January 2014, TruckingInfo.com - Editorial
Racing cars in the top echelons of motorsports have tons of horsepower. But could they pull a load of firewood out of the bush? No way. They can barely get themselves moving from a dead stop.
Compare that to driving a truck. You don’t have to feed the engine any fuel at all to get rolling, and you’ve only got maybe half the horsepower.
What gives? Torque, that’s what. And of course gearing. A racing engine might produce 900 horsepower but only 100 pounds-feet of torque down low on the tach, compared to the 1,400-plus pounds-feet that even a modest diesel churns out at 1,200 rpm or so.
What is torque? It’s pure twisting force – not a measure of how fast the 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.
The more horsepower you’ve got, the faster you could haul that wood – or climb a 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 that in one minute, it could lift 330 pounds 100 feet. Thus the term, “one horsepower.”
How much torque is involved there? That’s expressed as 33,000 pounds-feet. 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 pounds-feet of work in one minute.
Getting a little more technical, a Cummins document says “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 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 force multiplied by the distance at which the force is applied. For example, a torque wrench could be 1 foot, 2 feet or 4 feet long.
The bolt head is at the end of the wrench. The distance for determining the torque is measured from the centerline of the bolt head to the point at which the force – or load – is applied. If you apply a load of 50 pounds at a distance, or lever arm, of 1 foot, the equation would be: torque = 50 pounds x 1 foot = 50 pounds-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 pounds-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.
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.
But, hey, just remember that more torque is a good thing.