Hey Crank...

I think you ought to prop that engine to run about 11000 RPM's...From what I have read the sweet spot should be between 10500-11500...I'm trying to keep all mine well under 11k (static) and figuring the engine will turn up a bit more when it's in the air...

I have yet to run any of my SF engine's with a stock muffler...I'd imagine a 12x8 with 15% nitro should put you right in the ballpark...Maybe some of the guys who are running that same set-up can advise better...

All I've run so far have been rear exhaust pumped versions of that same engine (RF-P)...I'm also using PowerMaster YS 20/20 fuel and tuned pipes...I doubt much of my info will apply to your particular set up...I'm turning about 10500 on a APC 12x10 prop on the test stand...I'm almost happy with that but I'd really like to get the static RPM's down a hair more...I may try a 12x10w prop to see if that helps...(w=wide)

My advice would be to borrow/buy a tachometer and set your engine up to turn 10500 RPM's or less on the ground...(I'd almost bet your current set-up is spot on)

Keep in mind these are Long Stroke engines and they produce their power at lower RPM"s...They don't need to "wind up" or turn the kind of RPM's other engines do...I'm actually planning to keep my engines running on the low end of tolerance as an effort to preserve them...I'll try not abuse them by squeezing every last RPM out of 'em...

Have Fun and enjoy those Great Engines!! They are really nice...

Chuck