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Old 22-05-2022, 11:42   #1
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How big should my muffler be ?

I have a Yanmar 2QM20H, 20hp diesel, raw-water cooled. How big should the water lift muffler be on a diesel this size ? Mine seems really large. It's about 10" high, and about 10" diameter, round.

Does that sound about right ? I'm trying to research now...
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Old 23-05-2022, 00:58   #2
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Re: How big should my muffler be ?

Exhaust component sizes will depend, largely, upon the engine size [HP & it’s back pressure spec], and the exhaust piping configuration [distances, bends, etc].
Exhaust flow is determined from the amount of HP the engine makes, and the more HP, the more exhaust flow; therefore it will take larger piping [and/or less bends, shorter lengths etc], and larger waterlock muffler, to meet maximum back pressure requirements.
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Old 23-05-2022, 01:52   #3
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Re: How big should my muffler be ?

In addition to the comments by Gord, I would add the following:
Look at the exhaust pipe leaving the waterlock, calculate the volume of the pipe up to the point where the water will flow back into the waterlock (rather than running out the outlet)
Add to this the pipe volume of the exhaust pipe from the motor (at the point of the water injection) to the waterlock.
When the engine shuts down, all the water in these two pipes will run into the waterlock.
I have just reread Dave Gerrs' book (fort you) which goes into great detail about exhaust system design.
I will précis it here for you, but I suggest that the book "Boat Mechanical Systems Handbook" by McGraw Hill is a really good source of information.
Safe rule of thumb: The water lift muffler should have %130 MORE volume than the entire lift line from the canister to the highest point of the exhaust line, (both ways as described above.
It is possible to fill the waterlock if the engine is hard to start, allowing water to back up and into the engine. For this reason fit a larger water lift rather than a smaller one (if space allows) A drain off point is a good idea if the engine is hard to start, you can drain off water that might otherwise threaten to flood the engine.
Very long lift runs increase back pressure. Avoid lift runs greater than 30 times the exhaust line diameter.
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Old 23-05-2022, 02:44   #4
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Re: How big should my muffler be ?

I did find other information that does indeed indicate that my muffler is sized as it should be. I've never had an engine this size and I couldn't remember ever really looking at mufflers before so I just wondered.

But I do believe it's fine. Thanks !
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Old 23-05-2022, 03:46   #5
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pirate Re: How big should my muffler be ?

Back in 1996 the internal wall of the muffler on my Westerly Longbow Md2b engine collapsed and exploded.. Scared the crap outa me.
We were sailing along the Algarve at the time.. anyway I pulled into Vilamoura to find a fix but nothing available so removed the destroyed muffler and ran the hose directly to the exhaust outlet.
Talking to someone I learnt the problem with this was a large following sea could force water into the engine.
So, headed on to Gib which was still a great place for boats back then to get things sorted but once again no joy with replacement muffler.
Basically I solved the problem by buying new hose to existing spec in a length long enough to run from the trap to the outlet with a large loop suspended from the deckhead.
Served me well till I sold the boat 4yrs later.

By the way.. The muffler was huge..
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