It is like night and day. The evo motor with the stock muffler has midrange problems. The evo tuned muffler cleaned that up and raised the rpms about 1500. I was using a 10x7 on my kaos but changed to a11x6 for the P7. You have to get the motor up on the rpms or it doesn't run good. I am using 15% fuel because there wasn't much gain with higher nitro . Get the engine/prop combo to turn at least 15000 and it should run good. Check out the evo60 threads in the glow motor section .