TORQUE: USEFUL CONCEPT or AUTOMOTIVE RED HERRING ?
By Bob Palmer
VIN:
March 4, 2000
Basics:
Acceleration is a basic and well understood concept in physics. In fact, Newton’s second law, F = ma, gives a simple relation for the force (F), mass (m), and acceleration (a) of an object like, for example, an automobile. Written as a=F/m, we express mathematically what we already know instinctively, which is that acceleration increases as we add force and decreases as we add mass. Hence, our interest in little cars with big motors. (For the sake of simplicity, we’ll overlook effects like wind resistance and traction in this discussion.) So now we’ve come to the not so surprising conclusion that, for a given weight of car, we need to apply as big a force as we can in order to get the most acceleration. So what produces this force, torque or horsepower? Now here’s where the discussion gets a little tricky. In Fred Puhn’s book "How to Make Your Car Handle", he states somewhat sarcastically that "torque seems to be that characteristic of a motor people refer to below a certain rpm". In fact, torque is a concept that is best related to zero rpm but we can, if we like, abstract it conceptually from horsepower at finite rpms. Maybe one way to relate torque to what is happening with a motor is to consider a torque wrench and think about using it to apply torque to the motor’s crankshaft. The units of torque reflect its nature; e.g., foot-pounds. If I push on a torque wrench that is one foot long with a force of 100 pounds, the scale should read 100 ft-lbs. Of course, any combination of force and lever arm which, when multiplied, gives the same product produces the same torque. If the motor is kept from turning while this torque is applied then the wheels don’t turn, the car doesn’t move, and no work is done. The important point here is that you can apply all the torque you want, but work is only available when motion occurs in conjunction with this torque.
Discussion:
Seems like it just gets harder and harder to keep up with this fast-paced, high-tech world we’re living in. Just about the time you think you’ve become an expert at something, they go and introduce a radical new technology and you’re back to square one. One of the things I like most about my Tiger is its inherent simplicity; no fancy electronics or computers. The good part of this is that if something goes wrong, I can probably quickly diagnose and repair it. But, in spite of all the electronic controls and gadgets on modern engines, they are still basically the same as they always were. After almost thirty-five years in production, the venerable Ford Windsor small block is a case in point. With all of this experience with internal combustion engines, you would think we would already have learned pretty much everything that’s worth knowing. However, there’s always more to be learned and even the old lessons need to be retaught to a new and eager audience. Unfortunately, all too often the wisdom of the past has been abandoned in favor of trendy rheotoric inspired by a combination of ignorance and the profit motive. Now, there’s nothing new about this problem. Since even before the time of the chariots it’s been caveat emptor - let the buyer beware. Most of the bogus information is just hype for the purpose of getting a leg up in competition for your hard-earned bucks. Sometimes, however, the errors are of a more fundamental nature and reflect a basic misunderstanding of the issues. While it would be foolish to try and correct all of the misleading and bogus information being purveyed by even so-called experts, some of the more important issues should be confronted from time to time. A case in point are the concepts of torque and horsepower, their relationship to each other and to the performance of your car. Time was when the performance of engines was discussed in terms of horsepower. These days it seems more savvy to talk about torque. So, is torque really a better concept than horsepower for evaluating and comparing the performance of automotive engines in various applications, and in particular with respect to Tigers? To answer this question, we will start with some very basic concepts and develop these ideas into some specific recommendations for building engines and gearing them appropriately to suit your individual goals.
So how do we turn torque into work? This is also very easy to do mathematically. In fact, many of you probably already know that power is equal to the product of torque times angular velocity (i.e., rpms). The way we can derive this is by starting with the fundamental fact that work is equal to force times distance. To extend this idea to power, which is the rate at which work is done, multiply force times distance-per-unit-time, or simply force times velocity. Going back to the idea of applying a torque wrench to the crankshaft, suppose we let the crankshaft make one revolution while applying 100 ft-lbs of torque. Say we’re pushing with a force of 100 lbs at a distance of one foot. The distance we move in one revolution is 2p times one foot, so force times distance is equal to 100*2p*1 = 628 ft-lbs of work. Notice how easy it was to segue from torque to work. We even have the same units, but with torque the units were force at a distance and, with power, it’s force through a distance. Now, if we make a full rotation in one minute, just multiply by one rpm to get 628 ft-lbs/min for the rate at which we are generating power. So how do we relate to these units of ft-lbs/min? Well, the more familiar unit of horsepower is defined as 550 ft-lbs/sec (or 745.7 watt), so let’s convert our units to seconds by dividing by 60 seconds/minute; so 628 ft-lbs/min ÷60 sec/min = 10.5 ft-lbs/sec and then, dividing again by 550 ft-lbs/sec per horsepower, gives 0.02 horsepower as the rate we are doing work for this particular example. If you’d rather not go through this derivation every time you want to convert torque to horsepower, just remember the simple conversion factor which is 550*60/2p = 5252. If you multiply torque times rpm and divide by 5252 you get horsepower. Conversely, if you divide horsepower by rpm and multiply by 5252 you get torque.
Now I’ll bet your asking yourself how you managed to get along all these years without this important information; right? Hopefully, at least a few of you gear heads have been sucked into this discussion in the interests of intellectual curiosity and are hoping to emerge with a little better feeling for the concepts. For the rest of you, don’t despair, there’s more practical information at the end of the tunnel, so hang in there.
While there is more than one way for an engine to do work, producing acceleration is perhaps the most interesting to Tiger owners and drag racing is probably the purest expression of the quest for sheer acceleration. Consequently, a good question to ask is what characteristics of a motor will get you across the finish line first. Of course this simple question is complicated by factors like traction, weight, aerodynamics, and reaction time. But, keeping all these things equal, what do we want from our motor to get the best elapsed time and speed. Incidentally, most folks familiar with drag racing know that your speed is a better indication of your motor’s horsepower that your e.t. This is because hookup is so critical in drag racing since what happens early in the race is integrated over a relatively longer time. This fact is also not lost on the better track racers who will make adjustments in their driving to let them enter the straight-away at a higher speed, even though they may have to give up a little distance in order to do so. For instance, if you can exit turn nine at Willow Springs 20-30 mph faster than the guy you’re following, you’ll overtake him down the straight-away, even if he’s got somewhat more acceleration than you.
Note 1: Engineers reserve the term motor for devices that convert electrical energy into mechanical energy. However, in common use, the terms motor and engine are equivalent and will be used interchangeably here.
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