TORQUE: USEFUL CONCEPT or AUTOMOTIVE RED HERRING ?
By Bob Palmer
VIN:
March 4, 2000
Practicalities:
But getting back to the drag strip, how do we get down the quarter mile the quickest? Given an unlimited amount of either horsepower or torque, the thing that ultimately limits the acceleration of a dragster is adhesion between the tires and the asphalt, and this limit implies a maximum acceleration; i.e., smoke ‘em, but not too much, all the way down the strip. This idea implies that we want a motor that puts out constant torque; i.e., constant acceleration versus rpm. With this ideal motor we would have just one gear and just let it wind up and keep producing constant torque and acceleration, but also higher and higher horsepower from start to finish (remember, power equals force times speed). While an electric motor comes close to such an ideal, flat torque curve, unfortunately internal combustion engines behave quite differently. But we can learn something important by considering this simplified example of an ideal constant torque motor. Suppose we build a very powerful constant torque motor that has the optimum torque for maximum acceleration for a particular car. Then we could use one-to-one gearing and the torque at the wheels would be the same as the torque of the motor. Well, we put this all together and it goes like hell and we instantly start breaking drag race records. But some guy with a motor that has only half the optimum torque has a bright idea. He gears his motor two-to-one and - voilą! Now he’s got the same torque at the rear wheels as you do. The only difference is, his motor runs twice as fast. But since his is also an ideal constant torque motor, he goes just as fast as your motor with twice the torque. Let’s get just a bit more realistic and suppose that the torque falls off some with rpm. If you double the rpm but the torque drops by less than half, you still win. By playing this little gedanken game you quickly come to the realization that, by using the right gears, you get the most torque at the rear wheels by running the motor where its torque times rpm are a maximum. But torque times rpm is just good old horsepower! Horsepower (or watts or whatever other power units you prefer) is just a simple and direct way to indicate the ability of an engine to do work. It is certainly a whole lot simpler than thinking “torque times rpm, torque times rpm, torque times rpm ------.“
So, let’s get this really clear; an engine’s potential for producing acceleration is directly related to horsepower, so you get maximum acceleration when your motor is putting out its maximum horsepower; period! Where it happens to put out its maximum torque has no relevance whatsoever relative to maximizing acceleration. The point of maximum torque might be an indication of the rpm range where the motor is most efficient, but that’s another story. Regardless of whether you have a little tiny motor with small torque that can turn lots of rpms, or a great big motor with lots of torque, but limited rpm, if they both put out the same horsepower then they both produce exactly the same acceleration. In fact, little tiny motors have a distinct advantage in terms of weight which is why the highest performance cars like F1’s use little motors turning 12-14,000 rpms and making great gobs of power. That’s not to say there’s no drawbacks to this philosophy, but in F1 style racing the advantages outweigh the drawbacks.
Let’s press on a little further with the main point here. I’ve tried to convince you by building logically from the fundamentals that it’s really horsepower, not torque that counts in measuring what a motor can do. But like all stories, its a little more complicated than that because in practice, a motor must operate over a range of rpms. So the more complete story is that the average acceleration is directly related to the average horsepower between the shift points. Now I know it’s inconvenient to have to visualize a curve instead of just remembering a number. Wouldn’t it be nice if you could just say motor A makes 325 hp and motor B makes 350 hp so B is faster than A. But we are just quoting the maximum horsepower at a particular rpm and in a real situation the rpms of the motor keep changing through the gears. If I can gear my 325 hp motor to get its average horsepower between shifts higher than your 350 hp motor, I win. So the whole story involves both the horsepower curve of the motor and how well the gears are matched to this curve. In general, the closer the gears the better up to the point where you’re losing too much time shifting.
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