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wirewrkr 03-13-2010 06:41 PM

Inside An Airhead Muffler
I thought someone might appreciate what a 74-84 muffler looks like on the inside. I sliced one today before canning it.
Rob ert

walkingbear 03-13-2010 07:37 PM

just looks like a big chamber .. where is the baffle?

anonny 03-13-2010 08:36 PM

No wonder they sound like a sewing machine :D


bmwhacker 03-13-2010 08:41 PM

Thanks for the tour....I always wondered what went on in there.....quite a restriction at that one point........

Airhead Wrangler 03-13-2010 08:44 PM

Wow. That's pretty cool. I've never seen the guts of one of those before.

wirewrkr 03-13-2010 09:27 PM

neither had I, hence the picture.
pretty cool huh?

Wirespokes 03-13-2010 09:56 PM

I've also wanted to know how they worked! Thanks!

But... now I've got more questions, of course!

So... I see the inlet emptying into the first chamber. Was it meant to exit through just the one hole we see, and the rest of the tubing rusted away?

Then there's a tube connecting the first and third (end cap) chamber.

Now the question: how do the gasses pass from the third back to the second (middle) chamber?

There's a guy in the area here who used to build stainless mufflers - Tom Webb. One of his friends did some research on exhaust technology and came up with the idea that BMW built the mufflers backwards (whatever that means). So Tom built some as directed and with only a slightly throatier note, produced 7 more HP on a 90S in a dyno run. They give the feeling of being in one gear lower.

I've always wondered just what they did to achieve that result. I asked him, but didn't get much of an answer. Perhaps I'll have to have him draw me a diagram.

motu 03-13-2010 10:45 PM

An old theory goes,change the direction 3 times.So it goes into the first chamber,then through the little pipe to the end,turns around and comes back to the middle,then turns again to go out the out hole.And each time it hits a chamber it expands.....controlled expansion,the correct way to contain an explosion and make it safe for the public....and that way you don't get the loud bang.

danedg 03-13-2010 11:20 PM

Very cool...
Jezebel is a bit older than '74, but I assume that the theory and principles remain essentially constant even with new Asiatic repops?:1drink
Wirespokes' comments, allude to my longheld theory that a certain amount of backpressure created by the exhaust "system" will yield HP benefits.
I'd guess that's what the skinny chamber would do...:D
Stagehand, on the other hand, has poo pooed the notion that my exhaust system needed to be "tight"'s a/2!.. just stick 'em on there!
And between the crossover pipe and the "slip on" silencers, I assumed he was right....:ear
This year, I've actually achieved some gaseous integrity, and though I've not many miles yet, it seems to have made a large difference in performance...better....:clap
When I first got the bike it had a pair of beat up Sirwin's on it...loud as hell and straight thru...I think some PO put 'em on to improve performance...

Airhead Wrangler 03-14-2010 06:28 AM


Originally Posted by Wirespokes
Now the question: how do the gasses pass from the third back to the second (middle) chamber?


If you look really closely, the wall between these two chambers appears to be perforated. You can see in the cut that the cross section isn't continuoous.

Frank06 03-14-2010 06:55 AM

How did you cut it? It looks like it was done on a mill.

Very cool, BTW.

wirewrkr 03-14-2010 08:55 AM


Originally Posted by Frank06
How did you cut it? It looks like it was done on a mill.

Very cool, BTW.

I have a 4x6 bandsaw and I use it in the upright position ALOT.
I just fed it straight in and held on for dear life..
Same way I did the cylinders awhile back.

Bigger Al 03-14-2010 01:40 PM


Originally Posted by wirewrkr
I just fed it straight in and held on for dear life..



El Hombre 03-14-2010 03:06 PM

I've read about using a 3/4" hole saw mounted on an 18" bit extension and drilling the baffle. Now that I can see what's exactly in there, I might not do it. That would bypass almost all of the baffles.

It's supposedly only a little louder, but from you photo, I don't think so.

Anybody try it? Too noisy?

wirewrkr 03-14-2010 03:25 PM

Tried it with a long tapered punch, to about 1/2" or so. Not that noisy, but a little more bark.

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