How to repack a sealed muffler

By Andy Hollis
Sep 24, 2023 | DIY, Shop Work, muffler, GRM+ | Posted in Shop Work | From the Nov. 2014 issue | Never miss an article

Remember how great your car sounded when that straight-through performance muffler was brand new? Over time, though, the fiberglass packing burns out, changing the tone and increasing the decibels. This also has a detrimental effect on performance, as the straight-through design stops emulating a straight pipe’s flow and works more like an expansion chamber. 

Round mufflers–like those from Burns Stainless we’ve tested here in the past–can be unbolted and repacked at low cost, but not oval mufflers. Our Honda CRX uses a combination of both shapes to attenuate different frequencies, but the exhaust note has been getting louder lately–and we have sound limits to meet at some events. 

Repacking the Burns in the system’s center section helped, but just tapping on the case of our rear-mounted MagnaFlow oval muffler produced a nasty, hollow ringing sound. The gears in our head started turning: We had a hole saw and a welder. Who needs bolt-on end caps?

Replacing the oval muffler would have been the natural course, but being DIY types, we thought we could do better. Researching performance-oriented muffler repacking, an every-race task in motorcycle racing, brought us to Bristol Core as a supplier of premium stuffing. 

Simple, inexpensive, short-strand fiberglass easily finds its way out through the perforated muffler core, so long-strand varieties are the best for mufflers. Bristol supplies a wide range of products to suit your budget–in this case, we spent about $50 total. 

Step 1:

Sealed-for-life muffler? Don’t worry, you can still repack it. For our redo, we asked Bristol Core for their Spec 31 fiberglass, which is good up to 1300 degrees. A kilo bag was the perfect amount for our 4x9x14-inch oval muffler that has a 3-inch core.

Step 2:

Our muffler’s inlet is offset to one side, so we had to cut holes in the opposite end to access the cavities on both sides of the core. We tamped down the material in bunches until solid–rapping on the case produced only a dull thud. An electrical box cover from the “motorsports department” at Home Depot provided material to cover up our hole saw’s intrusions. A little grinding and some high-temp engine paint made it presentable and rust-resistant.

Step 3:

The results were astounding: The car was never quieter. The material was so effective, in fact, that we could now forego packing the center-section muffler. We’ll definitely use this material the next time we need to pack the Burns, which typically uses less effective fiberglass matting. Subsequent race weekends with sound controls in place showed our car is now 5 decibels quieter. As a bonus, data logs from our engine management software showed an increased demand for fuel at certain rpm, which indicates a little power pickup at resonance points. All this for $50 of material and an afternoon’s fab time.

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Noddaz GRM+ Memberand UberDork
2/11/22 12:48 p.m.

Nice muffler rebuild!

I went to the MagnaFlow website to see how this offset muffler was constructed as a straight through muffler was constructed, but all I found was a short animated video of a straight through muffler.

My question is:

What does that muffler look like inside before you packed it full of fluff?  Could you actually see through the muffler if you held it up?


John Prieve
John Prieve New Reader
2/11/22 1:50 p.m.

Too bad there is no information on how to get the old packing out. I've had motorcycle silencers were the packing was all carboned up and as hard as a rock. It was one solid chunk.

dean1484 GRM+ Memberand MegaDork
2/11/22 4:02 p.m.

Always wondered about this!!! I have a muffler from a race car that burned out and I have always wondered about re packing it.

A side note -  Welding galvanized metal like the electrical block-off plate that was used to close up the muffler in the article can make you feel a bit under the weather.  It is actually called Metal Fume Fever.    Kind of like a hangover or a mild case of the flue.   It is from the zinc burning.  I have had this and it sucks.

WIKI >>>

I am sure you all know this but figured it was worth posting up.  Better to go to the other end of the motorsports department at Home Depot and get a 12" square piece of regular sheet metal.  It is also sold at most local hardware stores.  They have a small rack with miscellaneous pieces of sheet metal and bar stock.  While not as cheap as the electrical box cover it is much better for welding.  


Pete. (l33t FS)
Pete. (l33t FS) GRM+ Memberand MegaDork
2/11/22 6:32 p.m.

When I did mine, I sliced the top and bottom off and packed very thoroughly, then spent about eighty pounds of wire welding it back together.  Worth it.

Could pack around the front (on right) but getting full wrapping around the rear required slicing the other side.

crankwalk (Forum Supporter)
crankwalk (Forum Supporter) GRM+ Memberand SuperDork
2/12/22 1:00 p.m.

Honest question: By the time you've bought the supplies, cut, stuffed, welded, and repainted the muffler, and paid yourself for the time cost....couldn't you just have bought a new one? I'm not part of the throw away society where the answer is to buy a new everything but this seems like a borderline exercise for $50 and a whole afternoon of fab time. I just repacked straight through muffler and it was $20 and 20 minutes of time but I don't know I would have messed with this. Hell, I probably wouldn't have even bought a new muffler and just lived with it being louder unless I had to meet some decibel requirement. 



Pete. (l33t FS)
Pete. (l33t FS) GRM+ Memberand MegaDork
2/12/22 1:40 p.m.

In my case, the mufflers are NLA. Racing Beat changed suppliers and the new ones are not the same build quality or sound reduction.

SupraFiend New Reader
4/6/22 2:55 p.m.

Yeah, depends on the value of your muffler for sure. A good quality stainless one will last forever though, and often are 200 to 300 dollars. But frankly, that looks horrible. I would go thru the top like the other guy did above if you can't do it in the back, no need to mess up the end you see. Also, just find steel that is the same type. Don't weld mild to a stainless muffler. You buy a stainless muffler so it will last forever, if you weld mild to it, it will rust out there eventually, regardless if you paint it. Hit a metal mart/recycler, it's often not that hard to find a little bit of stainless sheet steel laying around places like that. If you get some strips of stainless sheet, you can cut square sections off the top and then use the strips to cover the gaps from your cuts and just weld the same piece back on. 

So this packing material can survive being welded? Be carefull what you use for packing material, try and light it on fire before you stuff your muffler with it or you'll make a blazing inferno and engulf everything you just put back in there as soon as you start welding.

fearlesfil New Reader
4/6/22 3:08 p.m.

In reply to John Prieve :

Try CRC Intake Valve & Turbo Cleaner to "Dissolve Baked On Carbon Deposits"

fearlesfil New Reader
4/6/22 3:10 p.m.

In reply to dean1484 :

Welding supply stores have masks good for welding galvanized and stainless (the latter to avoid cancer)


Pete. (l33t FS)
Pete. (l33t FS) GRM+ Memberand MegaDork
4/6/22 3:17 p.m.

In reply to SupraFiend :

Depends a lot on the application.  Stainless is a much weaker material than mild and will break.  I repacked a Racing Beat muffler because the old ones were mild steel.  The new ones are stainless and they crack within one or two heat cycles.

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