w8ye,
Thanks for that observation. I've been planning to use my 91 on a Hobbico Su-31. The cowling on that ARF does not have the extra housing along it's bottom like is on the fullscale airplane. I had planned to mount the 91 at about 8o'clock so that the stock muffler would be positioned in that cowling bulge. Since receiving that ARF and discovering the lack of the bulge, I've been debating using a Pitts muffler instead. It would save me building the bugle myself and fitting it into the bottom of the mfg'd cowling.

The sticking point was that I didn't like the looks of the Slimline pitts muffler for the 91 nor it's price. And I'm getting a bit tired of using Pitts mufflers to solve cowling problems.

It actually shouldn't take much effort to layup that bulge, and now that I've read your observation, I'm actually a bit pumped on doing it. I've got good reason to stay with the stock muffler now. It's always bothered me to throw those things in the parts box. Seems a waste. And I kinda like the looks of that strange bulge on the bottom of those cowls. Was sorta disappointed when I saw they'd left it off.

Now I got good reason to do that modification and am looking forward to doing it..... thanks