Update on the OS 120 AX in the H9 Showtime.

Amazing engine, still only have about 6 full tanks through it but very happy. I'm spinning an APC 17x6 prop.


Exhaust report:
I was using a shortened Bisson muffler specifically made for this engine I then chopped down the expansion chamber about 2 inches to make it fit in the cowl, then narrowed the exhaust diameter to replace the lost of back pressure. With this combo I was getting about 8400 RPM far less than the reported 9200. This combo will hover at about 3/8 throttle and has great pull out even at these lower RPM numbers.

I then switched to the stock Powerbox muffler with the pitts adapter and immediately got 8900 RPM. I figure this will improve closer to the 9200 as the engine completes break-in phase. The sound is improved (quieter but deeper), at lower RPM it now doesn't have that irritating 2 stroke rattle/ratatattat sound that it had with the Bisson though full throttle does not sound as impressive. I figure the increase in backpressure will help transition performance (still breaking in) but I'm really excited about the extra 500 instant RPMs. I have not flown this combo yet.

All figures are using 15/15 PowerMaster fuel at sea level.

I may also try the APC 16.5x5W prop and switch to a plastic spinner to reduce weigh and improve transition performance.