exhaust pipe extension for more grunt

Discussion in 'Performance Mods' started by jaguar, May 23, 2011.

  1. jaguar

    jaguar Well-Known Member

    After reading a research paper revealing how straight exhaust pipe lengths affect delivery ratio (% the cylinder fills with air/fuel mixture) I experimented with pipe extensions on my 48cc piston port engine with 6.9" intake length (cylinder to carb slide). The research paper had specified this intake track length so I duplicated it. Here's my results:

    Pipe _ street 1 __ street 2 ___ flat
    16"___13.5mph____20.0mph____25.5mph (best)
    10"_____X________18.0mph____25.0mph (standard pipe length)

    The "X" means I couldn't climb the street without pedaling.
    Pipe length is actually the exhaust length from piston to muffler. (On mine the pipe extends 5.75" inside the muffler also.)
    Street 1 is steep for this engine.
    Street 2 is a moderate incline.
    "Flat" is just a flat street for testing max mph speed.

    I used automotive hose to test with but found out the heat is too much for it after a while. You should weld in the same diameter pipe for a permanent setup.

    Attached Files:

    Last edited: May 23, 2011

  2. jaguar

    jaguar Well-Known Member

    Advanced grunt pipe

    The next test was cutting an opening in the exhaust pipe right after it enters the muffler, and removing the baffles. (see first attachment) Then I attached a can to the end of the muffler with twelve 5/32" dia holes in the end of it as a silencer. This caused a gain of 4mph more going up the steepest hill. This is definitely the way to go if you are climbing hills (in addition to the intake extension). And it didn't rob speed on the other two streets. If you have a muffler with an internal stinger pipe (see second attachment) then you may not need a silencer. But then again it may not be as good climbing hills.

    Attached Files:

  3. rustycase

    rustycase New Member

    Thanks so much, Jaguar!

    I am very interested in broadband power increase and do NOT want to increase noise to an objectionable level. Top speed and hi rpm HP are not a goal, at this time.

    Yet it might be, for fun runs later this summer when I make further progress on my basket case project! The Grubee sponsor ad here on the forum always catches my attention with the pic of the fellow at the Salt Flats !

    I don't know that I'd ever be able to run through the traps at Bonneville, but I'd sure like to see how many MBers would be interested in speed runs in the vicinity of the Pozo Saloon. (google it)

    Tnx for all your tech information!
  4. jaguar

    jaguar Well-Known Member

    It's hard to get high rpm power out of this 48cc engine due to the limited area of the transfer ports so its best to try to make due with the rpm range available and maximize what you can.
    With a bit of course brass brillo pad inserted into the home-made silencer the noise level returned to about what it was before but it had a better/meaner sound to it.