Custom Expansion Exhaust

Discussion in 'Performance Mods' started by Gchdavid, Aug 10, 2014.

  1. Gchdavid

    Gchdavid New Member


    I am sure you have seen your share of expansion exhausts on here but I figure I would share my project. I still haven't ran it but will post once I do with any findings. I literary just finished welding it and based on my math it should give the engine some nice mid range punch. The exhaust is meant to function without interfering with the pedal arms from the jackshaft kit. It does require the use of a rear mounted kickstand. Also, I have plans to add the OEM muffler to the end of this pipe and remove baffles until it sounds just right as I would expect it to be quieter than stock if I just throw it on there. I already have a carb upgrade that I have also not tested (waiting for a replacement freewheel for pedals, switching to pull start from now on)

    The bike is a schwinn sanctuary 7. Plans for it are a banana seat so I can be further back and that's it. I've been using it to commute to the office since I got it running until the issue with the freewheel, plenty of thumbs up on the way.

    Would love to hear some opinions.



    Attached Files:

  2. jaguar

    jaguar Well-Known Member

    workmanship doesn't look bad but your short baffle cone is too steep. That causes a mid range slump as the price to pay for good (but short lived) top rpm punch.
    but kudos on the attempt. Everything we do is a learning experience if we have an open mind.
  3. Fabian

    Fabian Well-Known Member

    The workmanship looks nice and i like the flowing (frame hugging) lines of the pipe, even if the pipe architecture isn't as well designed as it could be for midrange power.
  4. HeadSmess

    HeadSmess Well-Known Member


    stuff the stinger up the rear cone. cuts a few dB off :) dont feel tempted to flatten that cone if clearance is an issue.

    the original muffler makes a great silencer, normally i gut the thing, hack it off about halfway up the hemisphere, and weld it straight onto the cone. replace the 10mm outlet with the same or slightly larger as the stinger pipe. then weld the end cap back on because im too lazy and see no point in having it bolted on.

    what welder did you use? if mig, try "stitching" rather than attempting full beads. if it was a stick...WOW! doesnt look burnt enough to be oxy, and if you had a tig... it would be near immaculate unless its really thin stuff!

    BRACKETS! preferably flexible type, as using rigid mounts will break. even tie wire gives enough flexibility to outlast a solid welded bracket, if not looking rather dodgy ;)

    final tip?

    ok...why doesnt "manage attachments" show previously uploaded pics anymore? whats going on here? pic...if its there below, shows....

    CARDBOARD! or 200gsm paper... light cardboard ;) make a model first.

    and whats that download? cone? hard to find the early version without the time limit thingy anymore...

    what did you use for the calculations? looks suspiciously like mh aerotools with bad input data...

    Attached Files:

  5. Gchdavid

    Gchdavid New Member

    Hello HeadSmess,

    The stinger is all the way into the center of the chamber. The welds were made with a Tig that I normally use to build turbo headers/manifolds out of stainless with. The EMT pipe welds ugly and that is most of what was used to make this combined with some aluminized exhaust pipe. I wasn't really going for super nice looking welds, just wanted to test the functionality of it so I didn't fill the pipe with Gas to weld it or clean off the galvanize/aluminized. The initial fitting wasn't that precise either so I filled some gaps with weld. If it works or doesn't, I will make changes and then make one where I make the welds look like droplets out of stainless material. I actually used alot of Jaguar's advertised pipe dimensions as well as reading two other articles with some simple math. It has the same major diameter and very close lengths as Jaguar's pipe up to where the end cone is where I did not add the extra straight piece. I wanted to try it this way, which I know is more peaky, before I do anything else to it.

    Also, thanks for the recommendation of using the stock muffler as a silencer. Can you be a bit more specific as what you did to it? Do you shorten it, or just strip it out?

    Thanks for the comments.

  6. HeadSmess

    HeadSmess Well-Known Member

    " normally i gut the thing, hack it off about halfway up the hemisphere, and weld it straight onto the cone. "

    i thought that would be fairly self explanatory :(

    maybe pics are better...


    standard calculated cone (rusty) next to the extended cone (black). next time im making a pipe ill sleeve at the belly so i can test the two against each other without hacking and welding.

    p.s. inside that long cone is another cone the same dimension as the rusty one, just made from perforated mesh. two stroke wizard v.5 with the duel add on. seems to do all that it claims to do. just takes some work to make the multiple tapers. yeah, the only straight sections are 30mm of header, and the belly. as for welding perforated gods!


    yet another pipe, with just the standard cone again. two stroke wizard again, but the older v.3.

    yuk.i hate exhaust pipe material... true, nothing welds that nicely, at all. meh, low resolution, isnt really that clear anyway ;) why i asked.

    the benefit of having sheet steel and a roller ;) its just making the long tapers that pose a challenge. one day if i finalise on a design ill taper turn some bar and after tacking, just wack it into shape using that. yeah, and one day ill get the plasma profile cutter up and day.
  7. Gchdavid

    Gchdavid New Member

    Here are some updated pics of the "pseudo" finished project. I am still working on a good mount for the rear muffler but at least it works. I modified/gutted it and it still made the bike super quiet which was good because when I first got it going just with the pipe it sounded like a dirt bike. I actually noticed quite a bit more power with the experimental chamber, specially in the mid range. It probably wants to generate even more at high rpm but vibration and lack of ignition strength hold it back...I'm guessing the ignition will be my next upgrade. Regardless, the mid range has definitely beefed up and you can feel the engine get "on the pipe" :)


    Attached Files:

    Fabian likes this.
  8. Fabian

    Fabian Well-Known Member

    That pipe has a nice organic looking lines as it follows the frame, and it looks quite stealthy when viewed from behind the bike.
  9. jaguar

    jaguar Well-Known Member

    ha! Fabian is being persuaded to give up his anti-expansion-pipe stance. love it!

    Cut the baffle in half and weld in a 30mm extension. That will lower the "hit" and lengthen the powerband.
  10. HeadSmess

    HeadSmess Well-Known Member

    the offset crank due to the pullstarter helps for clearance down there. plus the extra long distance between BB and the rear axle.

    not so much space with MTB's and standard cranks unfortunately. usually theres less than 30mm just where you want a nice 50-60mm belly :(

    amazing how much difference just an expansion chamber on the end does for noise.

    i call a tuned pipe a tuned pipe and a big bore hollow can an expansion chamber. regardless whether its tuned or not. it just allows expansion with no thought about pressure waves and length. let the gas cool, let it escape in a more steady stream, reduce the "pulse" or noise at the outlet.

    mh aerotools does have a nice lil calculator on expansion chamber silencers, just varying the amount the inlet and outlet are pushed in can make huge differences on which frequencies are attenuated or accented. and the resonator type, which are very selective on what frequencies theyll attenuate. use this applet, and play with the low pass/high pass demo files for an idea of whats actually going on in there ;)
    the tuned pipe dimensions given by mh aerotools do leave something to be desired but.:thinking:

    side note, port matching/flowing curves makes all the difference. changed the stock header on an rc car from a nasty right angled thing that killed all aspects of the following pipe, with a nice curved its a challenge to use on the dirt as it actually has a powerband!

    from this

    to this

    theres also the "swiss muffler" method... which is what most people actually picture when thinking of silencers. perforated pipe, wrapped in fibre, with no baffles. typical yz250 or similar type can...