ORIGINAL: Bigdaddy1963
Im also building a patriot xl with robart retracts and a brand new os 120 ax and thinking about a jett tuned muffler and a 14 x 12 apc. am i wasting my time?
OS120 is probably not the best choice here. Great engine, but that prop selection will be a real bear.
Play with it on the bench. With the jett-stream muffler, you want to prop the OS120 for around 10,000-10,500 ground peak rpm. Upwards to 11,500-12,000 is ok (engine is ok with it too).
You can squeeze out a 12" prop and clear the ground, so look more into a 12x12 or the 12x14 that APC has.
Again, you will need to properly characterize the engine on the test bench - so you know what to expect in the aircraft.
And, as noted, you should expect to add a bunch of weight in the tail end. Plan ahead.
Bob