I found the seamless tube on a site for kit telescopes, it's about 3 3/4" dia. The stainless screen, or mesh (1/8" by 1/8"), I found at a local welding shop that does some food grade work, it was a scrap and I had it sheared to length and rolled at a HVAC place I do crane work for, then stiched it together with safety wire. The muffler packing was a a little harder to find, but a custom bike supply house had some, sorry no info. As I recall I came up with the 38" length based on the happy coincidence of the stainless mesh, the aluminum tube, and the muffler packing all being around that length. The mild muffler tube was a simple job for a buddy who owns a muffler shop, one end was expanded and slit so it slips right onto the Rans supplied stock muffler outlet up in the cowl.
After some thought, I decided a car type muffler clamp was the way to go for clamping the 90 to the pipe in the cowl, I do double nut it though and just like in a car it works perfect, no use re-inventing the wheel here! The same type clamp secures the 1 1/2" flex regular old auto exhaust tubing. Now here is the first trick old Tony didn't mention: NO clamp on the other end of the flex, it simply slips onto the 4" length of 1 1/2" aluminum pipe (same welding shop, not anodized or anything remotely aircraft like, a little less then 1/8" wall, but easy to weld) when I clamped BOTH ends the flex would self destruct about every 75 hrs. Some scrap aluminum plate (1/8") was cut and drilled for the end caps, half the pipe stubs go in, half stick out, they are welded to the plate.The other big tip is the (again) auto type rubber muffler hangars, these are bolted to an aluminum angle that is secured via nutplates to the cockpit floor, actually tabs welded onto the tubing for securing the boot cowl. Now they do that plus hold the muffler up. Two all stainless hose clamps per hangar give me a little redundancy, and I can take the whole thing off in about 45 seconds. Once I did both the flex muffler hangers AND started only clamping one end of the flex, nothing wore out anymore.
I wasted a lot of effort on making both end caps with the pipe stubs, removable, thinking I’d need to get in there for maintenance and inspection. The nut plates and their brackets wore and cracked, finally at about 800 hours I welded one end permanent and at 1000 hrs welded the other end as inspection showed the muffler packing and the mesh was like the day I put it in.
Like Tony said, assembly is easy. The muffler packing was simply rolled around the mesh tube until the OD of the bundle just barely slipped inside the big tube, the pipe stubs engage the mesh tube (the mesh tube slipping over the pipe stubs. Now you have a (little) jet effect as at least the exhaust exits straight back, not at a 90 to the slipstream like the stock setup. And maybe, just maybe, I get some kind of accidental tuned exhaust effect....the only reason I say this is I have done static run ups and full throttle flights with and without the SBMS, and I can’t tell any difference! Sure it looks draggy as hell but I’ll be damned if I can quantify any degradation at the speeds I fly at. Total weight around 5 lbs. Total cost less then 200 bucks, time to build the next one if I ever wear this one out, about a half day.
Combined with the Kiev prop I use, (supposedly one of the quietest props out there) it is very effective, I can’t tell you how many times I’ve arrived somewhere and walked in or knocked on the door and surprised people, people that are attuned to aircraft noise, sneaky! Just the other day I was BS ing with a guy on the chairlift, and it turns out he is one of “the new guys” who lives down the road from me a bit over a mile. He’s been there 10 years now, and had heard I flew an airplane. Poor choice of words: as he asked me “so, you still fly? I understand you had an airplane”? Or words to that effect.....keeping in mind when I am climbing out to clear the ridge line to the east of me I am in his neighborhood and still pretty low. He claims he has NEVER noticed me going by, much less heard me, I find that hard to believe myself, but I’m doing something right I guess? 1200 hrs in the last 5 years and he wasn't even sure I flew? I can put up with the funny looks when other pilots see the thing hanging there and think maybe I didn’t know enough to put it in the cowling, as long as it’s so effective, cheap, long lasting, and has no effect on my performance.
