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**knottybuoyz** · 09-11-2009, 15:27

Question. I have a Yanmar 4JH-TE. Exhaust outlet from mixing elbow is 3" Dia. The muffler and exhaust hose that came with the engine has inlet/outlet size of 1-5/8". Somewhere along the line there must have been a reducer of some sort but that wasn't included. I have the service manual for the engine but there's no mention of minimum exhaust size requirements. I don't know much about diesel engines, well actually at this point I know nothing about them, but if they breath better, larger exhaust, they should run better no? Can I go down to 2" all the way back (approx 12' run through water lock, muffler & gooseneck)? That should be better for the engine or should I keep it 3" all the way back?

Vasco · 09-11-2009, 15:35

Rick,

Yanmar is very adamant about the exhaust hose size and it seems theirs is bigger than most. When I re-powered I had to increase the exhaust hose size. They will not warrant an engine if the exhaust is smaller than what they specify. I would stick with the 3".

sailvayu · 09-11-2009, 15:36

Best off sticking with the 3" I would not go smaller than 2-1/2" with that engine and then only if the run was short and did not have a lot of bends. I always try to stay the same size as what comes out of the engine. Back pressure will make a big difference in the horse power output.

Wayne Canning, AMS
projectboatzen.com

jkleins · 09-11-2009, 16:19

I just installed that engine in the "AE" version and had to increase the exhaust size to 3" to get the warranty. I had 2 1/2" and it wouldn't fly. The biggest PITA was the fact I had to replumb the fuel lines as they also won't allow 1/4" hoses which were sprinkled all through my system. I had to go 5/16" or better.

Jim

Chief Engineer · 09-11-2009, 19:35

Do not reduce exhaust hose size. period.

delmarrey · 09-11-2009, 21:40

My 3JH4E is also 3". And I'm re-plumbing too. I think 3" is Yanmars favorite #

Windblown · 09-02-2010, 10:32

I have a "new" Isuzu engine I am putting in my boat. It came with a 2 inch exhaust pipe coming out of the manifold, so I am planning on going 2 inch the rest of the way, with a waterlift muffler and 2 inch hose going out the back, about a 10-12 foot run altogether. I've just had it test run, and discussed this with my mechanic, and he feels it should work fine. The engine is rated at about 58 HP.

Any other opinions? Should I go bigger?

Kevin

never monday · 09-02-2010, 10:42

Quote:

Originally Posted by Chief Engineer

Do not reduce exhaust hose size. period.

^that. To include, don't use fiberglass elbows. Get a quality flexible hose. The fiberglass bends further reduce hose size.

Cheechako · 09-02-2010, 10:46

exhaust hose size is important..1.62 is really small

Windblown · 09-02-2010, 10:51

To clarify, I don't plan to reduce it, I just want to be sure that 2 inches is big enough. Since that is what comes out of the engine, I'm assuming it is, but I've seen other specs that say for 58 HP it should be 3 inch.

**s/v Jedi** · 09-02-2010, 11:10

@Kevin: If you need to put everything in new and it isn't a problem, I would go for three inch. I would do that because it works great and you can connect any engine to it.

Buy a 3" waterlift muffler, a 1' piece of 2" fiberglass exhaust pipe and a 1-2' length if 3" exhaust pipe.

You use the 2" pipe to adapt the input of the muffler. I use 2" wide fiberglass tape, wrapping it around a piece of pipe while wetting out with epoxy until it just fits into the muffler inlet. Support and let cure. After that I put the muffler with the inlet pointing up and use a syringe to put a nice bead of epoxy with high density filler in the cavity to cover the edge of the glass tape (see photo below)

Use the 3" pipe at your transom. Cut/change the hole so that this pipe fits through and epoxy it in. Leave it sticking out for 1" minimum and cut it to length so that on the inside you can easily get the 3" hose around it (good access). On the inside, I reinforce the thru-hull construction with 2" wide fiberglass tape again. Cut 1.5' long pieces and glass them around the pipe onto the hull surface, much like a neck-tie around a neck and onto a shirt. Put these pieces all around.
On the outside where 1" is sticking out, you can fit one of those rubber flappers over the pipe. The minimum size these come for is 3", another good reason to go for that size ;-)

ciao!
Nick.

klem · 09-02-2010, 11:54

If there is no spec sheet that gives this number, call Isuzu. Exhaust backpressure is very important on a diesel. Backpressure is a function of exhaust size, length and routing. If you have too much backpressure (small exhaust, restrictive elbows) you will stress the engine and your efficiency will be low. Too little backpressure and your turbo won't spool right. On a turbocharged engine, it is possible to have too little backpressure so call the manufacturer and get it right.

delmarrey · 09-02-2010, 12:07

Diesels need to breath. too much back pressure will not let the burned gases out so the next stroke will not have enough oxygen to burn the next round of fuel. The water in a wet lift can create back pressure as well so the right max. height for the loop is important also.

It's better to go over size then under, turbos call the manufacture.