Exhaust exhaust port header mismatch

Discussion in '2-Stroke Engines' started by emdude, Jun 28, 2010.

  1. emdude

    emdude Member

    Hi All:

    Yesterday I replaced the leaking exhaust gasket on my 66 cc China Girl. In looking at the patterns "embossed" into the old gasket it became far clearer than it had been that the exhaust port (big and square) to header pipe (small and round) match is really pretty bad...or non existent. And I don't think the header pipe can be matched to exhaust port with enough meat left to be safe but would gladly stand corrected by anyone that has accomplished this. I think the header pipes internal area is significantly smaller than even the beginning of the exhaust port right at the cylinder exit.

    But this leads me to the following question:

    Is anyone aware of ANY aftermarket exhaust that takes matching of the exhaust port and header into any consideration. I've looked but didn't find anything definitive so far.

    Seems to me this is a real weak point in the overall design. Even the stock mufflers could provide better high rpm performance if this mismatch were eliminated.

  2. AussieSteve

    AussieSteve Active Member

    You can grind the stock ones out to almost match, but as you say, there isn't much meat left.

    Here's a pic of the header section of the Sick bike Parts expansion chamber.
    It's a closer match and could be ground out a little more.
    (In fact, it needs to be ground out and filed a bit, at least, to square things up.)

    Attached Files:

  3. emdude

    emdude Member

    Thanks for the pic, that looks pretty good really. That attached to an appropriately sized header pipe on a stock muffler would probably be an improvement.
  4. AussieSteve

    AussieSteve Active Member

    If you ask Jim and Paul at Sick bike Parts, they might sell the header section separate.
    Be pretty easy to make up from plate and tube, though, to get a really good match.
  5. KCvale

    KCvale Motorized Bicycle Vendor

    I am headed out this morning to get a Deremel tool to do some exhaust port matching on my Grubee SkyHawk kits because they match is pretty bad on them too.


    The gasket fits the head port well, but you can see how poorly the tailpipe matches with it.

    I will post some pics and evaluation of how it goes as the exhaust system is actually a part of the intake system.
    This animated .gif on SpookyTooth illustrates it pretty well.


    You just don't want any obstructions in that system, it's tuned.
  6. Rob930

    Rob930 New Member

    Very good pic, KCVale. I read a few intake/exhaust engineering books a few years back when I was designing a new exhaust system for my turbo car. Designing an exhaust to scavenge and "supercharge" (compress) the chamber for the intake part of the cycle is a challenge but worth the efforts. I started messing with small-scale exhaust designs on RC cars and go-karts, but haven't had the time yet to address the poor stock muffler that comes with the Zoom kit.

    Yes, the port matching is terrible in all regards, but what can I expect for such an inexpensive kit! Even the washers aren't punched in the center! Oh, and one head nut out of four on my engine didn't have any lock washer. It's almost humorous, but also a lot of fun to try correct the mistakes.
  7. KCvale

    KCvale Motorized Bicycle Vendor

    I found a file that works at Ace and ported 2 tailpipes yesterday.

    I just used a C-clap on my waste high bench to hold it.



    I didn't go through the pipe and into the welds, after gringing away the welds keeping the washers from sitting flat I didn't want to risk it, I just removed all the extra flange and debris to match the inside of the pipe, which is actually a lot.

    It may not look like much in this comparison pic, but as the spooky help mentions, you can sure feel it with your finger.


    The one on the right is new, the one on the left I pulled off my ride so I had know power for comparison.

    After a brief ride that extra 20 minutes filing is worth the reward ;-}
    That will a standard feature on all my new builds too.
    lucianomi likes this.