RE: OS 33GT Gasoline Engine for Pattern
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I have not used any muffler on any of my gasoline engines so I don't have a first hand comparison. But judging from power/rpm numbers people are quoting for specific props on muffler, a pipe gives considerable boost in output over the quoted numbers. A pipe enables higher pitches to be used for the same rpm and diameter. OR larger diameter for the same pitch and rpm, but you will have to be careful with larger diameters because noise can increase fast.
A pipe also significantly reduces the noise footprint over mufflers. Expect to be drowned out by any muffled engine flying. Couple a pipe with a 3 bladed prop and a soft mount and you will hear even a larger difference in noise footprint.
I promised photos but just couldn't get to it last night. I may get to it tonight
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I've often wondered as we lengthen our pipes to "soften" the transition (or "coming on the pipe) that we hit a point where ultimate horsepower is not increased with a tuned pipe. I ran an OS 1.40 on both a tuned pipe and a muffler with good success turning a 4 blade prop (and extremely quiet on both). I did notice the mid range wasn't as strong on the muffler, but the ultimate rpm's were pretty close to the pipe, which means the engine was producing similar ultimate HP. The muffler gave a more linear response to throttle.