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How do I make an Ultra Cheap aluminum muffler?

(You may also want to see our page on soldering aluminum. Also, Air Hobbies Engine Accessories has silicone tubing and aluminum flex tubing.)

I must give credit where it's due--Rene Teo is the first that I've seen to implement this idea. He included one with an engine that I bought from him. I ran the engine with the muffler, but since he sold me the package NIB, he hadn't tried it out yet. I found several things that need minor improving, and include them in the description below. (I'll try to get some photo's soon--maybe there'll be a digital camera under the tree this year...)

An ultra cheap muffler that you can easily make is to buy a cheap die cast aluminum enclosure for $8-10. Just cut out a pattern that matches your exhaust port. You might want a piece of 1/4" flat stock between the muffler and the engine, to allow for some airflow. Alternatively, you could use a muffler extension. You can drill and cut the holes with a regular drill, a hand saw and a dremel to smooth it out. You might also want to have a backup plate on the inside, to transfer the force from the bolts to the box better, offering a better seal.

Now you need to insert some aluminum tubes for outlets and solder them in place. JB Weld might work, but the slow stuff it good for 500F, and the slow stuff is only good to 300F. Just make sure to seal the junctions, or you'll have oil dripping out of the seams. Tube termination inside of the muffler box is up to you. If you want some better muffling, cap inside the ends of the outlet tubes (run them into the opposite wall, solder plugs on them or pinch the ends) and drill enough small holes to allow the gas to flow. (Note that if your outlet tubes are 7/16", you'll need 13 holes, 1/8" each, to  make up for the same amount of flow. For 1/2" tube, you need 16 holes, and for 3/8" you'll need 9 holes. It's easy to set up an Excel spreadsheet to figure out the number of holes.)

I've found that 7/16" ID is the optimum size for 25cc gassers running in the 8k range. If you go smaller than that, you are restricting the flow. Dropping to 1/4", will cost you about 1,000 rpm. If you go too much larger--like 1/2" ID--you will get significantly more noise, and the midrange won't be nearly as smooth. I experimented with this on my Homey, and you get no RPM improvements going up to the 1/2" tubes. I've recently run a 46cc Poulan chainsaw engine with one of these mufflers. I left the two tubes (1/2" OD, .430 ID) removable, and when they were out, not only did the engine get louder, but it lost power as well.

If you are running a glow engine, you need to be concerned about muffler backpressure so you get adequate pressure to your fuel tank. I'd suggest you stick with the size that comes with the stock muffler, or in the case of the ST 2300, the size of ONE outlet tube. If you want two outlet tubes for cosmetic reasons, don't just cut the ID in half--remember, it's cross sectional area that you are concerned with, and that's a square factor. The 1/2" ID tubes have over 30% more flow of a 7/16" tube, despite being only 14% larger. Don't change dimensions unless you know what you will achieve. The formula for circular area is AREA=PI*R*R, or AREA=3.1416*R*R. (Notice that R is squared!) 

The size of the box also depends on your displacement. For smaller engines, a 1"x2"x3" box will work well. If you want a larger muffler, you can go up to 1.5"x3"x4". For most weedwacker conversions, one of the best size/weight boxes is the Eagle ME400-4591. With dimensions of 1.2x2.5x4.5, it's available from Mouser Electronics, and is a good size for weedie conversions. You can also get a similar box from Digikey, but unfortunately they have a minimum order of $25. Digikey also has a box that's 1.5x3.25x4.3", but it's nearly 8oz, and nearly twice as heavy at 7.9oz. 

One thing to watch out for in selecting a box is the wall thickness. You want it as thin as you can get it. The aforementioned Eagle box weighs 4.2oz with 1/16" walls, and will probably be close to 4.75 or 5oz by the time you add the spacer and outlet tubes. Not too bad for a $10 muffler, when a $45 BCM weighs 4.5oz. With a rough dimension of 1.25"x3.25"x3.5", the BCM is actually almost 1 cubic inch smaller. The larger box, on the other hand, has .125" walls, which is where all of the extra weight comes from.

Another thing to work on is sealing the box. I haven't worked with this yet because I only ran the muffler on the test stand before getting a stock unit, but I'm sure that some high temp silicone gasket sealer would work. (For a while, anyway.) If you don't seal the box, you end up with exhaust oil dripping down the box--and all over it. Just don't seal it so well that you can't open it to take the bolts out! Alternatively, you could seal it, but leave some closeable access holes on the lid for removing the unit. Even better yet, but it involves more work, you can build it BCM style with aluminum guide tubes going from the cover down to the bolt, with the tubes soldered into place. rcfaq

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Revised: April 18, 2006 .

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