http://www.physicsforums.com/showthread.php?t=153996Flott tråd som tar opp temaet, både fra gatebilsiden og for raser-bilen.
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#15That'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 modelled as a device which has some function LaTeX Code: \\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 LaTeX Code: 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 LaTeX Code: 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.
#20Just to clarify, all engines have their hp peak after their torque peak (at least if there's only peak for each curve). It's an amusing little math exercise to prove that.
Anyway, the answer to your second question is as follows. In a given gear, the instantaneous acceleration is highest at the torque peak. At a given speed, a vehicle allowed to select any gear ratio will have the highest instantaneous acceleration at the power peak. If that sounds contradictory, try reading it a couple of times. The first condition has you choose a gear and vary speed. In the second, you fix speed and vary the gear ratio.
So say you were at the torque peak in some gear. Even though you're accelerating as hard that gear will ever allow you to, you might be able to downshift and accelerate even more. A perfect CVT optimizing straightline performance would keep you at the power peak at all times.
Also, if you always maximize instantaneous acceleration, you'll also maximize average acceleration. So there's no need to worry about that (at least at the level of this discussion).
#29My point is that if you go to the track and ask the successful drag racers how they're tuning their cars, you will find that they want their torque (powerband) optimized for the higher rpms and they want the powerband wide enough to cover their shift points. They launch pretty hot and flog 'em down the stretch and they want to continue to accelerate strongly in each gear, so the torque has to be available over a range of RPMs that exceeds the RPM drop caused by shifting to each higher gear. Peak horsepower is a "nice to know" number, but to continuously accelerate, you need to deliver that power effectively over a usable range of RPMs. That's the importance of the torque vs RPM curve on a dynamometer plot. If the absolute value of your car's torque is relatively high and the high portion of the curve is wide enough to cover your shift points (gearbox dependent), you will outperform a competitor with higher absolute torque values if his curve does not adequately span the RPM differential at his shift points. His acceleration will not be consistent because his torque curve is narrow and peaky and since acceleration adds cumulatively to velocity, your car, with a slightly lower but broader torque vs RPM curve will win out.
