New Homemade muffler for HF 2.5

Discussion in '4-Stroke Engines' started by fleebell, Jul 13, 2011.

  1. fleebell

    fleebell Member

    I finally got my homemade exhaust pipe finished and mounted on the trike today. The main exhaust muffler tube is 12" long and 1 3/4" diameter. The inlet tube sticks into the main tube about 4". It is welded shut on the end and has many 1/4" holes drilled into the sides of it. After the inlet tube the rest of the main tube is filled with 4 of the stainless steel scrubbies used for washing dishes.

    It's not a whole lot quieter (I can't seem to make a real quiet one for some reason) but it does cut out pretty much all of the high frequency sounds and the little 2.5 hp engine now sounds like a regular 4 stroke motorcycle engine. It sounds a little better than the one I built for the pocket bike engine too. You can see my new spring loaded idler pully in the pics too. I don't why it looks bent one of the pics. Optical illusion I guess. It works much better than the other idler did. You can also see all the holes I drilled in the air cleaner cover on this pic too.

    Attached Files:

  2. HeadSmess

    HeadSmess Well-Known Member

    the trick is that the holes arent just randomly drilled anywhere at any size ;)

    cavity resonances :)

    its hard to explain but what you have created is, despite all the holes n chambers and whatever, a pipe. and the pressure wave at the inlet leaves the outlet in much the same shape... whats required is to "rectify" the pulses until there is merely one constant stream... the pulses create the noise. smooth them out? no more noise :) liken it to water hammer in your water pipes... you need a space to absorb the pulse, then release it gradually.gradually.repeat? gradually.

    so. resonant cavities.

    your pipe goes through a chamber, unobstructed(outlet can be a tad smaller...). there is one hole only in the pipe into the chamber. this is a resonant cavity! and, depending on the dimensions, will attenuate(dampen) one specific frequency :) big chamber low frequency, lil chamber hi frequency...

    sounds a bit like a boost bottle!

    sort of like blowing on a jug... :jester:

    back on track!

    then vary the diameter and number of holes in successive chambers. what you already have, but with a straight thru pipe, and broken up into 50mm/2"sections... :) then the baffle section is the last 100mm...all these frequencies being attenuated results in an exhaust that can be nearly silent...

    there is software and maths and theory n stuff out there to get lookin into if you run like that...

    someone mentioned it elsewhere so ill say it here too... then you have induction noise!
    Last edited: Jul 14, 2011
  3. fleebell

    fleebell Member

    I think it might be easier to just get a moped muffler for it. That will already have all the math work done for me. The only reason I haven't already got one is that the local shop wants as much as a new one cost for a used one and I'm on a tight budget. I do like the way this one sounds though.
  4. HeadSmess

    HeadSmess Well-Known Member

    tight budgets and home made contraptions are always sorta more satisfying...

    always nicer saying "i maded it!" than... i bought it :(