I found an old, .91, Ultra Thrust tuned muffler in my muffler box. It happens to be the long chamber one that I bought 4-5 years back to run larger props on an OS .91FX that never ran well so I sold it. I thought I'd give it a try since I still have SK 90 #2 on the test stand.

At this point I am running a big, black, heavy, Master Airscrew 15-6 and turning 9500-9800 rpm on Omega 10%. This is about what my #1 engine turned, too. I installed the Ultra Thrust and cranked it up. I leaned out for a peak of 10,650 and a nice easy 10,400 rpm. That's a good 800-900 rpm. I think that is pretty good on a big prop and at lower rpm.

For this weekend, I'll install it on engine #1 on the Ultra Stick and give it a try in the air. The needle seemed fairly insensitive so I don't think it will lean out in the air or in maneuvers, although you can't tell in advance. I have had engines just scream on a pipe on the ground, but lean out when I pull g's in the air. I'll report later.