While mufflers are primarily intended to reduce sound, they often do so at the expense of horsepower. You can build your own muffler for increased performance and better sound, but odds are you're not going to save any significant money over a cheap "glass-pack." Still, it's one more assembly you can point to on your custom car and say "I built that."
There are two basic types of mufflers, "glass-packed" and "baffled/chambered." Glasspacks (the kind used in most stock vehicles) have a perforated center core surrounded by fiberglass, which absorbs all sound waves regardless of their tone. More fiberglass packing makes for a quieter muffler, which is one reason why performance glasspacks are so small. Additionally, glasspacks are cheaper and easier to build than baffled mufflers.
Baffled mufflers use a series of internal metal plates (baffles) to reflect the soundwaves back onto themselves, which results in a phenomena called "sound cancellation." While they tend to flow a bit less than straight-through glass packs, baffled mufflers often yield a lower and more pleasant exhaust note.
Building a Glasspack
Start with a regular, oval shaped car muffler. First, cut both ends off of the muffler (where the pipe stubs are) as close to the outer case as possible. Once you have the ends cut, cut the internal tube to separate the ends. Cut the pipe stubs off of both ends, and weld a sheetmetal plug into the circular holes where they used to be. Cut a 2.5 to 3 inch hole in the center of both ends.
Measure the length of your muffler case, and cut a length of exhaust tubing (again, 2.5 to 3 inch outside diameter) exactly 6 inches longer than the case. Measure 4 inches from either end of the tube, and mark a line around its circumference. Swiss-cheese at the section between those lines with a 1/8 inch drill bit, spacing your holes no more than 1/8 inch apart.
Assembling the Glasspack
Slide your tube into one of the muffler-ends so that the perforated section is just below the end-piece, and weld it in place. Install that end piece onto the muffler, and then weld that in place. Tightly pack the entire case with dry, body-work fiberglass (you can also use stainless steel wool, but it'll be louder) available at your local auto-parts store. DO NOT use household "pink" fiberglass; the resins in the pink fiberglass will quickly catch fire while you're welding.
Repeat the end-capping procedure on the open end of your muffler, but don't run a continuous weld around the seams. Instead, spot weld the pipe onto the muffler end and the end piece onto the case. Keep the whole assembly as cool as possible to keep from melting or burning the fiberglass. You can just tack weld the end piece every 3 inches and seal the seams with lead solder or silicone, but it won't look as pretty.
- Photo Credit exhaust pipe image by A74.FR Ben Fontaine from Fotolia.com
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