It should be noted that engine's should not be simply compared in an across the board, rpm to rpm scale. Saying motor A makes X amount of X, Xrpm sooner than motor B isn't really saying much. Simple answer is becasue of gearing.
Say you have a motor (A) that revs to 5500rpm makes peak power at 5000rpm and peak torque somewhere near 2000rpm.
You then have motor B that revs to 7000rpm, makes peak power above 6000rpm and peak torque above 2500rpm.
Both motors make identical peak torque and horsepower numbers. Lets call it 200hp and 200lb-ft of torque, it doesn't really matter for our purposes however. Ratio's scale with whatever numbers you plug in.
Now based off the above information, many would say motor A is superior simply because it revs 25% lower to accomplish the same force and amount of work. In reality motor B is capable of getting everything motor A is capable of doing and more by simply gearing it 25% lower (higher ratio).
I'll give an example in a 'crawling' context. 1st gear (3:1 for simplicities sake) 1:1 transfer, 4:1 rear end (again simplicities sake). Trolling along at the motors torque peak at full load, 200lb-ft@2000rpm. Wheels are turning at ~166rpm (~16mph on 33" tires) and there is 2400lb-ft of torque at the contact patch. Pretty substantial right?
Now lets take motor B, same scenario, different gearing to take advantage of its different power band. 3:1 first gear, 1:1 transfer, 5:1 (25% lower) rear. Again, rolling along at its torque peak (2500rpm) at full load. We want to match motor A's 'crawl' speed (166rpm at the wheel) and hopefully have just as much, or maybe more torque.
We end up with the same wheel speed as motor A but the motor is putting out 3000lb-ft of torque at the tires.
The reason the higher reving motor puts out more torque is simply becasue by making its peak torque 25% later, it is making 25% more horsepower at its peak torque rpm, thus it can do more work. We could gear motor B to make as much torque at the wheel as motor A but it would end up 25% faster at that new ratio.
Obviously more than just peak torque and power numbers would determine which of the above motors are more capable offroad. Such as the rest of its powerband, available gearing, engine longevity and fuel mileage at 25% higher rpm through its lifetime, etc.
The point is, you cannot simply disqualify an engine becasue of the rpm at which it makes its peak numbers. Gearing is the great equalizer, if a motor makes the same torque at a higher rpm (either peak or throughout the rev range), you can gear that motor to make more torque at the wheels for a given speed due to the higher horsepower produced. Sure the engine will spin faster the achieve this by virtue of its higher rpm nature. But that is simply the nature of the beast. There is more than one way to skin a cat.
I'll also contest AllanD's assertion that engine torque is more needed by heavier vehicles, and lighter vehicles don't need as much torque. Gearing works no matter what a vehicle weighs. Its possible to put a low torque, high horsepower motor in a heavy vehicle and have it perform the same as a high torque, low horsepower motor. The inverse can be said for a light vehicle. gearing makes it possible. What counts is torque at the wheels, which can be attained more ways than one.
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