I had an exhaust diverter on my ASP .46 and had it at full throttle on the ground. When it popped off and the RPM's raised, I dropped the diverter in my flight box and never put it back on again.

My Tower Nitro ST-15 came with a standard muffler and exhaust diverter. After I got playng with it I took the diverter off of it and it was faster.

Exhaust diverters drop the power available to your engines. Leave it off and clean the goop off of the plane.