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The 2018 BMW M5 boasts 600 horsepower and 553 lb-ft of peak torque.

Jason Tchir

I’m shopping for a car and I’m overwhelmed by all the jargon in the ads. But one I really don’t understand is torque. What is it, and why should I care? – Alex

For far longer before Ricardo Montalban purred about the softness of Corinthian leather, car companies have used phoney buzzwords. But torque isn’t one of them.

“Torque isn’t a made-up thing,” said Robert Karwel, senior manager of the Power Information Network at J.D. Power Canada. “People understand what horsepower is, but torque is less understood.”

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Both measure an engine’s output. Torque measures an engine’s turning force and horsepower measures its power. Torque is a force applied over a distance. It’s measured in pound-feet (or in metric, newton metres).

For instance, think about trying to get the lug nuts off a wheel to change a flat. If you have to apply 10 pounds of force to push down on a wrench that’s a foot long in order to loosen them, that’s 10 lb-ft of torque (and those lug nuts should have been tighter).

So what’s turning on your car? The engine’s crankshaft and, ultimately, the wheels.

And what’s the power? Power is the amount of force over a specific amount of time. It’s a measure of how much work your engine can do.

To calculate it, you multiply torque by engine speed. And to get the horsepower, a measurement invented by James Watt to sell steam engines by showing how many horses they could replace, that’s then divided by 5252 – for math reasons.

Wheel torque

The torque produced by the engine changes by the time it gets to the wheels. Some is lost along the way.

But mainly, it gets magnified by the transmission, the same way a longer wrench lets you produce more torque on that lug nut.

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“Different vehicles will have different gear ratios and different gear-shifting control strategies, which will affect how much torque reaches the wheels during an acceleration,” Jennifer Bauman, an assistant professor at McMaster University’s faculty of engineering, said in an e-mail.

So which number – torque or horsepower – matters on the road? Good question – and one that’s often debated.

“You can’t have horsepower without torque,” Engineering Explained’s Jason Fenske said in a YouTube video. “But the most important figure if you’re looking at how fast ... a car [will] be able to accelerate – what will its top speed be – that comes down to power.”

There are plenty of analogies on how to understand the two. A common one is: “Horsepower is how fast you hit the wall, torque is how far you take the wall with you.” Technically, that’s wrong.

“Horsepower is a good measure of how fast you hit the wall, but torque is more like how much horsepower you have per turn of the crankshaft just before you hit the wall,” writes Sean Murray at Motorhub.

Car makers typically give you numbers for peak torque and peak horsepower – but they’re delivered at a specific engine speed.

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For example, BMW’s M5 performance sedan delivers 600 hp between 5,700-6,600 r.p.m. and 553 lb-ft of peak torque from 1,800-5,700 r.p.m. Electric motors deliver their peak torque at 0 r.p.m.

“Higher torque at lower r.p.m. means you have a lot more horsepower at lower r.p.m., which makes it easier to tow things from a standstill. That’s usually how trucks are geared,” Murray said. “High torque at higher r.p.m. means more power while you’re already underway, which usually results in higher top speed. This is how sports cars are geared.”

So what numbers should you be looking for? If you’re just looking at a light compact sedan, you may not need a lot of torque, Karwel said.

“If a four-cylinder gas engine can produce 150 lb-ft of torque and 170 hp – it doesn’t need a lot of twisting force to get going, so it can deliver that torque at a very quick rate,” Karwel said.

And, ultimately, because the transmission plays such a big part, those peak torque and horsepower numbers might not tell you a lot on their own, Bauman said.

“As a buyer, it is better to compare the specs that matter to you, rather than just engine torque or power,” Bauman said. “For example, I suggest looking at the 0-60 m.p.h. acceleration times and the max towing capability.”

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