Exhaust Size - Replacement Engine?

[Reg and Kerry]

Can anyone advise?
Am replaceing dead 45hp engine with a slightly used Yanmar 4JH2E.
Old engine had 2" exhaust system.
New engine has 3'' exhaust elbow on motor.
Am intending to replace waterlock with 3" 10 ltr unit and 3" hose to it.
Can I use the rest of the existing 2" exhaust system or do I have to replace complete system with 3''.

[s/v Jedi]

All has to go up to 3".

ciao!
Nick.

[donradcliffe]

No Yanmar owners have jumped in yet, so here is my 2 cents worth.

I looked at a Yanmar to replace my Perkins 4108, but they wanted a 3" exhaust. I went with a 50hp Beta because they were happy with a 2" system . If you are installing a used engine, you don't have Yanmar warranty issues. I suspect you will do fine with the existing 2" system unless you plan on running at WOT all the time.

[s/v Jedi]

Who, I own a Yanmar, but fail to see the reason that is important.

If you install a 2nd hand engine without warranty, you can just reduce the exhaust by an inch? You really think they spec 3" for warranty reasons? I think it's wise to have faith in the Yanmar engineers that decided the engine needs a 3" exhaust system. When you reduce to 2" you increase the back pressure. When 2" would have been fine, the spec would have said that and the engine would have had a 2" exhaust connection.

cheers,
Nick.

[Vasco]

When I repowered from a Volvo 2003 to a Yanmar 3GM30F I had to increase the exhaust to 3". Yanmar will not warrant the engine if you don't.

[David M]

The manufacturer will specify the maximum allowable back pressure, exhaust diameter size or both. Check with them to get the facts. Don't go cheap on your exhaust system, it can ruin your engine or sink your boat.

[imagine2frolic]

Originally Posted by s/v Jedi

Who, I own a Yanmar, but fail to see the reason that is important.

If you install a 2nd hand engine without warranty, you can just reduce the exhaust by an inch? You really think they spec 3" for warranty reasons? I think it's wise to have faith in the Yanmar engineers that decided the engine needs a 3" exhaust system. When you reduce to 2" you increase the back pressure. When 2" would have been fine, the spec would have said that and the engine would have had a 2" exhaust connection.

cheers,
Nick.

Doing exhaust for 33 years from Rolls to Tuckers. I am inclined to agree with this thinking. The back pressure may result in pump failures.......i2f

[Cheechako]

Go with the 3". Diesel manufacturers are pretty picky about their exhaust size for warranty, there must be a reason....

[Blue Stocking]

2" ID hose has 3.14 sq ins of flow area
3" ID hose has 7.07 sq ins of flow area
You are restricting the exhaust flow by potential 225%

I agree with Jedi

[John A]

When I repowered from a 4.108 to a new Yanmar, I had to also upgrade the fuel lines and fittings/pickup tube to a larger size due to the increased fuel flow required as a coolent for the injector pump. I didn't upgrade the return lines.
John

[Reg and Kerry]

Thanks everyone.
In my heart I knew that 3'' right through was the right answer. I guess you live in hope. Extra work and costs put us back a bit longer.

Once again, thanks for the promt reply. My first time at this and the results are pleasing.

Rags.

[Reg and Kerry]

The 2" system i am now replacing, had a stop cock in the line before the silencer then to skin fitting. Is this a good or bad thing? Do I need to replace it in the new 3" system. Exhaust is out stbd side about 300mm above waterline.
Rags.

[s/v Jedi]

Originally Posted by Reg and Kerry

The 2" system i am now replacing, had a stop cock in the line before the silencer then to skin fitting. Is this a good or bad thing? Do I need to replace it in the new 3" system. Exhaust is out stbd side about 300mm above waterline.
Rags.

You need to think carefully about that. I don't like these valves (you mean a valve, right?) but your exhaust isn't at your transom. I the transom is nearby and reachable with the hose, I would divert it to there. In that case, I wouldn't use a thru-hull fitting, but a piece of 3" fiberglass pipe (if your boat is plastic). When you epoxy/glass that into the stern, you can put the hose on the inside part and a flapper on the part that extends outside (need at least an inch or so sticking out).

No matter what you do, the hose should have a high loop before coming down to the fitting.

There's a 3rd option. I don't know the hp of your engine. But if it's not too much, you can use a water separator that replaces the high loop. I don't know if they are available with 3" fittings and the hp rating.

cheers,
Nick.

[Reg and Kerry]

Nick, thanks for your input. I have asked a diesel mech and he has never seen a valve/stop cock on an exhaust system. I figured it must have been for a very long haul on a port tack.
It has been suggested to leave it out and when I replace the skin fitting with a 3" one, get one with a flap. Also have the loop as high as possible.
Our boat is a ferro. This is my second one and have learned over the years to repair easily.
Quote" Ferro means more boat for your buck."
Rags.

[donradcliffe]

My minority view still stands.

Exhaust sizing is essentially a function of horsepower--every 100 hp will generate 200 cfm of exhaust at WOT. Engine exhaust systems are designed to keep backpressure below manufacturer's limits, primarily to keep the engine from overheating and secondarily to maximize output. A 100 hp engine will need roughly twice the exhaust flow area as a 50 hp engine to maintain the same back pressure.

Now lets look at Yanmar--I think their spec is 40" of water, and their 4JH3-DTE engine--turbocharged at 125hp, has a 3" exhaust. So does their 3JH4E at 39hp. Which design do you think is pressing the real limits and which design is very conservative??

Back to the real world, if your exhaust system is fairly short with long-radiused bends (right angle elbows are equivalent to 10-15 ft of hose), you could probably get away with not changing it out, especially if you are a cruiser who doesn't plan on using all 50 hp very often. One option would be to do the install, and be prepared to upgrade the exhaust if the engine runs hot at WOT.

OTOH, if you have an extra boat unit, an upgrade to 3" certainly won't hurt.