ORIGINAL: Firepower R/C
I actually ''tune'' my pitts mufflers by CAREFULLY crimping down both outlets while the engine is running full throttle until a slight decrease in rpm is detected, then open them back up with a screwdriver just enough to get the rpm's back. This has always given me a solid idle and performance that is at least on par with a stock muffler. As a plus, there is no risk of a plug EVER coming out. You will also need to retune as you do this, as the back pressure will change as you crimp, obviously.
For what it's worth-
Some years ago Sullivan used to send out there Pitts mufflers without a pressure nipple and they had pipes about 6 or 8 inches long. You just cut them to tune the muffler to the engine. I always cut them at an angle with the back pipe shorter then the front. Max RPM and as soon as it dropped a little you quit cutting. Most of my engines did not require the pressure nipple at all. If it did I just drilled and tapped the front of the box for it. My first choice is always the Bison Pitts style muffler though. I found them to work perfectly out of the box. I never tried to tune one at all.