Not many people understand what horsepower and torque really mean, let alone how it affects the performance of the vehicle. Nevertheless, almost every ad for a heavy-duty truck will mention those specs at some point. If you’ve never noticed, try listening for it next time you see one.
The power produced by an engine is called its horsepower. In physics, power is defined as the rate at which something does work. For cars, horsepower translates into speed. So if you want to go faster, and get up to speed quicker, you need more horsepower.
Torque, on the other hand, is the expression of a rotational or twisting force. In vehicles, the engines rotate around an axis, thus creating torque. Torque can be viewed as the “strength” of a vehicle. It is the force that rockets a sports car from 0-60 in seconds and pushes you back in the seat. It is also what powers big trucks hauling heavy loads into motion.
These are the basics of horsepower and torque, but how are these concepts measured and how are they interrelated?
Behind the Numbers
Mathematically speaking, horsepower is the force needed to move 550 pounds one foot in a second, or 33,000 pounds one foot in a minute. Engine horsepower is measured using a dynamometer, but what the dynamometer really does is measure the torque output of the engine, as well as the RPM – or “revolutions per minute”. These numbers are plugged into a formula (torque x RPM / 5,252) to determine the horsepower. Horsepower is found by measuring torque because torque is easier to calculate.
Torque, as previously mentioned, is the expression of a twisting force and is measured in units of force times distance from the axis of rotation. So for instance, if you use a 1-foot-long wrench to exert a force of 10 pounds on the end of a bolt, then you are applying a torque of 10 pound-feet (10 lb-ft).
Relationship Between Horsepower and Torque
Both horsepower and torque affect a car’s overall speed, so you can see why people mix the two up. However, in the real world of driving and hauling, their differences—along with vehicle’s design—have a significant impact.
For instance, the greater the horsepower an engine produces, the higher the torque potential. The way this “potential” torque translates into real-world applications is through the vehicle’s axel differentials and transmission. This explains why a race car and a tractor that have the same amount of horsepower can vary so drastically. In a race car, all the torque is used for speed through the gearing, whereas the tractor converts the horsepower into pushing and pulling extremely heavy weights.
Another way to understand horsepower vs. torque is twisting off the cap on a new pickle jar. When you are using all your strength to open the jar, you are applying torque—regardless of whether or not the cap comes off. Horsepower, however, only exists with movement. So you need torque to loosen the lid first, and then you can apply horsepower with your hand by spinning the lid of quickly.
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