It is automotivedom’s most oft-asked technical question. 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. Hell, most of the autojounalists I know get all tongue-tied when they’re asked to explain …
the difference between torque and horsepower.
Infographic: Where does horsepower come from?
The first thing you need to know along this path of understanding is that, while torque is a real, measurable force, horsepower is but an artificial construct. Indeed, torque is nothing more than that leverage ratio lesson we were supposed to learned in Grade Eight 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 — and, damn, I’ve got to remember to re-up my CAA dues — 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. Easy-peasy, right?
The force applied to a one-foot wrench is a simple way of practically demonstrating the principle of torque.
Kevin Van Paassen, National Post
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’ ability to actually 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, an acolyte of Sir Isaac Newton, proposed a more optimistic 44,000 foot-pounds of work every minute as the equivalent of one horse’s power.
Also read: These are the top 10 dollar per horsepower bargains
Besides being arbitrary, there was nothing high-minded about Mr. 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. “My new, improved Newcomen steam engine can do the work of three horses while Bob’s over there can only do the work of two.” Mr. 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 Mr. Watt’s pace for very long — Sir John’s estimate would seem to have been a more accurate assessment of how much coal one horse could reliably haul out of a mine. It matters not a whit. Mr. Watt’s machinations might have been artificial, but it is an artifice we have all come to accept.
The 3.5-litre V6 engine of the Lexus RC 350 produces 307 horsepower and 276.6 pound-feet of torque.
Russell Purcell, Driving
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 in an article about torque and horsepower — the one thing that should stand out is that horsepower is very much a speed-dependent quality while torque is not. What that means, in plain, simple English, is that an engine’s torque is the leverage that lets a diesel-powered Mack truck haul a fully loaded 48-foot trailer up a long, 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 in this issue of 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 2.0L turbocharged four. But the 2.0L 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 powerplant. Oh, given its head and a long enough straightway, 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.
*Aficionados will, of course, recognize this leverage ratio argument as the reason the proverbial long-stroke engine produces more torque and horsepower, the longer stroke (quite literally the distance the piston travels up and down in the engine) literally acting like the longer lever arm (or, in the case of the example in the story above, a longer wrench) that produces more torque. Diesel engines are renowned for their torque as well, in part because they are typically longer stroke engines. Throw in the fact that diesel fuel has 11% more energy per gallon of fuel than gasoline and that diesels usually run much higher compression ratios — typically 17.0:1 compared with a maximum of 13.0:1 for gas engines — and you have a recipe for mondo torque. But, because all that internal pressure requires more robust — that should be read heavier, sometimes much heavier — bits and pieces, they can’t rev very high, hence the reason most turbodiesels lag gas engines in horsepower production.
