I run 3 ST 61's (1 ring and 2 abc's) in King Kobra, TF (red box) P-51B mustang, and GP arf FW-190. They all run superb regardless if inverted, side-mounted, or semi-inverted. I use 11x7.5 or a 12x6 prop and 10% Cool Power. I also have a TT-Pro 61, it was on the FW-190 but was replaced with a ST-61. The TT-Pro 61 required tuning, retuning, etc and never gave a consistent performance. However, I must say the ST 61 ring or abc is a dying horse because of production down-time. All the new 61's at our field are OS 61FX and although there are more and more coming to the flight line I don't believe they perform any better than my ST -61's.