You know your muffler is too restrictive if:

1. The engine overheats with it on, but does not overheat when removed. Better cowling can offset some of this, but if the muffler was not originally designed for the engine, changes to the cowl may not help. Imagining that making the cowling air exit hole at least 17 times the size of the inlet would be an inspired feel-good move here, but it won't do much to reduce the fried egg drama.

2. There is too much power reduction to do the job. If the engine without the muffler--but with the right prop--will fly the plane nicely, but with the muffler and an optimum prop (may not be the same one) will not, then there is too much restriction. Chuck the engine and the non-standard engine into the junk barrel and borrow a "big block" from Proparc. They are quiet, and have enough mojo to pull the engine bearers right thru the firewall.

3. The pressure tap provides so much pressure that even though you run the needle in, practically to the seat, it still runs rich. Or it blows the fuel line off the fittings. Or it blows your Suvvillan super-clunk tank out at the seam. Or, finally proves that you did have a solder pinhole on that metal jobbie, and that's why the uniflow never worked.

4. You can't stand the strained, moaning noise it makes. Kind of an uh, uhhh, uhhhh sound. Like a dead animal trying to roll over and get it over with.

5. It is so restrictive that you can't hear it. At that point, you might as well go electric and keep the oil off your beautiful 20-pointer. But then, you can still glue the muffler on the side of the cowl so you get big boy points. While you are at it, spill some castor on the nose and let everyone admire the front end for half an hour while you clean up after your IC flying session. Go with the rituals. Impression points are real....

Divot McSlow

   Part of the reasoning for a chip or tongue muffler is less weight in the nose. It's better to start off with too many holes or have as big of an opening as possible. The sound will be less than an open stack and for fields where "mufflers Required" it will fit the bill, unless you have to pass a certain decibel level. I have made them from Du-Bro muffler extension stock. Cut them a little bit longer than some of the commercial units that you see just for the extra internal volume. Drill and tap mounting holes to fit your engine. Cut a plate to fit the other end and drill and tap holes to mount that. Then shim the plate out as far as needed to adjust back pressure. Drill and tap for a pressure fitting. You can regulate muffler pressure to the tank by making and inserting different size insets into the line. The best of both worlds is a Scott Dinger light tube muffler. They only weigh about 1/2 ounce and have a healthy volume to let the engine breath. Randy Smith makes a similar unit now and then., and they both take a nice, shiny polish if you so desire.
   Type at you later,
   Dan McEntee

Hello
Modern engines seem to tolerate a lot more back pressure/restriction as their brass liners expand as they get hot. A lot of current engines have mufflers stuffed with baffles and tiny exit holes and run really quite. A lot of modern mufflers appear to be heavy dead weights but some are very light for their volume with thin walls (weigh them and you will see big variations between similar sized units, some OS mufflers are very surprising! ).

 Old engines especially those with steel fined liners and steel/iron pistons do not like too much exhaust restriction and often end up too loud in an effort to keep cool . Swapping to a smaller prop ie 9x6 instead of 10x6 on a Fox 35 helps reduce the heat build up with a muffler .
I run a lot of old engines on muck about models for my sons and wife and use a lot of' 'stack', 'chip' or 'tongue' mufflers with great success but would rather have a nice tube or custom muffler with adequate volume on high performance motors and if weight is not an issue a factory muffler but sometimes drill these out too.

Regards Gerald

 Chip mufflers do not lower the sound much if you are within 100 ft but they have a big effect on sound at 500 and more ft. They break the pulse into shorter length and that shortens the distance the sound will travel. They are a pretty much an attempt to save a flying site.
Ed

This Biggerish Tongue & the like sized one on the Spitfire arnt to rowdy . About 10 2mm or 2.4 mm holes on the Spit .

The C.S.C. Hunt Genisis muffler copy , WAS rowdy . In fact it was quiter at the handle when it fell off . on a Magnum 45 OS FSR clone .
But it was outboard , and throws outlet downward . Gone its Ex. away . so mayve just been the outlet aiming away then .

This Fuse. is over 4 inch wide , so theres a fair bit of the du bro Extn. there . Ends are filed ' Vee'd ' . Blok Sanding cleans up scratches .

Main advantage is its far less disruptive to the airflow . Inboard mounted can blank the wing and even turbulance wake generated blanket inboard tailplane . . Which is half the reason for rear Ex enclosed mufflers .

Well....not exactly.

The compression ratio is actually a mathematical formula. It does not change value due to backpressure.

What you can say is that the scavenging is adversely affected. That means that because of the higher exhaust pressure, less exhaust comes out. More burned gases will stay in. Less incoming charge may make it to the combustion chamber, and so on. So no question that the performance is degraded, but to avoid confusion it would be better not to try to redefine the meaning of compression ratio.

Dave

Chris,
Where should I be looking? I see the Makarenko mufflers being offered.

I am assuming that relative to diesels, what you are saying is that if a more restrictive exhaust is used, the required compression setting is different. That makes sense.

Dave

    Counts for precisely jack-doo-doo, compared to having the engine run correctly. We go out of our way to use 13 ounce engines with 3-4 ounce exhaust systems to get it to run better, not to save weigh.

      Chip mufflers haven't been used much in stunt for well over 30 years, so I guess we agree.

    Brett

Brett, I dont see the word 'stunt' or any reference to it in the original post so perhaps wieght savings count as doo-doo?