The difference between horsepower and torque
Consumers are confused by it, slammed Civic-driving hipsters only pretend to understand it and even seasoned hot rodders — able to quote valve clearances ad nauseam and rebuild cylinder heads in the dark — can’t really explain it. Even most of the automotive journalists I know get all tongue-tied when they’re asked to explain the difference between torque and horsepower.
The first thing you need to know is that, while torque is a real, measurable force, horsepower is but an artificial construct. Torque is nothing more than that leverage ratio lesson we were supposed to learn in Grade 8 physics, the one teachers always illustrated with a fat kid on one end of a teeter-totter.
More practical to everyday life: say you’re on the side of the road with a flat tire that needs replacing. If you apply 25 pounds of force on the end of a one-foot-long wrench to undo the lug nuts, you’ll have, quite literally, generated torque: 25 pound-feet to be exact. If you’ve been particularly negligent about your maintenance — you didn’t rotate your tires either, did you? — and everything is rusted in place, you’ll have to flex your triceps a little more, say to the tune of 50 pounds, or get a longer, two-foot wrench. Either way, you will have generated 50 lb-ft of twisting motion, probably more than enough to unstick the most recalcitrant of wheel nuts.
That, my friends, is all you need to know about torque.
Horsepower, on the other hand, is a much more convoluted matter. For one thing, it’s a measure of power, which, for lack of a simpler explanation, is the rate at which work is done. If that’s not confusing enough, the very definition of one horsepower — the ability of a horse to haul a 33,000-pound load one foot in one minute — is completely arbitrary.
Though the inventor of the horsepower, James Watt (yes, he of the electrical watt), claimed that he actually measured various horses’ abilities to pull said loads, an entire raft of scientists of the same era begged to differ. Sir John Leslie, for instance, obviously employing less stout ponies, determined that one horse could move but 22,500 foot-pounds in a minute, while John Desaguliers proposed a more optimistic 44,000 footpounds of work every minute as the equivalent of one horse’s power.
Besides being arbitrary, there was nothing high-minded about Watt’s experiments. In fact, his sole motivation seems to have been that he was looking for an easily advertised method of marketing the steam engines he was quite literally hawking.
Watt’s definition may now be the universal definition of what a horse can do, but that has more to do with his ability to sell steam engines than scientific rigour. Indeed, even back in the day, scientists were well aware that few were the horses that could maintain Watt’s pace for very long..
It matters not a whit. Watt’s machinations might have been artificial, but it is an artifice we have all come to accept.
Without going into all the technical gee-whiz about gear ratios and high-compression pistons — though I will include the basic formula for horsepower (hp = torque X rpm/5,252) because it would be totally remiss not to in an article about torque and horsepower — the one thing that should stand out is that horsepower is very much a speeddependent quality while torque is not. What that means, in plain English, is that an engine’s torque is the leverage that lets a dieselpowered Mack truck haul a fully loaded 48-foot trailer up a long,
His sole motivation seems to have been that he was looking for an easily advertised method of marketing the steam engines
seven-degree hill while horsepower is what makes a Formula One Williams scream down the back straight of Monza at 370 kilometres an hour (in 2004, that is; F1 regulations have recently clipped the wings of the world’s fastest race cars).
As extreme as those two examples may be, the same basic premise applies in real-world driving.
The new 2016 Kia Sorento, tested by Driving, is a perfect case in point. The 3.3-litre V6 in Kia’s lineup boasts 290 hp, 50 more than the comparatively paltry 240 hp offered by the company’s secondary offering, a 2.0-L turbocharged four. But the 2.0-L T-GDI generates slightly more torque — 260 lb-ft versus the V6’s 252 — and, more noticeably, at a lower, more convenient rpm — 1,450 rpm versus the V6’s 5,300 rpm. The result is that, despite its significant 50-hp deficit, it is the little turbo four that “feels” like the top-of-the-line power plant. Oh, given its head and a long enough straightaway, the V6 will almost assuredly prove the speedier of the two. But in most circumstances, especially those that matter to owners of run-of-the-mill CUVs, the torquier turbocharged four is more responsive to the driver’s demands and passes slower-moving traffic with ease, while the V6 always feels like it has to gather itself before getting a move on.
Horses may indeed be for courses, but torque will get your minivan moving.