Inside An Airhead Muffler

Discussion in 'Airheads' started by wirewrkr, Mar 13, 2010.

  1. wirewrkr

    wirewrkr the thread-killer

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    I thought someone might appreciate what a 74-84 muffler looks like on the inside. I sliced one today before canning it.
    Rob ert
    #1
  2. walkingbear

    walkingbear Lets Play Chicken!

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    just looks like a big chamber .. where is the baffle?
    #2
  3. anonny

    anonny What could go wrong?

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    No wonder they sound like a sewing machine :D

    Cool!
    #3
  4. bmwhacker

    bmwhacker Still on 3 wheels

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    Thanks for the tour....I always wondered what went on in there.....quite a restriction at that one point........
    #4
  5. Airhead Wrangler

    Airhead Wrangler Long timer

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    Wow. That's pretty cool. I've never seen the guts of one of those before.
    #5
  6. wirewrkr

    wirewrkr the thread-killer

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    neither had I, hence the picture.
    pretty cool huh?
    #6
  7. Wirespokes

    Wirespokes Beemerholics Anonymous

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    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.
    #7
  8. motu

    motu Loose Pre Unit

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    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.
    #8
  9. danedg

    danedg Horizontally Opposed

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    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"...it'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...
    #9
  10. Airhead Wrangler

    Airhead Wrangler Long timer

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    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.
    #10
  11. Frank06

    Frank06 Been here awhile

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

    Very cool, BTW.
    #11
  12. wirewrkr

    wirewrkr the thread-killer

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    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.
    Robert
    #12
  13. Bigger Al

    Bigger Al Still a stupid tire guy

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    :photog
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  14. El Hombre

    El Hombre Banned

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    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?
    #14
  15. wirewrkr

    wirewrkr the thread-killer

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

    Max Headroom lost in the '70s

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    IIRC, that was in an article about hotting up airheads printed in the early '80s, along with a bunch of other performance tips. I'd been reminded of that tip when I was looking at the pics of the disected muffler earlier too.

    I'm quite reluctant to try that on my S/S Keihans, but am interested in feedback from anyone who has . . .
    #16
  17. bmwrench

    bmwrench Long timer

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    I did the hole saw trick a lot "back in the day". It does seem to offer a bit more power, and since Udo Gietl suggested it to me...It's not a lot louder, really, just a little throatier.

    I backed away from doing it when an older muffler started to come apart on me. That, and most of the guys who wanted fast bikes had moved on.
    #17
  18. walkingbear

    walkingbear Lets Play Chicken!

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    I've just installed a set of Pea-shooters from Epco and had the header's ceramic coated.a month ago.

    I'm experiencing .. low torque starting out and mid-range.

    Once it's going ..it's like a jet.

    Do I need to jet up or down? I need to get more air to the air box
    What is the best way to do this. I'm hoping to have this figured on on
    Tech day on Saturday
    #18
  19. wirewrkr

    wirewrkr the thread-killer

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    i Really like those pea shooters. These are the same set you got.
    Off of Ebay, "seconds"??
    I think they were a steal!
    Robert
    #19
  20. wirewrkr

    wirewrkr the thread-killer

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    Here's a better angle
    #20