John, sometimes things happen that just baffle us. Martin knows this story. The first time I tried my high compression Saito .80, (15.5 to 1) I ran a 13x6 prop, same as glow, It hammered and knocked would only turn 5,500 and sounded like it was going to become a Black and Gold grenade. I installed a Bolly 13.5x8 and voila, just like you experienced, the knocking went away and it turned the 13.5x8 8,790 or there abouts.