Although I've made quite a few of these, I've never giver much thought to the actual operating principles.

For example, is there a specific internal volume and shape recommendation for the type, size and operating characteristics of the engine?

And, here's something I've recently thought about. The hole size. For example, if it appeared that the muffler was too restrictive, would the proper fix be to enlarge the holes or add more holes?

When I make one, I calculate the area of the original (tube type) muffler discharge hole and then insure that the total area of all the holes in the tongue muffler is around 120% of that. For no specific reason, I normally start with twelve holes.

Opinions? This could get involved as it relates to thermodynamics and compressible fluid flow through a sharp edge orifice. 

                                   Bob Z.

I quit using tongue mufflers only because all of them are too short!  They don't do much muffling, but they should provide for squirting all the exhaust goo away from the fuselage.  The short ones don't do that.

Floyd

I used a Fox tongue muffler and modified it to fit my Super Cyclone. I drilled the holes out one size for more flow. It worked OK but was a little louder than I wanted. If I had it to do over I think I would have made more smaller holes and it would be quieter. It did knock off the bark somewhat the way it is now though.