What is horsepower and torque and how do they relate to a car being fast or a truck having a towing capacity?
In: Engineering
Horsepower is a measure of power and torque is a measure of twisting force, but let me expand on that. A waterwheel can develop a tremendous amount of torque, like 2,000 ft-lb of torque. But it does this at a low speed, say 10 rotations per minute. A BMW M3 makes about 400 ft-lb of torque, but it does this at 5,000 rpm.
So it sounds like the water wheel would be better for towing, right? Well, the difference is gearing. If you took that BMW’s 400 ft-lb of torque and geared it down from 5000 to 10 rpm, it will be making 500 times as much torque, or 200,000 ft-lb of torque.
Horsepower takes this into account. A car’s horsepower = torque * rpm / 5252 (the 5252 is just a normalization factor, the important thing is that it depends on torque AND rpm).
So really, horsepower is the much more important number. A car that makes 400 ft-lb of torque at 2,000 rpm will accelerate just as fast as a car that makes 200 ft-lb of torque at 4,000 rpm, provided the proper gearing.
Unfortunately, that doesn’t tell the whole story either. That BMW M3 makes 450 horsepower, but not always. The “horsepower” number you see advertised is really PEAK horsepower, and it’s only true ta certain RPM. If a car makes 450 horsepower at 7500rpm, then it’s making less than 450 horsepower for the rest of the rpm range. This is where the torque number comes into play.
When you want to tow something, you want something with a broad torque curve… you need to be able to pull it from a standstill, and might not be able to wait for the engine to wind all the way up to 7500rpm. So you want a lot of torque all the way down to idle rpm’s, like 1,000rpm. That’s where towing capacity comes in.
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