header pipe inner diameter

Discussion in 'Performance Mods' started by jaguar, Sep 15, 2012.

  1. jaguar

    jaguar Well-Known Member

    I had a 23mm I.D. header made and I just tried it out as a replacement for the 20mm stock one. (For my 55cc reed valve engine reving to 8500rpm with torque pipe). It gave better low end power and 6% more speed which means a broad range effect which may be attributed to better delivery ratio (% of cylinder space filled with fuel mixture). I think the too small header (for my higher rpm) causes a stronger suction after the exhaust pulse leaves which sucks out needed fuel mixture. The residue pattern on top of my piston backed up that belief. Some suction is desirable to help the mixture transfer up into the cylinder, but not too much. I think the calculator for header size on sheet 2 of my expansion chamber calculating Excel file is pretty right on. Now I need to make a few more runs and then look at the piston top to see how the pattern of residue has changed.
    By my calculator the stock 20mm header is right for these specs:
    48cc - 7900rpm
    55cc - 6900rpm
    66cc - 5700rpm
    So for the stock top rpm it looks like it's too big for the 48cc and just right for the 66cc.
    Last edited: Sep 15, 2012

  2. jigoku

    jigoku New Member

    will the inner diameter of header affect how long the header should be when tunning for low end and top end power? im trying to make my header pipe shorter without hurting low end, will increasing the header diameter do just that?
  3. jaguar

    jaguar Well-Known Member

    you would have to make a huge change in diameter but that would throw off power all thru the rev range.
    there is only one right diameter and one right length according to the port durations, exhaust temp, and peak rpm.
    shortening the header will shift the powerband higher and leave you with less power at lower rpm. You would have to lathe metal off the base of the cylinder to lessen the port durations to match a lessened header length. I just remembered that increasing compression lowers exhaust temp because the fuel burns quicker and is more "all burnt out" by the time it leaves the cylinder. Also advancing the spark helps. Also leaning the mixture.

    Last edited: Dec 6, 2012
  4. Scootmeister

    Scootmeister Member

    Hi Jag. Are your calculations specific to HTs? I have a Sachs moped with a 50cc engine that came stock with a 26mm header pipe and expansion chamber. It seems to me that most of the stock header pipes on HT and HS engines are too small to allow them to run at optimum power. I've started changing all of my standard header pipes to 1" ID conduit and the engines run a lot better. I read that any header pipe should be at least 20% larger than the exhaust port on the engine.
  5. jaguar

    jaguar Well-Known Member

    My Excel file shows the correct size needed for engine size and peak rpm. also the range of stingers you can try.
    It is not specific to HTs.