Perhaps it is also important to point out what is not relevant in the foregoing discussion; e.g., the torque curve, the rear end ratio, the weight of the car, etc., etc. Of these, the most irrelevant is the torque curve. In fact, my advice to those of you who may have both torque and horsepower dyno data is to tear that torque curve sheet up and toss it in the trash. Failing the courage to take this bold step, perhaps you could at least tape it to the under side of a drawer or some similarly obscure place where, hopefully, over time you will learn to live without it. Remember, if you ever get really desperate for the torque data, you can just divide the horsepower curve by the rpms and multiply by our handy-dandy, easy-to-remember conversion factor (5252).
At this point, the reader probably falls into one of three categories: (1) those of you who have fallen asleep; (2) those of you who disagree violently and are busy thinking of arguments to the contrary; and (3) those of you who already knew all of this and are wondering why I’m wasting your time. To the former and particularly to the latter, I apologize. To those of you who may remain unconvinced, but who are still with me, read on. I think I can anticipate at least one or two of your objections. Also, I may be able to add just a little more practical advice.
Some of you may be thinking that, even if torque doesn’t relate to acceleration, it must at least be an important characteristic of truck engines. At least that’s what common wisdom seems to hold. But I could just as easily have made the same argument and come to the same conclusions by merely substituting work in terms of pulling a weight up an incline, just another form of force times distance, instead of work in terms of acceleration. It just so happens that the practical attributes of truck engines favor lower rpms. In particular, diesel engines have good efficiency and long life. They also have a limited rpm capability, so running at high rpms is simply not an option. A high rpm, high efficiency motor (e.g., a turbine) could easily be a good truck power plant. Although gearing might present some problems in the extreme case, fundamentally there’s no advantage to a low rpm, high torque motor versus a high rpm, low torque motor if both make the same horsepower (although there may a valid issue comparing longevity). Certainly, I’m not suggesting that it would be practical to replace a diesel truck motor with a F1 motor with the same horsepower. For one thing, the cost of a F1 motor would be prohibitive. But if we were to have a hill climb contest between two identically loaded trucks, the truck with the most horsepower between the shift points would win, regardless of the supposed torque advantage of a typical truck engine.
What does all of this mean in terms of building an engine for your car? Using the best information available, try to build your engine with the end result in mind. Do you really want a motor that develops its best horsepower at 7,400 rpm, especially if it means a loss of horsepower at lower rpms? And, how long will it last at those higher rpms? Remember, to get the most out of the 7,400 rpm motor, you would need to be shifting it at over 8,000 rpm! It only makes sense to build the heads and cam to work best at high rpms if you are also willing build the bottom end to be reliable in the same range (plus some safety margin for over-revving). Resist the temptation to go into uncharted territory with bitchin, go-fast parts that happen to fit your budget. You won’t know until it’s all together how it’s going to work. Instead, use the same parts as a previously built and tested engine that most closely gives the horsepower curve you are trying to achieve. This will also save you the cost of a dyno test, although a dyno test is the only way to confirm that you are getting what you expect.
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