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Old 09-13-2016, 02:41 PM
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question regarding exhaust mainfold/risers

This is kind of a dumb question, but I've wanted to know this for a long time, so I'm going to ask. What are the general design differences between exhaust manifolds and risers on our boats versus those on an I/O?

I ask because my brother owns a 24ft saltwater boat that he uses up in the San Juan's. He says that he has to replace his mainfolds/risers like every 3-4 years. If he doesn't, they will end up leaking and he'll get water in his cylinders. He just had an issue this summer where he got stuck on an island and had to pull the plugs to get the water out just so he could get back to the dock and do the repair.

He acts like I'm crazy for not looking at mine more closely. He just got done re-doing his on his own and the parts alone were over $1000. To me this seems just silly, but he swears everybody up there just accepts it. I get that saltwater plays a HUGE role in this, but he is acting like it wouldn't matter a great deal even if it was fresh water. It's just an expected expense.

So what's the major difference in design and why don't we see more posts about this topic here? Is this something dealer's/shops look at or inspect when they're servicing our boats?

Can someone set me straight so I can explain to him why this isn't a common problem for us?
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Old 09-13-2016, 02:55 PM
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Not sure the difference between DD and IO but manifolds and risers can split or crack but mainly do to freezing. If this question came up a few weeks ago I would say that my manifold/risers are 22 years old and just fine. Which I believe they are...

But I am working through an issue now with water in Cylinder #1 only and still have to do a heat test on my manifold and riser, with new gaskets, but I feel if it was leaking internally I would have water in all cylinders.
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Old 09-13-2016, 03:00 PM
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It has to have something to do with cooling and exhaust design differences between an I/O and an inboard. I just don't know what those differences are in layman's terms. I wish I did, I'd love to explain it to him. He can't believe I've owned my boat for 10 years and never had to address or change them.

I also asked him if he could convert to closed cooling and he said yes, but that nobody up there runs theirs that way (~24-28ft). They just repair/replace them every few years. Seems so silly to me.
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Old 09-13-2016, 03:11 PM
88 PS190 88 PS190 is offline
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Do you run your boat in Salt? Salt murders manifolds.

There is not much/any functional difference in I/O exhaust manifolds. You have a jacketed exhaust manifold bolted to the block, this has short sections on the bottom which get hot, bake off paint, and usually look rusted. Then they turn upwards and have water jacketed areas, usually these are fed from the front of the manifold, and a drain plug is in the back to drain water out of the manifolds. Water goes through this lower manifold pushes up through into the "riser" thing of this an upside down pipe trap, it lets exhaust fumes go up and around the bend and then dumps the water into the exhaust stream where if flows downhill through the rubber pipes and into the exhaust.

The only real difference on an I/O is that many have a muffler that crosses between the two risers above the bell housing. This muffler acts as a Y pipe and connects the two risers together into a single exhaust that goes through the stern drive and usually pushes the exhaust out near/through the propeller. This keeps the exhaust down in the water and quiets the system.


But on the engine side, there is really little to no difference.


On big boats it is not uncommon to have a "dry" manifold - that goes into a wet exhaust/through hull.
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Old 09-13-2016, 03:18 PM
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So, it IS all just because of the saltwater then. Interesting. What is the typical lifespan on our boats assuming 100% fresh water, generally speaking? 10-15 years? Longer? I swear, I never read much about them here, but maybe I just haven't been paying close enough attention.
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Old 09-13-2016, 04:12 PM
Blackhawk36 Blackhawk36 is offline
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Quote:
Originally Posted by brucemac View Post
So, it IS all just because of the saltwater then. Interesting. What is the typical lifespan on our boats assuming 100% fresh water, generally speaking? 10-15 years? Longer? I swear, I never read much about them here, but maybe I just haven't been paying close enough attention.
My friend has a Nautique, fresh water only and it is 17 years old and the manifolds and risers are pristine. Look like new in and out. We pulled them to inspect them this summer.
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Old 09-13-2016, 05:17 PM
88 PS190 88 PS190 is offline
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Yes salt is awful on boats that aren't designed for it.
My 88 I have no reason to believe has anything other than factory manifolds.
Similarly my 94 I also have no reason to suspect the manifolds have been replaced - however it gives me more scale out of the drain plugs every year than does the 88.

There are riser gaskets - it can be interesting to separate the risers off the manifolds because you can directly inspect the passages.
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Old 09-13-2016, 05:20 PM
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Quote:
Originally Posted by brucemac View Post
So, it IS all just because of the saltwater then. Interesting. What is the typical lifespan on our boats assuming 100% fresh water, generally speaking? 10-15 years? Longer? I swear, I never read much about them here, but maybe I just haven't been paying close enough attention.
Owned both DD/IOs in saltwater environments. As the others said, there is no difference between DD and IO except that the IOs send the exhaust through the outdrive but that has nothing to do with the manifolds, just the exhaust routing after them.

Saltwater wise, the manifolds definitely get abused by the salt. 3-4 years is to be expected. It used to be common in saltwater environments to only cool the block with coolant and have the manifolds cooled with raw saltwater. Replace the manifolds every 3-4 years as your friend does. Don't understand why people do that anymore, there are plenty of good fully closed systems that keep the saltwater out of the motor and the manifolds. A little more $$ up front but will save tons on manifold replacements.
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Old 09-13-2016, 06:05 PM
88 PS190 88 PS190 is offline
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Quote:
Originally Posted by gambier View Post
Don't understand why people do that anymore, there are plenty of good fully closed systems that keep the saltwater out of the motor and the manifolds. A little more $$ up front but will save tons on manifold replacements.

Pretty easy issue - full closed loop systems have to handle ALL the exhaust heat. Which is a lot of heat.

Think about your car - it has a pressurized closed cooling system - but the heat of the exhausts is lost to the air - with no cooling largely because it is not confined in a engine box.

If you use a "half" loop cooling system your heat exchanger only needs to be sized to deal w/ the engine block/heads heat. You use "cold" raw water for the oil cooler, trans coolers etc. You use cold raw water for the exhaust manifolds/risers. which means you can have much smaller exchangers installed.
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Old 09-13-2016, 09:31 PM
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fresh water exhaust manifolds can last indefinitely.. but they should be inspected after 500 hours and then every 2 season...

especially id they are seeping at the gasket.
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