When assessing a car’s “on paper” specs, the first thing that most people look for are the car’s horsepower and torque ratings, and rightfully so. These two figures are the best ways to assess how powerful, quick to accelerate, and generally fast the car will be (although weight and transmission type do play big roles here, too). Most consumers are aware of this, and take horsepower and torque ratings at face value.
However, most consumers don’t know what these two performance metrics actually mean, i.e. what they are actually measuring. In this article, we’ll try to fill in this information gap and explain exactly what horsepower and torque really are. Many articles on this topic get bogged down with math and formulas. Rest assured, I hate math as much as you do, so we’ll stick to simple, practical explanations.
First Things First: How Does an Engine Work?
Before we can understand how horsepower and torque work in the context of cars’ engines, it is first important to develop a working understanding of what goes on inside of an engine. This is a quick and dirty explanation, but it’s all we need to get the job done. Very basically, an engine works by igniting a mixture of air and fuel.
In gasoline-powered cars, this mixture is ignited using a spark plug. This ignition creates a small explosion inside of the cylinder, and the pressure it creates then pushes the piston downwards (in most engine configurations). The piston is attached to the crankshaft, and because of this, when the piston is pushed down, the crankshaft spins. This rotational motion of the crankshaft is then transferred to the car’s wheels via the transmission, and associated driveline parts.
The Difference Between Horsepower and Torque
Now that we know how an engine works, we can talk about the differences between horsepower and torque. As promised, I will stay as far away from math and formulas as possible, and stick to only what you need to know in practice. That said, there are two very basic formulas that are necessary in order to properly flesh this out. So, let’s start with torque, the formula for which is this: torque = force*distance.
That’s it, just two variables. In simple terms, torque is a rotational force, or how forcefully you can get something to spin. For example, think of when you are tightening a lid onto a jar. You’re applying torque to that lid in order to close it: the force of your muscles twisting the lid, multiplied by the distance at which the force is applied (in this case the distance would be very low, because your hand, which produces the force, is presumably on the lid). So how does this rotational force apply to engines? In an engine, the torque figure is measuring how hard the piston presses down and thus spins the crankshaft. The more torque an engine has, the more forcefully it rotates the crankshaft.
Now on to horsepower, which is very closely related to torque. The simple formula for horsepower is: horsepower = torque*rpm. In practice, horsepower is how quickly work can be done, or in this case, how quickly torque can be applied. In a car engine, think of horsepower as how quickly the crankshaft is spinning. As we know, the crankshaft’s rotation is a result of the piston pushing down—and that force is torque. That’s why we say that horsepower is a function of torque. To sum these two up, think of it like this: torque is how forcefully the piston pushes down, and horsepower is how fast it can do it.
Horsepower and Torque in Practice
Now that we know what horsepower and torque are, let’s take a look at what they mean out on the road. Your first question is probably which one is more important for acceleration? One could argue, with some degree of correctness, that horsepower is more important, because you could have all the torque in the world, but if you’re only applying it at 1 revolution per minute, you’re not going to be getting anywhere very quickly. While that is true, saying horsepower is more important is a bit misleading, because you do need torque to get horsepower. So one is not really the end all be all of acceleration, because you need both.
Where the rubber meets the road
To round things out, let’s take a look at a real life example. You’re sitting at a stop light in your 2009 BMW 335d. Your car is equipped with a 3.0L I6 diesel engine that makes 265 horsepower, and 425lb-ft of torque. All of a sudden, a 2009 BMW M3 pulls up next to you, looking to race to the next stop light. His car has a 4.0L V8 engine, which makes 414hp and 295lb-ft of torque.
You’re paying close attention to the numbers as you decide who’s going to win this race, and realize that your cars have very close to opposite horsepower and torque outputs: his car has about as much horsepower as yours does torque, and vice versa (M3: 414hp, 295lb-ft / 335d: 265hp, 425lb-ft).
Your cars are both equipped with the same 6-speed manual transmissions, and weigh about the same, you think it should be close. You wring your fingers tightly around the steering wheel, a bead of sweat dripping down your brow as you wait anxiously for the light to turn green. The M3 driver revs his engine to show his engagement with the competition. It’s the ultimate showdown of horsepower vs. torque. In a frenzy you dump the clutch as the light turns green, and with a squeal your tires grip the pavement and you’re off!
Despite your efforts, shifting as fast as you can and flooring the pedal with all your might, the M3 wins, and by quite a lot, too. What happened? Well, in an example like this where two cars weigh roughly the same, have a similar transmission and have roughly opposite horsepower and torque ratings, the car with the higher horsepower will win every time, because while its pistons aren’t quite pushing down as hard as the others are, they’re doing so much faster, which is enough to overcome the torque deficit.
Now, if this were a battle of which car could tow the most, the car with more torque would win just as handedly. That’s because towing requires force, not speed. The car whose pistons can push down the hardest, not the fastest, will be able to get that heavy load moving more easily.
As we can see, horsepower and torque do serve different purposes, but are very much interrelated. You can’t have horsepower without torque, and you can have tons of horsepower, but without adequate torque, it won’t equate to much. So, the next time that you look at a spec sheet, those numbers will be more than just meaningless marketing tools, because now you actually know what they mean!