How to quiet noisy exhaust?

Discussion in 'Diesel Engines' started by Brands01, Jan 12, 2010.

  1. powerabout
    Joined: Nov 2007
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    powerabout Senior Member

    There will be little to no restriction with the muffler on the inlet. ( although ugly and it will rust inside as well)

    I and many other have used them on air compresser inlets to make them quiet when they are close to your workshop.

    You need a tapering hose to help kill the sound thats why even small diesels have tapering snorkel that the air goes in.

    Having a paper filter on a small diesel is very common, these also help with the sound.
  2. apex1

    apex1 Guest

    Every bend in a in- or outlet hose or tube restricts the gas flow, there is absolutely no doubt possible. To which extend that restriction is tolerable is another question, sure.

    A 90 degeree bend and a oil bath filter (as it was described here) is too much in my opinion, compared with a open inlet.

    A Muffler has two 180° turns in gas flow, that chokes the inlet, no doubt possible. In this case one needs no calculations to state: leave it!

    The compressor was not a real comparison, right?

    The taper is helpful, and already recommended in my former post.

  3. Yellowjacket
    Joined: May 2009
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    Yellowjacket Senior Member

    It depends on the muffler that you are would use on the inlet, but that really isn't the right answer.

    The noise is an acoustic resonance in the inlet pipe. Putting a different length pipe moves the resonance up or down in frequency (speed), but doesn't really change the nature of the resonance. A tapered pipe won't help a lot either, since it will increase the area of the pulse, but reduce the pressure of the resonance. Noise is pressure over area, so you are simply trading one for the other, and that won't help a lot.

    There is a pressure loss in bends, but if the velocity is kept low (below 0.1 Mach), there is, as a practical matter, no real measurable loss. Bends themselves don't help much since the acoustic pulse just follows the pipe. Better than a straight open pipe, but not a whole lot.

    The best approach is to find an air filter module from an existing engine of similar displacement and simply use that. Automotive filter boxes and filters are well designed for this purpose and any losses will be minimal. The best approach would be to go to a wrecking yard and find one that is designed for a similar size or larger engine and simply put it on. The pressure loss is going to be on the order of 6 inches of water, so the loss is not going to be an issue.

    Most inlet silencing systems turn the flow 90 degrees from the filter inlet direction to the tube that goes to the engine. This is done to reflect the pressure pulse (noise) back into the engine. Any type of straight thru system is not going to be very effective at reducing inlet noise. Another approach is to find a tall round filter that has a bottom diameter close to your inlet pipe, find a length of inlet pipe that minimizes the resonance and then add the filter and put a cap on it that closes the top and bounces the pulse back into the engine. Still, the easiest and most quiet approach is to find a automotive filter and use that.
  4. Brands01
    Joined: Nov 2006
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    Brands01 Senior Member

    Thanks yellow jacket. To answer frosty's previous post, the eBay auction for the landrover oil bath was for 10 days and finishes today. I think I'm going to be the proud owner for the grand sum of .99 cents (plus postage, of course!).

    I'll try the rover filter and see what happens, but perhaps a scout of a wrecking yard for something more modern could be wothwhile.

    The existing air cleaner is one of these wire mesh filters which keeps only rats and t shirts out. It has a 90 degree bend from the filter body to a tapered inlet pipe. It is that pipe that makes the racket which caused this post.

    Yesterday a friend lent me an oil filled foam filter that I've put on the inlet. It isn't bad at all. No cap on the top to reflect the sound back into the engine, but the length of it naturally makes the end of it sit flush up against the side of the engine enclosure, which probably performs the same function.
  5. jimcro55
    Joined: May 2010
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    jimcro55 Junior Member

    Brands, what size is the opening in this wire mesh filter? Is the opening about 1"? Or is it smaller? Just curious as I may have another solution for you.

  6. Brands01
    Joined: Nov 2006
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    Brands01 Senior Member

    Hi Jimcro,

    The air intake diameter is 2", and the wire mesh filter has a tapered tube at 90 degrees. The tube flattens as it tapers to keep the cross sectional area of the opening the same.

    I'm all ears about any other solutions.


  7. Brands01
    Joined: Nov 2006
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    Brands01 Senior Member

    This post is fairly old now, but i now have an excellent solution. Thought I would write it up here for anyone with similar problems in the future.

    I bought a truck air cleaner - similar in shape to a rover oil bath, but has a big paper element inside. Its designed for an engine much bigger then mine i assume. It has a 2.5" inlet, whereas my little bukh is only 2".

    The inlet of the air cleaner has a bend in the pipe, and the pipe is split in two around the bend, then joins up again at the expansion chamber. This has the effect that the air must travel further around the outside bend than the inside bend, and when the two air streams meet up again, the pulses are out of phase and cancel each other out. This cancelling obviously works better at various RPMs, and not so well at others but on the whole is very effective.

    I initially had the truck air cleaner plumbed in to the manifold via a 90 degree elbow, but this restricted the air flow significantly enough to produce plenty of black fumes and an oil slick on the water out the back of my boat. By removing the elbow, the air flow has been increased enough to remove the fumes and the slick.

    So the earlier comments about bends in the pipework restricting flow in my case have proved correct. Keeping in mind that all this pipework is at a larger diameter than the engines intake (pipework = 2.5", manifold intake = 2").

    Anyway, the end result of all of this is that the engine is purring like a well fed kitten and the problem noise is not a problem any more.
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