Hey airbus --

Back in the day, as they say, the Rossi rear exhaust would turn close to 14.5 - 15 on a RevUp 11X7 prop using Cool Power 5% -- remember the Rossi's are/were European engines set up for no nitro. I was a copy cat in those days (maybe still am) and never tried a different prop on an MK Arrow and a few other planes after that prior to going to the YS inthe Aurora type setup. I wish I had numbers for my setup length but I don't. I recall going too short with the header and putting back inside the coupler the piece I had cut off. The needle gets touchy and the engine may burp/surge in and out of being on the pipe when flying. A little long means giving up a slight amount of RPM but with the side benefit of never having to worry about an engine run. I had the luxury (and expense) of the inflight mixture but I hardly ever used it. At the conservative setting/length it was very consistant and easy to deal with. The go fast days were fun -- unfortunately all my stuff is in other hands now or maybe in landfill...

Have fun and good luck!
Tom