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OT..sort of: Info on back pressure on small engine mufflers

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  • OT..sort of: Info on back pressure on small engine mufflers

    I have 7.5 kw electric start generator for the home. I set it in the garage door, point the muffler outside. Because of how the house is situated to the prevailing winds it's a safe scenario. However, it's loud as all get out.

    I know back-pressure is supposedly a big deal on car engines to get them to run smoothly. (Is that to keep the valves from floating?) How big a deal is it on a 14 hp generator 4 cycle engine? If I make a new muffler for it maybe 5-6 inches in diameter and a 1 1/2 foot long and maybe run about 7 feet of 2" pipe vertically up will I screw up the performance of the engine enough to hurt or seriously degrade performance?

    That would take care of the exhaust noise and I know the mechanical nature of the engine will still produce a lot of noise if not baffled with sound absorbing walls.

    I don't want to spend a lot of energy or money only to find out it won't work if someone here already knows that it won't
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    Thank you to our families of soldiers, many of whom have given so much more then the rest of us for the Freedom we enjoy.

    It is true, there is nothing free about freedom, don't be so quick to give it away.

  • #2
    Muffler

    That will help.
    Just be sure that you do not restrict the exhaust flow and no bad effects will happen.
    2 inch pipe should be fine, thats probably larger than the exhaust port is now and the large expansion chamber will muffle the sound of the exhaust..
    Enclosing the generator is a bad idea because it needs a free flow of air to cool it.
    I enclosed an 8kw propane powered generator but put a large fan on the enclosure to make sure the unit did not overheat.
    Last edited by Seastar; 10-08-2008, 10:21 AM.
    I cut it off twice and it's still too short!

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    • #3
      YOD..a place I still work at from time to time..has a gas powered air compressor. Due to vandalism they decided to move it into the shop.
      They run a 2" pipe up about 10 feet, then an elbow...thru a car muffler then outside with about 8 feet more 2" tubing.
      Thing is amazingly quiet now. It's a 12hp Kohler that used to be really loud.
      Didn't seem to hurt the performance one bit. This motor will run for hours at a stretch as they use a lot of air...for gouging, painting, air tools or whatever.
      Russ
      I have tools I don't even know I own...

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      • #4
        Dig a hole in the ground and bury a 2 lb coffee can or similar. Use a flexible metal hose and run the exhaust into the can and back out again. If you can't dig a hole then build a box big enough to contain the can buried in sand. A major part of the noise that isn't eliminated by a muffler is radiated by the walls of the muffler itself, like a loudspeaker. Surrounding the muffler with a thick damping medium is very effective and this method is, ahem, dirt cheap. It also produces very little back pressure.
        Free software for calculating bolt circles and similar: Click Here

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        • #5
          Visiting an old relative in Scotland ages ago i was given the grand tour - the power for the place used to come off a
          big Kelvin diesil, the exhaust went into the ground into a
          50 gallon barrel sized iron muffler and ran out a long iron pipe to an exhaust pipe a distance away.

          so underground mufflers were quite common practice.

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          • #6
            I used the buried muffler trick back in the 70's when we built a lakeside cabin. I had an old 4 cylinder Kohler for a genset that was very theft resistant since there was no road access and the unit weighed 600 lbs. Buried a 5 gallon can and ran the exhaust into that and out again. From 50 feet away the loudest sound was the mechanical noise of the engine, not the exhaust.
            Free software for calculating bolt circles and similar: Click Here

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            • #7
              Gee, thought I responded to all your ideas but don't see it here now?

              At any rate, thanks guys. Think I'll make something for the RV's generator out of 2" muffler tubing and a tractor muffler from Tractor Service Supply.

              The hole in the dirt thing will work nicely for the generator in the garage for the house power outages. We have a garden right next to the garage door. I'll ask the wife how she'd like to have some of her plants warmed up during inclement weather! Suppose I could use the woodchucks hole and not have to do any digging at all. Don't they usually have more then one hole?

              Thanks again.
              - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
              Thank you to our families of soldiers, many of whom have given so much more then the rest of us for the Freedom we enjoy.

              It is true, there is nothing free about freedom, don't be so quick to give it away.

              Comment

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