Image: Paccar

Image: Paccar

A little while ago I had one of those immensely annoying – and unwinnable – arguments in an online forum with a guy I’d never met. Nowadays just about anything you write in such a context is met with vitriolic anger if you happen to present an opposing opinion. Now there is only confrontation. Discussion has become impossible.

You’d think the touchy topic was the wisdom of wearing a face mask (I’ll never, ever be able to figure out why that’s contentious), or maybe that old and seemingly forgotten one, climate change. Nope. Nothing political at all.

It was torque, specifically its definition. I wandered into this discussion thread and immediately saw a big mistake in one fellow’s explanation of engine power, so I gently, respectfully offered my alternate view. In response he exploded in anger. I knew I was right because I’d written a long feature story on the subject 22 years ago, with the help of several engineers. That story drew a letter of praise from Roberto Cordaro, who was then president of Cummins’ on-highway engine business, and another from a Meritor executive.

I quickly saw there was no reason to continue an ugly "chat," so I dropped it, but it occurred to me that if a gearhead in an automotive forum misunderstood torque, he couldn’t be alone. So, whittling that 1998 article way down, here’s how I explained things…

I started with racing cars in the top echelons of motorsport. They can accelerate to 60 mph in little more than a second or two and reach incredible top speeds three times that fast. But do you think they could pull a load of firewood out of the bush? Or haul a house trailer up the Montreal Hill in northern Ontario? No way. The fact is, they can barely get themselves moving from a dead stop.

Compare that to yourself at the wheel of a truck. You don't have to feed the engine any fuel at all to get rolling in most cases. You can probably just idle away, even though you might be pulling 80,000 lb or more and you've only got half the horsepower of that race car.

What gives? Torque, that's what, and of course gearing. That racing engine might produce 900 hp but only 100 lb ft of torque down low on the tach, compared to the 1,800-plus lb ft that your diesel churns out at 1,200 rpm or even lower.

Torque is pure twisting force — not how fast an engine can do work, which is horsepower — but just the bare potential for work arising out of that twisting motion. As the torque figure rises, so does the amount of firewood you could haul with Richard Petty's race car.

Add the element of time to this mix and you're now talking horsepower. The more horses you've got, the faster you could pull that wood or climb that grade. It's a calculated value, directly tied to torque, that measures the rate at which the work gets done. Oddly enough, it has its origins in Scotland.

Nearly two centuries ago, Scottish inventor James Watt decided that the industrializing world needed a way to measure the output of his steam engine. So he measured how much work a good horse could do and found it could lift 330 pounds 100 feet in one minute. Thus the term, ‘one horsepower’.

How much torque is involved there? That's expressed as 33,000 lb ft. We get that by multiplying 330 pounds (the amount the good horse can move in a minute) by 100 feet (the distance he can move it). Put another way, one horsepower is the ability to do 33,000 lb ft of work in one minute.

Getting a little more technical, Cummins said "the torque output of an engine is a measure of the amount of turning force it produces which will move a load. Torque is a force, or load, applied in a circular path and measured in pound feet. One example of torque would be to loosen a screw-type lid from a tightly sealed jar."

Torque is the amount of load multiplied by the distance at which the load is applied. For example, a torque wrench could be one foot, two feet or four feet long. The bolt head is at the end of the wrench and the distance for determining the torque is measured from the centerline of the bolt head to the point at which the load is applied. If you apply a load of 50 pounds at a distance, or lever arm, of one foot, the value would be: torque = 50 pounds x 1 foot = 50 pound feet. Make that a load of 25 pounds at a lever arm of 2 feet, and you'd have the same result: 25 pounds x 2 feet = 50 pound feet of torque.

In an engine, torque is generated by the pressure load of the expanding gases on the top of the piston times the stroke, meaning how far the piston moves.

Two basic principles apply:

  1. torque is stronger at the lower end of an engine's operating range, while horsepower is higher at the upper end; and
  2. a bigger displacement engine will produce more power than a smaller one, simply because there's more area for combustion to force down those pistons.

OK, now fire away with your side of the story!

About the author
Rolf Lockwood

Rolf Lockwood

Executive Contributing Editor

Rolf Lockwood is editor emeritus at Newcom Business Media, which publishes Today’s Trucking.

View Bio