I finally got fed up with the loud pipe on my '07 450EXC and decided to see if I couldn't do something about it. I removed the muffler from the bike and started the disassembly. Notice the hole in the middle of the can. After pulling the end cap and removing the innards, this is what I found. Only half the EXC muffler is actually packed for sound deadening. The packing is only wrapped around the smaller perf pipe shown on the right. The hole half way down the can holds what appears to be a large perf pipe, seen on the left. This is actually the Krizman spark arrestor. It is nothing more than a hollow pipe with a set of angled vanes welded in the center. Evidently, these vanes cause the exhaust to spin, keeping any sparks from exiting the chamber . Anyway, the arrestor unit has no sound packing except a thin layer between the screen and the outside muffler can. I assume this is simply to keep the outside can cooler. The stock EXC muffler can is 18" long but only half of it is actually a muffler :huh. No wonder it's so damn loud. I can see the solution is to ditch the Krizman and pack the entire can. I picked up a used 2-smoke muffler with a full length perf pipe from the local KTM dealer. I pulled it apart and removed the perf pipe. This perf pipe is 1 3/8" diam, which is smaller than the 1 3/4" diam stock perf pipe, but it matches the diam of the pipe in my end cap. This will solve two problems. It will easily attach to the end cap and it will slide inside the stock perf pipe. This will allow me add a screen type spark arrestor inside my new muffler. I cut the smaller perf pipe just long enough so it would extend about 2" inside the larger perf pipe. I welded a small ridge around the smaller pipe which will hold a washer in place. I then wrapped some stainless steel wire mesh around this section of the pipe and slid the washer on to hold it in place. Volla, a spark arrestor . When assembled, the mesh slides inside the larger pipe and the washer seals the connection between the two . I then wrapped the entire assembly in the remaining wire mesh from the Moose packing kit. This was then wrapped with the fiber packing and slid back into the stock can. I have not yet had it measured with a sound meter but this is how a muffler should sound . It's very much quieter and what was a loud braaaap when you twisted the throttle is now a smooth purr. It's not a constricted sound, but rather a well muffled straight pipe sound. It may not be perfect but it's a hell of a lot better .