Why is it that if you over bore some 4 stroke engines the performance gained is more than what you would expect with the minor increase in actual displacement. As an example a YZ250F with a 2mm larger piston gains only 13cc in displacement yet will likely gain a few horsepower throughout the rev range with no other modifications. The gain actually comes from the larger bore"unshrouding" the valves in the cylinder head allowing for increased air flow. The 5 valve head is especially bad due to the crowding. All the power from a 4 stroke is in the head.
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