What Jim is trying to convey is that if you go from 14x6 to 15x6 your engine will not be turning as many rpm's. More pitch and less diameter is associated with speed, and the converse for more acceleration. For instance, I like to fly some of the smaller profile fun fly type planes. For a .40 size engine we usually use more diameter, say an 11x4 instead of a 10x6. This gives more rpm, better throttle transition and better hovering ability and slower flight speeds when we need them. Sort of like a helicopter, a long flat blade. What particular plane, engine, prop and plug are you flying?