641 Calories in one hour doing its work if it were 100% efficient.

Measuring Horsepower
If you want to know the horsepower of an engine, you hook the engine up to a dynomometer. A dynomometer places a load on the engine and measures the amount of power that the engine can produce against the load.

Imagine that you have a big socket wrench with a 2-foot long handle on it and you it to apply 50 pounds of force to that 2-foot long handle. What you are doing is applying a torque, or turning force, of 100 foot-pounds (50 pounds to a 2 foot long handle) to the bolt. You could get the same 100 foot-pounds of torque by applying one pound of force to the end of a 100 foot handle or 100 pounds of force to a one foot long handle.
Similarly, if you attach a shaft to an engine, the engine can apply torque to the shaft. A dynomometer measures this torque. You can easily convert torque to horsepower by multiplying torque by RPMs / 5252.

You can get an idea of how a dynomometer works in the following way. Imagine that you turn on a car engine, put it in neutral and floor it. The engine would run so fast it would explode. That's no good, so on a dynomometer you apply a load to the floored engine and measure the load the engine can handle at different RPMs. So you might hook an engine to a dynamometer, floor it and use the dynomometer to apply enough of a load to the engine to keep it at, say, 7,000 RPMs. You record how much load the engine can handle. Then you apply additional load to knock the RPMs down to 6,500 RPMs and record the load there. Then you apply additional load to get it down to 6,000 RPMs. And so on. You can do the same thing starting down at 500 or 1,000 RPMs and working up. What dynomometers actually measure is torque (in foot-pounds), and to convert torque to horsepower you simply multiply torque by RPMs / 5252.

If you plot the horsepower versus the RPM values for the engine, what you end up with is a horsepower curve for the engine. A typical horsepower curve for a high-performance engine might look like this (this happens to be the curve for the 300 horsepower engine in the Mitsubishi 3000 bi-turbo):

Any engine has a peak horsepower - an RPM value at which the power available from the engine is at its maximum. An engine also has a peak torque at a specific RPM. You will often see this expressed in a brochure or a review in a magazine as "320 HP @ 6500 RPM, 290 ft-lb torque @ 5000 RPM" (the figures for the 1999 Shelby Series 1). When someone says an engine has "lots of low-end torque" what they mean is that the peak torque occurs at a fairly low RPM value, like 2,000 or 3,000 RPM.

Another thing you can see from a car's horsepower curve is the place where the engine has maximum power. When you are trying to accelerate fast, you want to try to keep the engine close to its maximum horsepower point on the curve. That is why you often downshift to accelerate - by downshifting you increase engine RPMs, which typically moves you closer to the peak horsepower point on the curve. If you want to "launch" your car from a traffic light, you would typically rev the engine to get the engine right at its peak horsepower RPM and then release the clutch to dump maximum power to the tires.

Horsepower in High Performance Cars
A car is considered to be "high performance" if it has a lot of power relative to the weight of the car. This makes sense - the more weight you have, the more power it takes to accelerate it. For a given amount of power you want to minimize the weight in order to maximize the acceleration.

Hope this helps.

54regcab 01-04-2000 11:06 PM

Here's an idea: http://www.howstuffworks.com/horsepower.htm

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54regcab 01-04-2000 11:09 PM

Another idea:

Torque vs Horsepower

Horsepower is a calculated value. Torque is a real, physical, rotational force. (Think of a torque wrench.) It's important to remember the dyno measures torque and rpm and then from these calculates horsepower. On the dyno it takes more water flow to the water brake to increase the load on the engine being tested. As the test engine's torque rises more water flow is needed. As the test engine's torque drops less water flow is needed. The dyno's water brake does not respond to horsepower. Major adjustments to water flow are needed as an engine crosses its torque peak but none are needed as it crosses its horsepower peak. In other words the water flow to the brake during a dyno test follows the engines torque curve and not its horsepower curve. Torque is what twists the tire, prop, or pump. Horsepower helps us understand an amount or quantity of torque. (Torque + time)

Here's an interesting bit of motor head trivia; below 5252 rpm any engine's torque will be higher than its horsepower, above 5252 rpm any engine's horsepower will be higher than its torque.

Horsepower = torque x rpm / 5252

[This message has been edited by 54regcab (edited 01-04-2000).]

Good_Guy 01-05-2000 01:04 AM

Torque vs Horsepower
As far as as physics are concerned, this is what I know.

Torque = Force perpendicular x Radius
= amount required to agularly accelerate a mass rotationaly

Power = Torque x angle deflection / time

It seems that torque is what makes the trucks accelerate as well as keep it at speed. My question is, where does horsepower become important? If torque is more important than horsepower, why doesn't car dealers emphasize this?

Leo dC

Indyfan 01-05-2000 11:09 AM

To answer your last question Good Guy, I'll tell you what some of my racer friends like to say:
"Torque wins races and horsepower sells cars"

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fast46triton 01-05-2000 09:02 PM


Thats where I got it from!

54regcab 01-05-2000 09:19 PM

I knew even you didn't have enough time to type all that http://www.f150online.com/f150board/wink.gif

Pastmaster 01-05-2000 09:57 PM

I always thought it was

torque-gets it up the hill faster


horsepower-something that Gearheads and Rich people know the definition of!

P38 01-06-2000 08:06 PM

The simplest difference i can grasp between torque and horsepower is this:

hp is force applied over a time interval.

torque has no element of time (instant), that is the 'tendancy' to rotate something.

an engine only works if you have both.

high hp moves a lighter vehicle rapidly, high torque moves more weight.

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