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bore and stroke have much more to do with it. Generally, stroke=torque and bore=horsepower. My guesses why: for a cylinder of a certain displacement, if the bore is greater in proportion to stroke (oversquare), the piston has less momentum to overcome during each stroke, and can rev higher. This produces a lot of peak hp per liter. On the other hand, for a cylinder of the same displacement, having the stroke proportionally longer than the bore (undersquare), the pin that the con-rod/piston assembly connects to on the crank is farther from the rotational axis of the crankshaft, which provides leverage. Since two 500cc pistons, regardless of bore x stroke dimensions, should make roughly the same force, having the application of this force at a larger radius would increase torque at certain moments because torque is force times radius. I also read that piston velocity has an effect on torque production (not sure why, though), and longer stroke would mean greater piston velocity.
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