TORQUE: USEFUL CONCEPT or AUTOMOTIVE RED HERRING ?
By Bob Palmer
VIN:
March 4, 2000
Practicalities (cont.):
Based on our discussion so far, we should now be able to make some pretty good choices relative to what we’re trying to achieve. If we want to build a drag motor for a Tiger we can start by assuming you are going to be using either a close or wide ratio top loader four speed. At the starting line you only need enough torque at the wheels to keep the tires on the edge of adhesion, assuming you’re not running big wide gummy slicks that work best with a certain amount of slipping. In any case, as you go down the track you will need to build horsepower output from the motor in order to maintain constant torque at the wheels to hold them near the breaking point. At some point a few hundred feet or so down the track you will no longer have as much power available from the motor as you could use and from there on you will want to keep the motor making its highest possible average horsepower. Ideally, you will be somewhat past the peak rpm in fourth gear as you cross the finish. How much past depends on how fast your power falls off after the peak. The ideal, as I said above, is to maximize the average power between the shift points. I read somewhere that you should be making your peak horsepower just as you cross the finish line. That’s wrong. Gear a little lower (higher numerically) so as to get the highest average horsepower. The same idea holds in track racing. In general, you should be past your peak horsepower rpm at the end of the straight-away. Let’s get down to some brass tacks by starting with the transmission ratios, which is what determines the rpm spread between shifts. For Ford toploader four-speeds, the following gears are standard:
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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