I cannot give you any technical data on running no muffler. I can tell you when I first started with model glow engines there was no such thing as a muffler. At least not one I had ever heard of. Then a few years later people started talking about the day all of us would have to run with a muffler. You should have heard the screaming and shouting. "We will be so underpowered with mufflers our planes will never fly", and the likes.

Bob