tuned pipe and prop choices

You have found the problem with the mini pipes. They are picky about the prop load. You have to keep it the same as the 11*4, so you will need to go down on diameter as you raise the pitch. Try 10*6 or 10*5. If you are looking for more thrust, cut down on the diameter of the 12.25*3.75 until it comes up on the pipe. There's been many posts on do's and dont's of prop shaving, btw. Someone here is more knowledgeable at tuning the mini pipes than I am. I never got the same overall performance out of any I've tried to modify.

You webt up in diameter, but did not decrease pitch. The mini tuned pipes only work from x-rpm up. If you load the engine too much, ground rpm will be too low, and adjustment of the carb is not possible. The header length of the minis is tuned to the pipe volume. It is what makes them work. If you want to go down in rpm, increase both pipe volume and header length. Check out my Helmholtz resonator program at

That engine/pipe combo likes a 10x5, 10x6, or 11x4. It likes to turn at 15K and more. No 12" prop is effective for a .40 sized engine on normal aircraft.

Bob