joema said:
This is a frequently-debated topic in automotive performance circles.
That's certainly true. It comes up so much in some other forums I frequent that I've thought about writing up a canned response. But I haven't done that yet, so here's another try:
People often confuse power and torque because car enthusiasts tend to (unknowingly) use these words for different concepts. This is a physics site, so I'm going to go ahead and use the definitions from physics.
The full-throttle behavior of an engine can be approximately modeled as a device which has some function \tau(\omega) associated with it. This fixes the torque it can produce as a function of engine speed (rpm). This function is not at all constant, although engineers often strive to make it as flat as possible.
Regardless, given the torque function, there is an associated power P(\omega) = \omega \tau(\omega). So if the torque is known at all speeds, the power is known at all speeds (and vice versa). You can't have one without the other.
Despite this, it is common practice for engines to be advertised only in terms of their peak torque and peak power. The engine speeds where those conditions may be found are also usually given. The peak power is very important for reasons I'll get to later, but the peak torque is essentially useless all by itself. The reason is that the gearbox can multiply the torque to (essentially) any amount whatsoever at an appropriate speed. But an ideal gearbox cannot change the power.
Staying with the ideal case, the maximum forward force that a car can produce is entirely determined by the power its engine is producing and the car's overall speed. So fixing speed, maximum acceleration is always reached by maximizing the engine's power output. It is the job of the transmission (and driver) to use the gearbox to keep the revs as close to the engine's power peak as possible if full acceleration is desired.
Modern transmissions have many closely-spaced ratios, so except at very low speeds (at the bottom of 1st gear), an engine may be kept close to its power peak for as long as desired. That means that a well-designed car that is driven well may produce a force F \sim P_{\rm{peak}}/v. This depends only on the peak power (and velocity), and explains why the power-to-weight ratio is such a good predictor of acceleration performance.
Having said that, the torque peak is not completely irrelevant. Its position relative to the power peak is usually a good indicator of the size of the car's "powerband." Essentially, how high do you have to rev it in a given gear before the engine really gets going? Having a wide powerband is extremely important in everyday (or moderately aggressive) driving where you're not going to redline in every gear. It makes the car feel much more powerful even if the maximum performance is the same. Of course, a wide powerband is also useful if your have a poor transmission or don't want to shift as much.
Russ, differences in drivetrain inertia between reasonable designs are not usually not a huge effect. They're certainly significant, but I don't think I'd include them given the approximations already inherent in this sort of discussion.
), and defined it as 100 ft-lb/sec, then the graphs would cross at 52.5 rpm.
