You're Still Wrong About What Torque Is

Illustration for article titled Youre Still Wrong About What Torque Is

The internet has been crafting explainer videos and quippy analogies about how to explain the difference between an engine’s horsepower and torque figures. According to Jason Fenske of Engineering Explained you’re still just not getting it. You’ve likely heard the explanation “horsepower is how fast you hit the wall, and torque is how far you drag the wall with you.” I’ve been around cars a long time, and I’ve heard it a bunch. Nah dude, SCIENCE.

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Fenske is doing a yeoman’s job of attempting to teach the layman about the science of cars. It’s difficult to beat that information into our tiny little brains, but little by little it’s happening. This time he’s breaking down the science behind vehicle torque figures, what they mean, and how they effect our driving experience.

In the video, you’ll see the difference between power, torque, power-to-weight ratios, and torque-to-weight ratios. It’s a short video with some valuable information held within, so click that big red play button and learn a thing or two about torque.

Jalopnik contributor with a love for everything sketchy and eclectic.

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DISCUSSION

I’m a mech eng with a rather profound knowledge of engineering dynamics.

I have sat myself down a dozen times the last 20 years, intending to write up a good explanation what torque really is compared to power.

Each time I have instantly got lost in terminology. Linear vs polar, etc. Once I reach a certain wall of words, I give up.

So I resort to analogies. Power hauls ass. Power is torque times speed. Power is like volume. Torque is like area. Speed is like height. If you want a box with a lot of volume, you want both large area and height. Lacking one you can increase the other, and you know what? It became a bloody wall of words again!

Perhaps I should keep it as simple as possible. Engine A has 150 BHP @ 6000 rpm. Engine B has 300 BHP @ 6000 rpms. Engine B must have twice the torque as engine A. Period. Nothing special there.

When you are awed that engine B accelerates twice as hard as engine A, it is not thanks to the torque, because it is power that accelerates masses. You are experiencing twice the power, even if it is down the rpm register. Say that you are doing 2000 rpms - engine B will produce 100 BHP, while engine A produces 50 BHP.

The torque number is academic and uninteresting outside the engine design department.

This is a semi-truth, because it ignores the fact that different engines have different torque curves. An ideal engine (like A and B in my example) has a perfectly flat torque curve, because any deviations down from peak means suboptimal combustion and timing.

Real engines have dozens of various nonlinear optimizations to tune in, to they are usually set to perform best down low or up high (or use various tricks to widen the optimal band).

Up high is better for maximum performance, but requires a lot of gearing to keep in the power band.

Down low is convenient for towing, because if the rpms drop a bit due to increased resistance (inclines), the engine just gets a bit stronger. Less gearing required. Such engines put out less top power though, compared to their weight.

And large truck engines are designed to produce an order of magnitude more power than the average car engine continuously, for a million miles. Hence they are really large and coarse, and hence they operate at rather low rpms, and hence they have tremendous torque in order to produce the desired power.

Thus the immense torque of a Scania is an unfortunate necessity, not a goal in itself. I say unfortunate, because it requires a lot of steel to survive that load. It would be way more convenient if it could operate at high rpms, because then crankshafts, gearboxes, driveshafts and so on could be a fraction of the size they are now, but such an engine would wear out prematurely.

Hell, it became a wall of words anyway. Nobody will read this.