I was having problems hovering a Pizazz with a 13/6. Plently of thrust, but it was hovering below the "pipe" with the std muffler. I changed the prop to an APC 12.5 x 6 and now I can control the hover much better. It raised the RPM about 600 over the 13/6. Actually did my first controlled torque roll the same day I switched to the 12.5 x6!