Expansion Chambers "tuned length"

Discussion in 'Performance Mods' started by jaguar, Jun 9, 2011.

  1. jaguar

    jaguar Well-Known Member

    Is your expansion chambers "tuned length" in harmony with your engine? (I doubt it.)
    (The following info was derived from the technical info in Graham Bells Performance Tuning guide.)
    Just measure the length from the piston (at the exhaust port) along the centerline of the pipe to halfway down the second "cone". This is the "tuned length" in inches.
    Next you need to know the exhaust duration in degrees. That is the total degrees the port is open. A standard 48cc Grubee engine has 136 degrees duration since it opens at 112 degrees ATDC. ((180-ATDC)x 2)
    To figure the rpm at which the pipe will deliver peak power just multiply the duration (# of degrees) times 1675 and then divide the result by the tuned length (in inches).
    The top rpm of a standard 48cc is only 5700, whereas most pipes being sold for them have a tuned length that delivers peak power at around 9000 rpm. That doesn't mean they will up your rpm to that level to deliver peak power. That means you aren't getting the peak power your engine is capable of delivering. To get peak power you need to extend the length of the pipe by adding pipe before the first cone (where it starts to expand).
    If you measure 25" as the pipes tuned length and you have a standard 48cc Grubee engine that hasn't had its exhaust port raised (which requires an even longer pipe to maintain the same peak power rpm) then the peak power rpm of the pipe would be 9112 rpm which is very hard to achieve by porting on these engines because of the limited size of the transfer ports. So if the pipe increases engine power between 6,000 and 12,000 rpm and your engine only revs to 6500 then guess what? You're not getting hardly any benefit from the pipe other than a cooler sound and better looks. You don't even know what you are missing!

    The proper tuned length for a 48cc Grubee engine is 40" so that the peak power rpm of the pipe is the same peak 5700 rpm of the engine. That is the minimum length. If you make it even longer then you get an engine that gets a power boost even earlier in the rpm range and will still go strong to its maximum rpm.

    Attached Files:

    Last edited: Jun 9, 2011

  2. rogernelson

    rogernelson New Member

    Is it? Then it is a worrying fact.
  3. jaguar

    jaguar Well-Known Member

    If you cut off the muffler you will notice a big gain in low end power. I believe this is what most people experience when they mount an expansion chamber close to the cylinder. So its not without its benefits but its far from what can be had by making the distance long enough for it to be in tune with the engine.
  4. srdavo

    srdavo Active Member

    What gains could be expected by using a 40" header pipe connected to the standard kit muffler? (baffle removed)
  5. jaguar

    jaguar Well-Known Member

    Its not the same resonance effect with a muffler as with an expansion chamber.
    My tests revealed that adding 6" before the muffler is the best.