Bill, from the picture you sent, it isn't clear to me where the 'cool tube' ends inside the muffler.  Give me a clue!

 

Tracy

Tracy, On the small muffler (2 rotor) the cool tube goes all the way thru. The front of the tube needs a feed from the cowl. The exit needs a pipe to the cowl exit although it could be very thin. This is a sealed style like Jerry Hey and Ed Klepis have come up with. On the 3 rotor pipe the cool tube stops just inside the tailpipe. The section might be a little confusing as it shows a small support bar that was suggested to prevent the internal pipe from sagging. The sections I included should give a good indication. The intent is to form a annulus that should draw air through the tube venturi style. This needs only some clear space in front of the muffler. The idea is that you should mix air in the tailpipe to A; cool the exhaust and not burn the paint off the belly, and B; lessen the exhaust note by absorbing some of the exhaust energy by mixing cooler air with the exhaust.

Bill Jepson