Exhaust Sound testing the" how-to make" muffler

Discussion in '2-Stroke Engines' started by professor, Oct 1, 2009.

  1. professor

    professor Active Member

    This style muff will go on my weedwacker engine so I decided to adapt it- temporary for this test- to the featherlight weedeater that is going to power my gas-electric bike.

    The muffler has only 2 little gills to get the exhaust out of the stock muff. The actual muffler is inside a heat baffle (pretty cleaver, cooling air from the fan is blown thru this baffle around the muff.
    I had to seal up a pipe around where the gills go to be able to attach it to an added on muffler.
    Here is a pic of the amazingly small outlets:
    The inlet is about an inch by a half.

    I pryed open the gills some and added (maybe shouldn't have) a hole next to them. Covering these is a section of weeedeater shaft (super thin 3/4 outside dia.) that I split for a bit and braized onto the muffler. I had not realized the silver color is aluminization. The alum. does not like to burn off. I thought the stuff was zinc (don't breathe the fumes for that either), after seeing the miserable time I was having, I ground some of it down - finished brazing and sealed the rest with hi temp ultra silicone Like so-
    To the right is the other half of the baffle which I needed to cut-out a place for the pipe to fit and have go back together.

    The muffler would not connect to the smaller tube so I wrapped a cotton rag around it for this short test:

    Wacker with just my pipe added to the stock muff:

    idle-80 ( dead stock was 78.5)
    3/4 throttle- 90.6 (about the same as stock)

    Wacker with my muffer added:

    3/4 throttle- 85

    This is a very informal test, checking the with the exhaust aiming right at the meter about 39 inches away. A worst case deal.
    Decibels are not like temperature graidents. I don't remember the details except that something like a 3 or 4 difference in Db is a difference of 1/2 sound pressure.
    This muffler may have been quieter with smaller driller holes - something about the size of the waves that are able to go thru.
    All in all, I am pleased.
    I do wonder if those tiny gills were a factory idea to control the rpm of the engine?

  2. HoughMade

    HoughMade Guest

    That is a pretty impressive sound reduction.
  3. loquin

    loquin Active Member

    each 3 db is half the sound pressure.

    Also note that the original baffle muffler is VERY frequency dependent - the noise reduction varies widely as the RPMs (and frequency) goes up. The charts I've seen have a 'sawtooth' type of response. There may be 10-20dB peak to valley differences. Weedeaters are designed to run wide-open throttle most of the time, and the mufflers are sized to provide maximum noise reduction at the frequency being generated AT wide-open throttle while eating weeds...
  4. HoughMade

    HoughMade Guest

    Do you have a vid of this so wecan hear it for ourselves?
  5. professor

    professor Active Member

    I thought about it at the time but was recovering from being sick. Besides, it is the muff that went on the 6hp/24" bike (it's now put away and am working on a gas/electric conversion).
    The muffler on the 24 sounded like a normal dual purpose dirt bike from the factory. Had that popcorn popper sound.

    I would like to see the internals on one of those super quiet Honda generator.