I'm running an APC 12.25 X 3.75 Power MASTER 15% stock muffler at 12,600 on a Morris Hobbies sledge 540 great combo for 3D Hovering and Torque rolling. and with my JR 10 X I can desensitize the throttle response RPM around the hover stick position for precise RPM control 1 ft off the ground in a hovering toque roll. good engine choice, very reliable once set never quits until you want it to!!!!