Hello Walter. The prop is 12.25 x 6.4 nominally three blade carbon. Todd has tweaked it to his liking but that's what comes out of the mold. Today he was using 12 1/2% Nitro fuel. Not sure the rest but experience tells me it was 22-23 % oil. The muffler is a custom carbon fiber one. He has told me the name of the guy who made it but ..........The run is/was flawless. The .67 is superb as is the .61 and .76. The .67 might be the best choice for running on a muffler but they will all do that. IC lives on- three out of the top five.
Dave
The 61 runs just fine on a muffler, Ted ran it that way for a while on the first Trivial Pursuit using a header muffler and something like a 12.5-5 3- blade. Running the lower revs reduced the performance as expected, but this case did not take any advantage of the weight difference that you could use if you built it for this from the start. The “mistake” version I like was made, apparently when trying to make it turn a bigger prop with more 4-2 action, the mistake actually making it run faster, reduce the break, with a smaller prop- which is exactly what I would want and why I like it better.
If I was planning on doing like Todd, the 67 is a much better starting point than the 61.
Brett
p. s. As a comparison of the rev difference Todd ran 6+ inches of pitch and I am running 3.75