Stock muffler and an APC 13x6 Sport was 11,800 and with the pipe I was able to get it to 12,900. Stick a 1/2'' piece back in and I can run a 13x8 at 12,000. Throttle response with the 13x6 is sick! This is going on a 6.5lb Rookie which has a 59'' slightly swept wing. Should be a fun combo. I need to get a 12x8 APC and try that but I had a limited number of props with me today since I wasn't sure what this thing would like.
5% Ritch's Brew with 22% oil (11/11) and it is 86 degrees here in the Dallas area.
Is 12,900 decent RPM for this motor or does this thing like more RPMs? The size of the carb suggests this is a lower RPM torquer but I'm just guessing.
Those numbers look impressive. Do post your numbers on the APC 12x8 - for comparison, my Jett .90L with Jettstream muffler gets about 13K with the 12x8.
Here's a tip - for each of your runs for each prop/pipe length combo, use Pe Reivers' spreadsheet to calculate the horsepower you are getting. Plot those on a graph and you'll see where the sweet spot is rpm wise. We did this with an MVVS .77 and found that it was happiest piped to do between 10,500 and 12,000 rpm. Anywhere in this band, we could pipe the engine to get a little over 1.9 HP calculated.