OK, I finally decided to try a new system, primarily to try to improve the hot weather/dead air performance, where I think electric, with the heavy line tension, seems to have an advantage. I have a few flying sessions on this system and today, at 102 degrees, it was incredibly good. The primary difference is the prop
STOCK RO-Jett 61 BSE "Brett version"
STOCK head clearance
Stock "long" venturi, drilled with #5 AWG drill, and SIG eyelet spigot with about .030 projection
Eather 13-4 3-blade flat back (blue dye) cut to 12.5 and pitched to 3.75 helical.
Billy/Randy 51-61 Slimline pipe, 17 5/8 to first baffle
Powermaster 10% "Air" or Powermaster 10% RO-Jett or equivalent. 6.3 ounces at 102 degrees and sea level (use 15% to start in the Midwest/Southeast)
Launch revs = 9600-9700
Engine should operate in deep 4-stroke, and just touch about 50/50 4-2 in the tops of the square 8. The break is as smooth as you could possibly imagine, as good as a 46VF.
This setup required a further-aft CG to make up the turn lost by the wider-blade prop, and the line tension is substantially heavier than the standard system. It works incredibly well in dead air and hot conditions. It will work pretty well in dead and cool conditions. I do not know if it will work as good as the previous baseline in heavy wind (where the previous system was nearly miraculous, basically, I was hoping for 20 mph most of the time).
If you want to switch, use the same setup with the 12.5 3.75 (helical) with the same settings, I tried it and the one difference (pipe length 3/16 longer) isn't different enough. My next test is to pull it out to 17 3/4, which probably is too long for this prop but might work even better with the cut-down 13-4. The test after that is to try it at 12.25 instead of 12.5 to improve the cornering, or at least try to get it back to the same CG. David has some buzzed-down 13-4s that I am going to trade with him, since there's no point to cutting up props if you don't have to.
Note also that it will also spin the unmodified 13-4 3-blade with no problems in terms of the engine, but my airplane didn't like it very much, and neither did my right wrist. This is the same prop that David uses on the 75, at 4.2". Obviously the 61 doesn't have the power of the 75 so it runs it leaner, but still in a 4-stroke in level flight.
Brett