Yanmar 3YM20 exhaust diameter

[pmdarrow]

I recently bought a used Yanmar 3YM20 for repowering my C&C 30. My existing muffler and exhaust hose is 1-5/8”. The 3YM20 exhaust outlet is 2”. Will the existing muffler/hose + an adapter be sufficient, or should I look at replacing them?

[Compass790]

I'm not an expert on this but it does not seem a good practice to go smaller than the manufacturers outlet size. I think any Yanmar service person would tell you to use 2" if thats the OEM outlet size. Hopefully you can get a reply from someone who has that engine with a sub 2" i.d. exhaust.
I didnt help much there sorry

[Wotname]

I concur with Compass790 and add - it also depends on the length of your existing exhaust. If really short, you might be OK.

Your existing exhaust was probably designed for your previous engine. Was the previous engine a 20 hp diesel? If not, replace with 2".

[Frankly]

My IP32 came from the factory with a 2" connection from the mixing elbow to the muffler and 1-7/8" hose from the muffler to the exhaust port. 3GM30 engine (predicessor to 3YM30). My take is that IP would not have done that without Yanmar's approval. As to can you go down even further, length has got to come into play.

I realize not a yes no answer but my engine is 30 YO 4,000 hrs and working fine. I have replaced mixing elbows that were almost blocked. That has also got to be part of the equation.


Frankly

[Squanderbucks]

Take it from me that you should not restrict the exhaust. I tried that with a couple diesels in my power boat when I repower Ed from gas to diesel. Ran it with the old gas exhaust and mufflers for two years and could not figure why I kept blowing exhaust gaskets.
Finically saw the light and increased the exhaust size and put in new larger mufflers. No more exhaust gasket issues.

[Wotname]

If you decide to use the existing exhaust, you should check the back pressure. Yanmar specs are a maximum of 0.15 bar, measured within 10" of the exhaust manifold inlet.

[sinnerman]

I have a Catalina 30.

It has 1-5/8" hose and a very LONG run with some interesting twists and turns from the amidships-mounted Universal diesel engine to the transom where the exhaust dumps overboard.

"As the story goes"...the 1-5/8 diameter was a hold over from the original Atomic-4 gasoline engine installed in these boats when they started production.

I read in a blog "somewhere" that the 1-5/8 " is a tad undersized for the diesel engine especially with the long hose run and bends in the hose.

Not sure what effect it would have other than robbing some horsepower at higher RPM's due to the back pressure.

My friend had a C&C34 (Atomic4 engine ) and the engine, per my recollection is under the companionway stairs, so it is a straight short run from the engine overboard. In this case, the hose diameter would not make much difference EXCEPT trying to mate a 1 5/8 hose to a 2" exhaust diameter.

Enough said.

My two cents.

Cheers..

[Niall leslie]

You may be OK and you may not, it depends upon the exhaust back pressure when measured close to the exhaust elbow with the engine under load at rated maximum RPM. If the back pressure is within specifications then you are OK, if it is more than specifications then no. In any event the complete exhaust system should be inspected and any sub standard components replaced as appropriate.
If the back pressure is more than specified it can play havoc with the combustion process, the valve life, emissions etc. and in very extreme cases with some engines, the raw water flow through the engine system. It will also make the possibility of exhaust leaks more likely due to the increased exhaust gas pressure.

Good luck.

[Boatyarddog]

Originally Posted by pmdarrow

I recently bought a used Yanmar 3YM20 for repowering my C&C 30. My existing muffler and exhaust hose is 1-5/8”. The 3YM20 exhaust outlet is 2”. Will the existing muffler/hose + an adapter be sufficient, or should I look at replacing them?

2" is plenty of diameter for a high speed 20 hp diesel.
No need to "measure back pressure".

Change the muffler, hose, outlet to 2"
Be done with it.
Most diesels want little back pressure, and if you keep your outlet diameter consistent, at 2" all the way back, no problems.
Boatyarddog

[Wotname]

Originally Posted by Boatyarddog

2" is plenty of diameter for a high speed 20 hp diesel.
No need to "measure back pressure".

Change the muffler, hose, outlet to 2"
Be done with it.
Most diesels want little back pressure, and if you keep your outlet diameter consistent, at 2" all the way back, no problems.
Boatyarddog

Agreed that using a 2" diameter will be satisfactory and you can ignore measuring the back pressure.

However... the OP is asking if he can use the existing 1-5/8” diameter hose. This is much smaller than the Yanmar exhaust outlet. The maximum allowable back pressure may be exceeded by using a smaller diameter exhaust and the only way to know is to measure it.

[Boatyarddog]

Originally Posted by Wotname

Agreed that using a 2" diameter will be satisfactory and you can ignore measuring the back pressure.

However... the OP is asking if he can use the existing 1-5/8” diameter hose. This is much smaller than the Yanmar exhaust outlet. The maximum allowable back pressure may be exceeded by using a smaller diameter exhaust and the only way to know is to measure it.

I've seen most leaning to the conclusion, if the outlet is 2".
Don't constrain it.
And it's a standard size for needed components.

As to the OPs question...
Best way to know for sure is hook it up.
Can't initially do much damage.
I'd upgrade personally.
Boatyarddog

[Searles]

Bigger is better ,if turbocharged even bigger again ,diesel exhaust I.d is determined by swept volume of engine and operational r. P m. overall length plus bends must also be considered ,most engine manufacturers will have parameters available on line .⛵️⚓️

[pmdarrow]

Thanks for the great discussion everyone. It sounds like the consensus is upgrade to a 2” exhaust system. For context, the engine I’m replacing is a Universal M-18 which was a replacement for the original Atomic 4. Saw the cost of a new waterlock, exhaust hose, transom outlet, etc. and wondered if I could get away with keeping the current system, but it sounds like it’s not worth the risk.

[Wotname]

^^ wise choice IMO.

[Compass790]

Originally Posted by pmdarrow

Thanks for the great discussion everyone. It sounds like the consensus is upgrade to a 2” exhaust system. For context, the engine I’m replacing is a Universal M-18 which was a replacement for the original Atomic 4. Saw the cost of a new waterlock, exhaust hose, transom outlet, etc. and wondered if I could get away with keeping the current system, but it sounds like it’s not worth the risk.

Well if you dont mind doing the exhaust replacement with the engine in situ you could try the existing exhaust first & see how it performs. I agree with boatyard dog, doubt you'll do any harm trying it.
Obviously harder to do replacement with engine in tho.
Depends on time & $$ budget if the experiment is worth it to you.