The older Oilhead mufflers have a tendency to crack around the end of the outlet tube. I had mine TIG welded to save sourcing a salvage muffler only to have it happen again someday. This got me to thinking about what could be done with the stock muffler, to get a better sound. I got another muffler with the same cracked end tube at the dealership where I work part time.
I carefully cut the canister off just behind the weld at the CAT outlet elbow. Then the internals gave an indication of how BMW designed it for very quiet operation, but with obvious restriction. More sawzall work got me to a starting point, in other words I hacked it all off. I then designed my own internals based on the baffled/reverse flow of a Flowmaster muffler. The new internals include a center divider, angled diffuser and reverse flow 1.625"OD outlet tube, all in stainless steel. I designed the internals based on what I could hand-fab with a hacksaw, sawzall and an air cut-off wheel.
The muffler I got for nothing (dumpster diving at its best), the stainless steel materials cost me $15 and the TIG welding I got done for $30, so I have $45 in a new solid muffler. But what did I get for my scrounging and fab work? A unique, Remus like deeper "twin" sounding exhaust that seems loud at idle, but is not a problem at all when riding. It seems to make most of any added sound below 3-grand. But at highway speeds it is barely noticeable. People riding with me have made no complaints. It is definitely nothing near the "straight pipes" sound popular with Italian and American brand twins. Not obnoxious in the least. More "performance" sounding in town or when I'm on the throttle.
I got definite better throttle respone at 2-grand to 4-grand, and great roll-on after 4500 rpm. No loss in mid-range power that I can tell. My plugs (Autolite 3923) are a nice light tan color, and reduced decel "popping" with a slight TPS adjustment. I'm very pleased with the results. See this post post; "http://forums.bmwmoa.org/showthread.php?t=18777 for details on the internals and the approximate exhaust flow path in red lines and arrows. If anyone wants more details just PM me for the additonal pictures and measurements. If you are a tinkerer, and have some fab ability, and access to a TIG welder, this is a great low-buck project for your bike. It keeps the CAT intact, still uses the O2 sensor, and it lost two pounds to boot.
It's said the cheapest part on any BMW cycle is the owner/rider. HEY! THAT'S ME!! Proud of it too.