I am not sure what APC's different designations mean on their 8.75 pylon props, except perhaps for variances in blade design. However, I don't think the designation is going to make up for 9 inches of pitch though.

If I follow this right, you got 115 on the 8.75x7.75 and 142 on a 9x6. 142 is pretty darn good, and the 18,500 is respectable. It sounds like the 9x6 might be your prop. Did you try a 9x7?

Paul