“The Aston Martin Vanquish produces 824 horsepower and 738 pound-feet of torque,” the car journalist confidently rattles off before cutting to B-roll of the grand tourer speeding off into the distance, presumably with the might of 824 perfectly coordinated stallions.
If you’ve consumed any sort of content about cars, you’ve heard these two terms: horsepower and torque. On vibes and context alone, you probably know that they’re both measures of how strong an engine is and, transitively, how fast a car can go.
But what does each word actually mean on a scientific level? What’s the difference between the two? Lots of you have been asking, but we’ve got the answer. So let’s crack open our textbooks and sharpen our pencils—Motor101 is in session.
What Is Torque?
Photo by: Chevrolet
At its most basic definition, torque is the measure of how strong a twisting or rotating force is, and in the context of cars and engines, the twist we’re measuring is that of the engine’s crankshaft. Mathematically, torque is the product of multiplying force and distance.
To visualize this, picture yourself twisting a torque wrench: force is applied by your hands while the distance in this situation is the length of the wrench. The longer the wrench, the more leverage there is, multiplying the amount of twisting effort (or torque) produced by the same force exerted by you. The more torque there is, the easier it is to get that seized bolt off. The more torque there is, the easier it is to get your car moving away from a red light.
Torque = Force x Distance
In the context of an engine, the “force” part of the equation refers to the combustion that moves the pistons up and down (or side to side), while “distance” is the size of the bit connecting the conrods to the crankshaft (also referred to as the engine’s “throw”). When displacement is increased, the pistons are made bigger, so the force part of the equation increases, resulting in more torque. Turbochargers and superchargers that force more air into the combustion chamber also increase force, thus increasing torque.
In imperial (read: American), you’ll have noticed that automotive torque is measured in a unit called “pound-feet”—a term that I embarrassingly still cannot read printed out like that without picturing a dude physically punching a pair of human feet. Instead of having anything to do with bruised toes, though, pound-feet literally measures the torque you’d get from X pounds of force multiplied by a distance of Y feet.
For example, if a car makes 300 pound-feet of torque, that means the twisting force it can put out is equal to if you applied 300 pounds of force to a one-foot wrench. Or 30 pounds of force to a 10-foot wrench. Or 150 pounds to a two-foot wrench. Or simply one pound to a gigantic 300-foot wrench. Or—actually, you get it.
What Is Horsepower?
BMW B58 Inline-Six Engine
Photo by: BMW
If you only read a single sentence from this entire explainer, make it this one: Torque measures how strong the engine can twist the crankshaft, while horsepower measures how much work that torque does by accounting for how quickly the engine is spinning.
Mathematically, horsepower is a product of torque multiplied by revolutions per minute (rpm), and the reason why we even bother counting horsepower at all is that the amount of torque produced by an engine varies depending on how fast it’s spinning. Where torque is largely a measure of force, horsepower handily captures both force and engine speed in a single number.
Engine speed matters because your engine almost certainly produces less torque (and in turn, less hp) at idle than it does, say, approaching redline. The big torque and hp numbers quoted by car manufacturers, spec sheets, and reviewers, by the way, are the engine’s peak torque and hp—the car’s not actually making all of that all of the time.
More precisely, horsepower is always equal to (torque x rpm)/5,252. For the math whizzes, this is why dyno charts always have the hp and torque lines crossing at 5,252 rpm as long as torque is being measured in pound-feet and hp is being measured in, well, mechanical hp. Look for this the next time you visit a dyno. It’s a neat rule.
Horsepower = (Torque x RPM) / 5,252
Unlike torque, horsepower is its own unit of measurement, and contrary to what the word suggests, 1 hp isn’t really analogous to the power output of a single adult horse, at least not in the way you’re probably picturing. Disappointing, I know.
Back in the late 18th century, Scottish engineer James Watt needed a way to convey the power of the steam engines he was selling in a way that made sense to people who were familiar with, well, horses. Lore says he observed ponies moving coal in mine hoists and approximated (very approximately) that one full-size horse was able to steadily, sustainably lift 33,000 pounds of coal one foot per minute—there’s 1 horsepower, defined.
Plug that definition in along with the formula for the circumference of one revolution, and you get the “5,252” formula above. And, yes, just to confirm, James Watt is the same guy that that other unit of measuring power, watt, is named after.
ELI5: What Does It All Mean?
In practice, a car’s torque is what pushes you back in your seat from a standstill or from low rpm, since horsepower is unlikely at its peak here, given the latter’s mathematical definition. Once revs climb, though, torque begets horsepower, which is what keeps you accelerating at highway speeds and beyond.
This is an admittedly crude way to think about it, but holding gearing, aero, and weight constant, you can think of horsepower as a better measure of how fast a car can go in the “top speed” sense of the question, whereas torque arguably matters more in the “0-60” sense.
Electric cars use motors that can hit peak torque practically in an instant, but often taper off at the top end due to battery limitations. Big displacement trucks and turbocharged sports cars have huge torque as well, mostly because the force part of their equations is that much bigger. Naturally aspirated performance cars, meanwhile, often lack torque low down and require hard revving to access prime horsepower.
- Torque = Force x Distance
- Horsepower = (Torque x RPM) / 5,252
Of course, carmakers can and do manipulate how quickly a car feels through gearing. It’s also worth noting that because drivetrains aren’t perfectly efficient machines, the hp and torque that come out of the crank won’t be the same as the amount actually getting onto the road. The former is referred to as hp or ” torque “at the crank,” or brake horsepower (bhp), while the latter is referred to as wheel torque or wheel horsepower.
For those who instinctively scroll to the bottom to get the one-paragraph summary, here’s what you need to know: In theory, torque measures engine twist and horsepower measures how much work that twist can do at a given engine speed. In practice, torque gets you going while horsepower keeps you pulling as speeds climb.
