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Engine basics => Engine set up tips => Topic started by: Robert Zambelli on August 19, 2011, 08:22:49 AM
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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. LL~ HB~> :X
Bob Z.
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IHMO
Tounge mufflers are not mufflers at all and nothing more than exhaust extensions. The minimal sound abatement they provide is exclusively created by forcing the exhaust to exit 90 degrees to the source. I Imagine that they significantly mess with engine performance.
Below is a link to a interesting discussion on muffler design.
http://www.supercoolprops.com/articles/expansionchambersmufflers.php (http://www.supercoolprops.com/articles/expansionchambersmufflers.php)
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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
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I had a situation where I wanted to use a stock muffler on a K&B 4011, but it was a half inch too long. I cut it off and plugged the back end. With a 3/32 drill (first one I picked up, I guess) I did what Bob said, and drilled holes equal to 120% of the outlet area. It worked fine, just like a stock muffler. I wonder if drilling several different size holes would make any difference.
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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.