I don't remember where I read it, but I read something on a forum about a 1/4 wave tuned side branch resonator that will all but eliminate the resonance performance mufflers can generate. I think I saw this on a Mustang site, but I can't find it at the moment.
Anyway, you branch out on a "T" (90* to the pipe), using the same size pipe, capped on the end. You make this branch 1/2 the length of the tail pipe. It seams that the resonance we hear actually comes from the tail pipe, not the muffler. Accordning to the stuff I printed out, "As the acoustic wave from the sourse reaches the branch, the energy splits equally. The wave travelling down the side branch reflects off the closed end and is reflected back down toward the branch. When it arrives back at the branch it is excactly 180 degrees out of phase from the source wave because the branch has a length of 1/4.
This was all based on a Mustang tail pipe being about 5' long, 2000 RPM drone, V-8 pulses, etc.
Ah ha! Here is the forum link.
http://www.corral.net/forums/showthread.php?t=496793
Shiny Side Up!
Bill