I disagree with statements to the effect that the motor should operate between the torque peak and the horsepower peak, at least not when optimum performance is the goal. This advice appears to be the result of some kind of confused compromise between two competing independent parameters, which is certainly not the case. In fact, torque is simpy a component of horsepower, but since horsepower directly relates to performance, just focus on trying to maximize the average power between the shift points and simply ignore the torque curve. (Am I repeating myself?)
I’ve read in more than one place that, in drag racing, you should gear your car so as to just reach the maximum horsepower at the end of the quarter mile. Wrong! Whether it’s drag racing or optimizing your gears for a particular road track, in general you need to be somewhat beyond your peak horsepower rpm at the end of the straight. The exception to this would be the case of a very long straight-away where you reach your top speed well before the end. In this case the optimum converges to the peak horsepower point. But certainly in the case of a drag race, you are still accelerating at the end of the quarter mile and probably the optimum rpm is still pretty close to your shift point; e.g., 6,400 versus 5,800 rpm in the above example. With some real data and a little calculus, we could solve these problems exactly, but I think that’s a little more detail than we need to get into here. But, to at least give you an idea how to gear your car for the quarter mile, let’s assume we have the motor we discussed above where, to get maximum horsepower through the gears, we need to shift at 6,400 rpm. What rear end ratio should we use? To achieve this rpm at the end of the quarter mile, we need to know our speed, which we can only find out exactly by actual experience. But, knowing our engine’s horsepower and the weight of the car compared with other similar motors and cars, we might expect to be able to get to about 110 mph, a rather ambitious goal, but definitely achievable in a Tiger with a good performing engine. To select the correct rear end ratio to achieve this particular speed at the end of the quarter mile, measure the circumference of your rear tires. With a Tiger, the tires will be right around six feet in circumference. At 110 mph you are going 9,680 feet per minute. Divide 9,680 ft/in by the tire circumference of six feet (which equals one revolution) and we find that the rear wheels are turning 1,613 rpm at 110 mph. So, we have 1,613 rpm at the rear wheels and 6,400 rpm at the motor; divide 6,400 by 1,613 and we get a rear end ratio of 3.97:1, or maybe slightly less to account for tire slipping. (assuming 1:1 fourth gear like top loader four speeds). A Tiger geared like this wouldn’t be very practical on the street, but it would sure go like hell in the quarter mile! (Remember, this example is for illustrative purposes only. Your results may vary!)
Well race fans, so long and remember; when the green flag drops, the b.s. stops!
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