The Thunder Tiger .91 Four stroke I have is almost identical to the OS .91 I think it's a copy of the engine. I might just throw that in instead of the KnB .61 I'll keep that engine for a P51 as it has a Pitts muffler and I don't think the Cessna 177 can take a Pitts muffler. I'll try the 13" x 3 blade prop I have one of those also. The sound of the Four Stroke engine is nicer it sounds like a real airplane motor.