I just switched Pitts style pipes on my plane, previous pipe broke. And seems I have lost about 300-400 rpm's. The new pipe is a little larger. Is it a loss in back pressure causing the loss in rpm's. And if so would reducing the outlet's help bring it back.

Did you retune the engine? I'm going to assume you are running a two-stroke, and they can be very responsive to muffler changes of any kind. I would be certain to retune properly before modifying the muffler.

With a general type "muffler" Pitts or can with down tube, it's usually more about tuning. There's a small difference in back pressure but not as much as one might thing. It's probably more about refelction, or when that back pressure bounces back into the cylinder. part of the intakeexhaust charge timing cycle. Where is becomes a lot more apparent is when you use pipes and the header length is wrong. You can end up with an engine that's hard to start or runs badly because of back pressure reflection,

For what you have focus more on re-tuning. You may still end up down a little bit, that's common with Pitts style mufflers. Pinching tips or reducing exit size won't get all that's lost back.

Thank you gentlemen.

Something you might find interesting. Run the engine witout a muffler and see what RPM you develop. You may find that ALL mufflers sacrifice some power

Due to many people not understanding the principles of tuned pipe operation, I have heard many people say that "two stroke engines NEED some back pressure". This misconception may have also come from the fact that exhaust back pressure can help a glow plug stay hot during idle operation for glow type engines.

AV8TOR

What type and size engine is it? I'm ordering a Bisson pitts muffler for my OS 55AX. Interesting that Bisson now makes one specifically for the 55AX.

Ernie

 Had to do some minor repairs due to a stall of the engine on Sunday. My falt, was in too big of a hurry to fly and did'nt retune again. Getting bad about that. Got it back out today. Ok. First thing is the old pitts was not made for this engine. It was much smaller than this one from RCGF. After initial start up gave it about 1/4 throttle for a couple of min's to warm up, then comensed to tuning. And now it run's even better with the right size pipe. I think the other smaller pitts was boggin it down.

Bisson has always pinched the tips of their muffler exits. Cutting off the pinch has always provided a hundred or so more RPM. Just sayin'.

T.o.m  you hit the nail on the head with ' when that back pressure bounces back into the cylinder".  Thank's</p>