Here is the ultra thrust I am using: http://www.pspec.com/results.asp?category=8 It is the sport muffler & not the racing. I also have a 3/4" spacer & that may help with the lower RPM tune.

The muffler is louder than stock, and works fine for 3D on 8 to 10 pound plane. That being said, I would go for the AX120 on a 8 pound plane and a FX160 on a 10 pound plane. My favorite 3D prop for the .91FX on an 8 pound plane is a SK wood 15x6slightly less thrust, but instant spool.