I am using an O.S. .91FS with an optional 90 deg. elbow between muffler and engine. Muffler is hardly noticable as it now can be swiveled "mid-belly". Keeps the model clean too. Very little cutting to accomodate the engine head. Lots of cooling area available at the "cheeks". As others have said, " it's fast", very slippery. Landings give me fits sometimes as it doesn't want to stop flying unless I can put it into a pretty good headwind upon landing. Great airplane!!