on my 91fx... i am not getting the rpm that i would like to get with a 13x8 prop... this is recommended prop... what would happen if I go to lower prop like a 12x8 or so? would it have too much revs? is it bad to under prop an engine but still tach below the max rpm of the manufacturer? I want to get closer to 15K rpm... I am using the stock muffler which came with the engine...