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Torque Explained. Fast Friday #40!
As anyone who knows me will tell you, I like to talk cars... A LOT. So naturally I attend many car meets and events and have had a chance to talk to lots of car enthusiasts. I often hear people mention that such and such car has x horsepower and X torque, but I should be extra impressed with the torque number because that's what's really important. If you consider yourself a car enthusiast than you have certainly heard some version of this, or maybe the saying: "horsepower sells cars, but torque wins races." In reality, this misconception couldn't be farther from the truth.
Horsepower and torque are related in that horsepower is a function of torque and rpm. In other words, horsepower is the rate at which torque is delivered. The equation for horsepower is:
hp = (torque x rpm) / 5252
To explain how this works, let me first give a basic physics lesson. A unit of torque consists of a unit of distance followed by a unit of force (or in some cases a unit of weight, which on earth can be considered a unit of force due to gravity). The unit of torque defines a situation in which the unit of force is attempting to rotate a hypothetical lever of length defined by the unit of distance.
So 1 ft-lb describes a situation in which a hypothetical lever of length 1 ft has a force of 1 lb trying to rotate it about a pivot. The force and distance values are multiplied together to give the total torque, for example a 2 lb weight sitting at the end of a 2 ft long lever would provide 4 ft-lbs.
Imagine for a second that you were to attach an 8 ft long 2x4 to the wheel of your car and you stood on the far end to attempt to turn the wheel. I weigh 150 lbs, so in my case this would provide 1200 ft-lbs (8 x 150) of torque. So if you can make 1200 ft-lbs of torque with your body weight and a 2x4, why do we need engines? Because even though you are making as much torque as a Mack truck for an instant, as soon as the wheel begins to turn you will hit the ground and have to reposition the lever in order to move the wheel again. Since horsepower is the rate at which torque is supplied, and torque cannot be continuously supplied in this situation, horsepower is essentially 0.
Building on this example, you can imagine that if you could find a way to continuously turn that 8 ft lever at high speed you would be able to get the car to move quite fast. Since torque is continuously supplied at a high rate in this case, you would be making high horsepower. This example shows how you need both torque AND rpm in order to accelerate a car, in other words the only thing that matters is horsepower.
Since horsepower is the only thing that matters, you want as much horsepower as you can get throughout the rev range. Engines that make more torque at lower rpm consequently make more horsepower at lower rpm, which results in higher average horsepower throughout the rpm range. However, this same advantage can be accomplished by spinning the engine to a higher rpm to make more horsepower up top. The reality is that it does not matter if horsepower is made by increasing low end torque or high end rpm, all that matters is the average horsepower the engine makes throughout its useful rev range, or to put it mathematically, area under the curve.
-Rye
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Matt
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