7.4 Merc Horizon inboards questions on Exhaust Mani/Risers and water ingestion

[rothfm]

Helping out a family member (I'm the outboard type) but do alot of the maint on his boat...He purchased this 1yr ago and got a successfull season out of it so far......1999 Sea Ray 34 Amberjack w/dual 7.4's.

These are Coolant cooled blocks, with Raw water to the mani's and Risers...Its time to replace them. Ive done extensive research and have learned a ton, including known water ingestion issues related to this motor setup.

I dont now what the original design was that might have had issues, But his now has a fairly tall riser, with a 3-4" adapter block between the Mani and riser. The riser output angle might be the 7degree model (but not sure yet), and the tubes going down to the mufflers have a significant length and angle pitch to the mufflers. The exhaust openings are on the side of the boat vs transom. He runs wet style mani/riser's. I havent measured but the waterline to riser top is appx 18"+. Apparantly dry joint manifolds/risers are not available for the 454 7.4's. Would like to move to that as it makes alot of sense, but so far bieng told not available. Unsure why.

The biggest questions we have if anyone has knowledge about this or experience:

* Does this setup sound exposed to the design issue with water ingestion problems?
* Is there any recommendation further?, such as riser output angle increase?

 

[tpenfield]

The water ingestion issues come when the gaskets that mate the manifold, riser (3-4" adapter block), and elbow start to fail and leak water into the exhaust chamber of the manifold. Water then usually fills the lower part of the exhaust manifold enough so that water gets to the exhaust valves and even into the cylinders.

I suspect that the dry joint manifolds have not been available for the 7.4 engines, because they have been phased out, even though there are plenty of them still in operation. It is worth a Google search to see if any of the aftermarket suppliers have introduced a set.

The output angle is pretty much standard, and they mate to 4" tubes, not sure if you will find anything different. Usually the output angle is not the issue if there is water ingestion, it is more likely the gaskets.

 

[Scott Danforth]

the 8.1 manifolds are dry joint and bolt on the 7.4 motors just fine if you want to switch to dry joint.

 

[rothfm]

Thanks for the replys...yes, im up on the maint interval and how they can leak, and wnd result.

The web is full of issues storys of design issues on these particular motors 1998-2001 reaulting in ingestion issues with seawater backing up into the risers.

Not sure of exactly sure of the original configuration, but between Mercruiser and some of the boat builders, there was a widespread problem

Just want to be sure this boat has already been "updated" and addressed, thus the questions.

We will be doing close measurements later this winter, waterline to riser distance and angles of 4"pipes etc....to be safe.

One parts guy we talked to said Dry riser/mani combination isnt any beatter than Wet and stated they are not worrh the extra money

How can moving the water flow to "outside" that joint, be a major improvement? If gaskets leaks, or a corrosion compromise, there is NO path intp the exhaust chamber and into the motor?

Cool to hear the 8.1 is dry and compatible-is this a definite?

Anyone have additional thoughts on moving to a dry mani/riser combo?


Thanks again!

 

[alldodge]

Dry or wet is no difference if they are working as designed. The difference comes about IF one is past time to change and/or the gasket starts to leak. The dry joint takes the gasket out of the equation but the leaking back from corroded exhaust remains the same.

The height of the elbows has to do with water washing back into the elbows, then the ingestion of water in the engine. If water ingestion due to a quick stop then this to is of no concern, so long as your exhaust water shutters are in place and working.

Only other reason IMO for upgrading exhaust is if the motor HP is increased and reversion comes to mind with larger cam, but this is another question for another thread

 

[rothfm]

"so long as your exhaust water shutters are in place and working"

Do Tell....(i'm an outboard guy)...Where is this shutter? I thought this was an outdrive thing like a flapper. I'm not sure if his setup has a "shutter" or not. None visible that I can see. The exhaust openings are on the sides of the boat vs the transom, and pretty much even with the water line, open with no outter flapper etc..

After verifying the entire setup is appropriate heightwise, angles etc.....I think we will go with the standard WET like it has. Unless he opts for the extra expense of tyring the DRY 8.1's providing they are 100% compatible. I will pull either style at the end of second year to inspect and do the gaskets for safety.

 

[alldodge]

Do Tell....(i'm an outboard guy)...Where is this shutter? I thought this was an outdrive thing like a flapper. I'm not sure if his setup has a "shutter" or not. None visible that I can see. The exhaust openings are on the sides of the boat vs the transom, and pretty much even with the water line, open with no outter flapper etc..

Hey ya got me, sorry was thinking in the all over area of what might happen if, and not majorly specific of your exact issue, my apologies, and will do my best not to try again

So you were worried about what would happen if you didn't have dry joint exhaust and was asking for other opinions, with mufflers for an inboard. While your buddy does not have shutters, it does do have exhaust mufflers. So if the mufflers are not compromised, as in there not rotted out to allow water to rush back in, then your good.

So maybe the next guy that want's to offer a thought, needs to stop and think more about what you have and not be so concerned as to what might happen. Needs to make sure it fits your question 100 percent before answering. Thanks for the thoughtful insight and hope you and your buddy have a great boating year without issue.

 

[rothfm]

Come now....all responses are highly welcome, everything in responses helps the community, gets the gears churning and me thinking. Its all good!! I"m appreciative.

yep, mufflers are in good shape...The issue is well posted and documented, I've gotta believe "some" of these engines, in particular boats were positioned too low, with no adapter, and Angle issues. Thus all my concern with a new to him boat, and planning this Mani/riser job. We read alot about it, but little about the ultimate cause/fix.

From these forums, and research, We now know what to look at closely, measure and check....If alls well will keep the Wet, with quality parts, and check at 2 years in.

Big thanks Alldodge.

 

[Scott Danforth]

the current installation has 4" extra riser blocks because the boat installation needs them. the extra height is because the static water line on the boat is a bit high

so what ever you choose on dry vs wet, you need to maintain that elbow height.

if you go with the dry joint (8.1 style manifolds), they make different height elbows for this reason. there are about 10 different elbow heights/styles for the 8.1 that I know of, maybe more

 

[rothfm]

I've learned a lot more about this I thought I would put it here in case it might help some others.

The documented issue pertains to big blocks in particular the 454 due to the valve overlap design....meaning an intake and exhaust valve is open at the same time for small periods.

So there are 3 areas of water ingestion concern:

1. As stated above by others- lack of riser height, and down angle piping, and the use of in-line straight mufflers.

2. Harmonic Resonance reversion- its proven and documented that some tuned exhausts develop resonance, or sound Harmonics if you will, that causes beads of water in the outgoing exhaust to flow back, thru elbows into engine. Mercruiser had vacuum breaks cast into some risers, then didnt.

In lieu of that cast vac break MC now recommends Resonators post-riser in the exhaust tube, to help stop that Resonance effect. MC has a bulletin about it.

3. Condensation at idle speeds:
More prevelant at low speeds with cool (raw water in manifolds) vs warm (coolant in manifolds) due to the wider temp difference of that cooling water and the hotter riser metal.

Condensation droplets form and drip down into elbow, an issue after "key-off" at the dock. This drip-back affect, runs down to the exhaust valve and over time corrodes it and/or gets into engine.....remember, valve overlap on these blocks means an exhaust port AND intake port can be open at the same time.

This water corrosion/rust messes with the valve seat, eventually bringing big problems.

The fix for this is "Turbulators" between Mani and Elbow. They are like a tray that catches this condensation, and it is burned off with the higher temp of the mani and blown out due to its designed vacuum affect, keeping the condensation from going down the hole. They need to be replaced periodically.


Sorry for long post, lots of research all confirmed with the MC Help line too.

We now have some other things to look at when the job happens.

Remember, the Resonance and Condensation issue can take a long time to Manifest itself....but when it gets to that point, it will be too late

I see posts where people are replacing blocks multiple times , with same results. Im not sure they are truly understanding that there are 3 seperate water intrusion issues on some engines, that need to be understood and looked into.