Mark, I’ve got a couple of the oval exhaust Stels that I ran for several years on some RC speed projects.
I ran them with an APC e prop. I think a 4.5-4? On 40% nitro, with a custom made header and tuned pipe. Globee or Norvel plugs. I’d run a Kamtecnik today.
Incidentally, I got over 150mph out of the final iteration in that design series, and was beating q500 pylon racers in runwatchabrung races.