Aluminum vs. Cast Iron exhaust for salt water boat-gctid360716

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I've been searching for the merits of aluminum vs. iron exhaust... but there just seem to be alot of speculation that aluminum should corrode faster and that the gasket interface would be prone to leaking because of the dissimiliar metals issue. In theory this makes sense, but I haven't seen anything from people who actually used aluminum on a saltwater boat. What about the aluminum kits that have ceramic coating? At Perfprotech, they offer ceramic coated aluminum w/ a 5 year warranty for $1065. I can get something like an Osco cast iron (same as I have now, which is fine but heavy) with only a 3 year warranty for around $680. Based on the warranty, seem like the aluminum would last longer. Really, for me, the weight difference is the selling point.

Anybody have experience with aluminum exhaust in the salt... or experience w/ the ceramic coated type?

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The oem volvo Y-pipe or downpipe(4-cylinder) is aluminum coated with some kind of paint or powdercoat system, and they last a long time.

If you are trailered, I would consider the aluminum units and just keep things flushed out. Just looking at the warranty, it's almost the same money per year for the cost vs. life if they were both no good right at the warranty end.

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Good question, I am also researching new manifolds and risers for my 5.7 merc gas engines. I am leaning toward aluminum but for the reasons you mention I am trying to get smarter. It seems that mercruiser is pushing the cast iron manifolds/risers that have been ceramic coated.

I will share whatever i find out.

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No experience, but don't understand the dissimilar metal thing since the metals are not touching.

Mine are on their 5th season and still running VERY cool, so I would not be inclined to go aluminum. I'd also be concerned with overheat issues, should it happen. That cast can take some serious heat with no ill effects... not so sure about the aluminum.

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My aluminum manifolds I can carry around with 2 fingers. Yeah the weight savings is significant.

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Good question is right, I have also tried to research aluminum mani/risers for salt water use as mine will be due in the next couple years, the main selling point for me is also the weight...

what I keep coming across is alot of hear-say and theory mostly around longevity in saltwater and expansion/dissimular metals, I found a couple forums where the guys (who actually run the aluminum) rave about them and are on the 5-7 years no issues, BUT they are in fresh water...It really would be nice to hear some first hand experiance and hard evidence of the aluminum manis in salt...

here is my 2 cents on some of the common "stereo types" of alum manis, which I question...again this is theory as I have no experiance with aluminum manifolds

- first if you have fresh water cooling the most that salt water will touch are the risers which are cheap and easy to replace (glycol in manis)...if raw water cooled I would be more concerned

-for dissimular metal...there are gaskets, (i don't believe they have direct contact)

-for expansion issues, automotive have been using alum heads on cast blocks and then cast/steel exhaust manis off the alum head again...there is occasionally an issue with nuts and bolts backing off but this is rare...I check torque as regular maintenence on my manis so I think this would be a non issue for me

again this is my theory, not real world experience

I would love to hear from someone with experiance in saltwater

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Went Through the same process. Now one year into using Ceramic coated, Aluminum Risers & Elbows on a closed cooled 7.4MPI...each riser has a short 1"x3/4" round anode...I removed them to check for wear and found them to be 98%...The Risers themselves are perfect. If your doing the change out yourself then I would definitely recommend Aluminum, so much easier to manhandle. The weight saving is considerable I can't remember exactly, but lugging the old cast iron ones out of the Engine Compartment was murderous. Absolutely no issues with the intake manifold mating surface (And mine required re-surfacing by hand) so at 175 degs running temp the dissimilar metals do not seem to expand at vastly different rates. (Used Permatex Ultra Copper on the Gasket Surfaces....Brilliant Stuff!) I am very happy so far with the Aluminum ones......They're Guaranteed for 5 years so..... we'll see???

Hope this helps?

Dave

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I should have qualified myself sorry....I worked for a marine heat exchanger outfit and saw alot of the nasty stuff that would come back for replacement or repair.

The biggest problem we saw with aluminum was in the gasket area where the aluminum met cast iron in a salt environment. Gasket selection was important here as the metallic versions would almost weld themselves to one side or another and getting them apart sometimes broke the casting. The aluminum would be pretty nasty and corroded.

The overheat thought is a good one, but aluminum melts at ~1800*. Your exhaust couplers, and motor for that matter....are going to be long gone before 1800* comes along. If it was my boat, and I had some $$$ to spend, I'd install the aluminum manifolds, a closed cooling system(you have this already?) including the manifolds, then a set of the fabbed stainless elbows to handle the saltwater. The only necessary maintenance from then on would just be the zinc anodes in the exchanger and giving the SS elbows a soak in some CLR maybe once every 1-2 years, they will build some white crusties over time.