Editor’s note: Written by Grant Feek, chief executive of Tred.
When assessing a vehicle’s specifications, one of the first thing car buyers look at are the horsepower and torque ratings. These two figures are the best ways to assess how powerful, quick to accelerate and fast a speed a car or truck can reach.
Most consumers know that and take horsepower and torque ratings at face value.
However, most consumers don’t know what these two performance metrics mean. And they often forget that the weight and transmission type of the vehicle also plays a role in speed and acceleration.
To understand how horsepower and torque work in the context of cars’ engines, buyers need to know what goes on inside an engine. That is a quick and dirty explanation, but 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 the cylinder, and the pressure it creates then pushes the piston downward in most vehicles. There are some other 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 is then transferred to the car’s wheels via the transmission and associated driveline parts.
Simply put, torque is a rotational force, or how forcefully something spins. For example, think of tightening a lid onto a jar. You’re applying torque to that lid to close it. Torque is the force of your muscles twisting the lid, multiplied by the distance you apply the force. 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? The torque figure measures how hard the piston presses down and thus spins the crankshaft in an engine. The more torque an engine has, the more forcefully it rotates the crankshaft.
Now on to horsepower, which is 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 an engine, think of horsepower as how fast the crankshaft is spinning. As we know, the crankshaft’s rotation results from the piston pushing down—and that force is torque. That’s why we say that horsepower is a function of torque.
Think of it like this: torque is how forcefully the piston pushes down, and horsepower is how fast it can do it.
Now that we know what horsepower and torque are let’s look at what they mean out on the road.
Which one is more important for acceleration?
One could argue, with some degree of correctness, that horsepower is more important. That is because you could have all the torque in the world, but if you’re only applying it at one revolution per minute, you’re not going to be getting anywhere very quickly. While that is true, saying horsepower is more important is misleading because you need torque to get horsepower. So one is not the end all be all of acceleration because you need both.
To round things out, let’s take a look at a real-life example. (We will use sporty sedans for the example because it makes the point better than a couple of pickup trucks. Also, this is an example – we don’t recommend drag racing.) You’re sitting at a stoplight in your 2009 BMW 335d. Your car is equipped with a 3.0L I6 diesel engine that makes 265 horsepower and 425 pound-feet of torque. Suddenly, a 2009 BMW M3 pulls up next to you, looking to race to the next stoplight. His car has a 4-liter V8 gas engine, which makes 414 horsepower and 295 pound-feet 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.
Both cars have the same 6-speed manual transmissions and weigh about the same. You think the race will 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. It’s not close. 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 vehicle 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 would tow the most, the car with more torque would win just as easily. That’s because towing requires force, not speed. The vehicle whose pistons can push down the hardest, not the fastest, will be able to get that heavy load moving more efficiently.