Torque vs Horsepower: Which Is More Important?
Any gear head or car guy loves power, and the more the better. Few experiences are as satisfying as the neck-snapping, back-pressed-into-the-seat feeling of acceleration that can be had at the expense of some gasoline and tire tread. Engine power has to be adequate for such acceleration, however, and that’s the subject of this article.
Engine output has for decades been quantified by “horsepower,” although in recent years manufacturers have been listing torque values as well. The reason is that, frankly, torque is more important. Let’s look at what these measurements are and then put them into practical use.
First, let’s not confuse power with work. “Work” is the force used to lift or push or pull over a distance. When you lift the coffeepot and hold it there you are doing work. Power, by definition, is the act of producing work over some specific time. The familiar “horsepower” is a term that stems from the 1700s and was created by James Watt to sell his steam-powered water pumps. He calculated (optimistically, as things turned out) that one horsepower is the ability to lift 550 pounds one foot in one second, presumably the power an average horse can exert without killing itself.
Just before the turn of the 20th Century carriage makers turned to internal combustion engines – rather than the trusty, if not smelly and high-maintenance horse – to propel their new carriages, but they used the tried-and-true measurements of horsepower to describe their new engines’ capabilities. Everyone understood it, because they could relate to it. In today’s world few of us have any regular interaction with horses, so the term “horsepower” is somewhat esoteric. Besides, it really doesn’t explain how a car accelerates.
That takes us to Jan and Dean.
What? Hang on, readers, because this will all make sense. Jan and Dean wrote and performed what is, arguably, the greatest car song of the 1960s, Dead Man’s Curve. In it, a race between an XKE and a Corvette ends in disaster. The gist of the story is that a mindless idiot driving a Corvette is challenged to a drag race by another mindless idiot driving an XKE. They end up going too fast and, after dramatic sound effects of crashing cars, the singers end the song thusly: “Well, the last thing I remember, Doc, I started to swerve, and then I saw the Jag slide into the curve. I’ll never forget that horrible sight, and I found out that everyone’s right…Won’t come back from Dead Man’s Curve.”
The important point to the song is that it is technically accurate. That is, the Corvette out-accelerated the XKE, although both cars were virtually equal in power-to-weight ratios, at about 10.5 pounds to the horsepower (3,200 lbs/300 hp for the Corvette, 2,800 lbs/265 hp for the XKE). Both cars had nearly identical gear and rear axle ratios.
So how could the Corvette have been so much faster than the Jag? In a word: Torque. The Corvette had 100 lb-ft more torque than the Jag, making it over one second faster from 0-60 mph.
We need to discuss the way engines actually accelerate cars. Obviously, a certain amount of power is required to keep a car rolling in the first place. In the case of a that Corvette, let’s say a force of 400 pounds is required to push it along at 60 mph on level ground. To translate that into horsepower (all we have is a force at the moment), we need to add the time element.
Since 60 mph is 88 feet-per-second, to calculate how much horsepower is needed we just multiply the 400 pounds of force times 88 ft/sec and we get 35,200 pounds-feet-second. Since we know that 550 pounds-feet-second equals one horsepower, we just divide 35,200 by 550 to get 64 pounds-feet-second, or 64 horsepower.
But that 64 horsepower isn’t accelerating the car, just moving it along. We need torque to accelerate. Torque, by definition, is a moment of force that produces rotation – or torsion – and is the product of tangential force multiplied by the radius of the part rotated.
Confused? Look at it this way: If you picture a horizontal arm one foot long and hang a one-pound weight on the end of it, you will have a torque of one pound-foot acting on whatever the other end of the arm is attached to. If we want to express this work as power, we must add the time dimension to it. Thus, we would have one pound-foot-second.
Back to the Engine
The engine above, while producing horsepower, is also producing torque. That’s because it is rotating, and we should be able to calculate its torque. In the case above, let’s assume the Corvette’s engine is spinning at 3000 rpm. By dividing the 3000 rpm by 60, we get 50 revolutions per second, right?
Okay, since we know from above that the engine produces 35,200 pounds-foot-second, all we need to do is eliminate the time factor to get the torque number. We do so by dividing 35,200 pounds-ft-sec by 50 revs-sec and we get 704 pounds-ft-revs.
So what the hell does that number mean? Not to worry. We’re talking about circular motion, right? All we need to do is remove the circular component from the number above and we get what we want. To do so is simply to introduce the concept of the “radian.” A radian is the length of the radius of a circle laid onto the circumference. Without going into painful – and boring – geometry lessons, just take our word for it that there are always 6.2832 radians in any circumference because of the relationship of Pi, or 3.14. Therefore, if one revolution has 6.2832 radians, we divide the 704 pounds-foot-revs by that number to get 112.04 pounds-foot-radian. Since radians have no actual value, we drop the word and end up with 112 pounds-feet of torque.
Okay, So Why Is The Corvette Faster?
It should be apparent that any engine produces torque because it is turning. Horsepower is produced by the exploding fuel in the cylinders, which turn the crankshaft, etc., creating the twisting force (torque) that you have at the flywheel. It follows that the greater the torque at the low end of the rev range, the less resistance the car’s weight has against the engine’s tendency to turn. The less resistance, the faster the car moves.
In the case of the Jag vs Corvette, the Vette’s engine has 100 pounds-feet more torque than the Jag’s. On top of that, the V8 configuration of the Corvette’s engine produces far greater torque at the low end of the rev range, hence greater acceleration. Of course, eight cylinders produce two more power strokes per revolution than the Jag’s six cylinders. It all adds up.
The real measure of how fast a car will accelerate is its torque, and at which rev range the maximum is developed. Horsepower is of secondary importance actually, but it sounds better than “torque.” Today’s cars with small displacement engines are fast and agile because they produce torque in another way: gearing. Five and six-speed transmissions use lower gears to multiply engine torque, creating terrific acceleration and good fuel mileage, although the “super cars” still use big engines.
Pounds-Feet or Foot-Pounds?
Technically, it’s “pound-feet,” because work is defined as force over distance. However, decades of usage of the term “foot-pound” by nearly everyone – including engineers – makes either term acceptable.
Anyway, who cares how torque is expressed, as long as there’s plenty of it!