5.7 manifold errosion

[cdavisdb]

Just pulled the manifolds and risers off my 5.7. Engine was installed in 1998, almost unused till I bought it in 2009. I replaced the risers then, but the manifolds looked almost new, so I did not remove them.

4 seasons later is now. Manifolds still look ok to me, but the seat on the block is erroded around the bottom of the exhaust for the center two cylinders on both sides, see pics. The erosion is about half way across the sealing surface. I don't remember seeing this on previous engines.

Is this normal? If not, do I need to to more that just bolting a new set of manifolds on? I could clean it up real good and marinetex it and sand smooth, but that's a lot of work. Ideas?

[Entourage]

You should replace the manifolds and risers every 2 to three years. You are asking for trouble by not replacing them. You cannot see the passage inside and could be rotted. If this rotts out while you are on the water or even out of the water with water in the manifolds it will end up going in your motor (cylinders) and will hydro lock, and you will have even bigger problems.

[cdavisdb]

New manifolds and risers are bought and ready. The old ones look amazingly good, but I'd prefer not to gamble.

Erosion on the block is what is worrying me.

[pelican]

looking at the picture:

see the small rust blobs ??

last time you did a compression test ?

[Islandtrader]

Side question here...is there any difference between OEM manifolds and the "other" type that are always less expensive?

If so what are the top 2? brands?

[cdavisdb]

Good question. That's one of the reasons I left the Volvo manifolds on there when I bought the boat and replaced the risers. This time, both old risers and old manifold are Volvo OEM. The manifolds I've got to replace them are HGE Marine Engine Products, Chinese made, bought through Jerrys Marine. Looks like a bit thicker metal, but I'm a little worried,thought I was buying American made. New risers are Volvo. Looks like everything should fit together fine.

[cdavisdb]

Compression test just before pulling manifolds, 186 to 200 all the way around. Suspect my meter is reading high, but its reasonably even. not sure what you mean by rust blobs?

[db3155]

the metal is thin there in the middle I would throw some high temp sealant on the gasket and go fishing.

[Bushwacker]

Originally Posted by cdavisdb

. . . Manifolds still look ok to me, but the seat on the block is erroded around the bottom of the exhaust for the center two cylinders on both sides, see pics. The erosion is about half way across the sealing surface. I don't remember seeing this on previous engines.

Is this normal? If not, do I need to to more that just bolting a new set of manifolds on? I could clean it up real good and marinetex it and sand smooth, but that's a lot of work. Ideas?

Connor, what's going on there is pretty much a classic case of what we used to call oxidation/erosion in the gas turbine business. The metal gets hot (I'd guess about 1200F in this case), oxidizes (rust in the case of iron) and then the brittle oxide spalls off or erodes away when the metal expands and contracts during throttle cycles. Used to see it all the time on turbine vanes and segmented outer air seals. Mechanism is the same regardless of alloy. Pratt & Whitney has some fancy plasma-sprayed ceramic coatings that would fix that, but it would cost more than the cylinder heads! Don't bother with Marine Tex . . . I suspect it will just start turning to carbon at about 600F!

One thing that I'd look at is how the exhaust gasket and manifold fit over the port. If there is a mismatch where a portion of the exhaust gasket and/or manifold sticks up in the "breeze" at the bottom of the port and blocks the flow, you'll get "stagnation" conditions where the flow comes to a stop at the obstruction and that surface can be quite a bit hotter than one with gas flowing smoothly over it. How much hotter it would get depends on the velocity (Mach Number) of the gas . . . at Mach 0.5, it would only be about 5%, but at Mach 1.0, it would be 20%! Anything you can do to smooth out the flow going into the exhaust manifold would reduce the temperature in that area.

I'd say the main concern is reduced wall thickness in that area where it's eroding and how far it is from the water jacket. If you had access to a scrap head that you could cut up, that would give you some idea of how much margin you have. Maybe Pelican or some other guys on the forum have cut up a head or have run one like this to failure and could give you a better idea of the ultimate failure mode. I suspect it would crack well before it actually eroded into the water jacket. If it 's a FWC engine, loss of coolant would be easy to detect, but if it's raw water cooled, a leak there would quickly kill the engine. If it just eroded through at the gasket surface to create an exhaust gas leak to atmosphere, that would be easy to spot . . . what us old turbine guys would call a "graceful" failure mode! Denny

[Entourage]

Originally Posted by Bushwacker

Connor, what's going on there is pretty much a classic case of what we used to call oxidation/erosion in the gas turbine business. The metal gets hot (I'd guess about 1200F in this case), oxidizes (rust in the case of iron) and then the brittle oxide spalls off or erodes away when the metal expands and contracts during throttle cycles. Used to see it all the time on turbine vanes and segmented outer air seals. Mechanism is the same regardless of alloy. Pratt & Whitney has some fancy plasma-sprayed ceramic coatings that would fix that, but it would cost more than the cylinder heads! Don't bother with Marine Tex . . . I suspect it will just start turning to carbon at about 600F!

One thing that I'd look at is how the exhaust gasket and manifold fit over the port. If there is a mismatch where a portion of the exhaust gasket and/or manifold sticks up in the "breeze" at the bottom of the port and blocks the flow, you'll get "stagnation" conditions where the flow comes to a stop at the obstruction and that surface can be quite a bit hotter than one with gas flowing smoothly over it. How much hotter it would get depends on the velocity (Mach Number) of the gas . . . at Mach 0.5, it would only be about 5%, but at Mach 1.0, it would be 20%! Anything you can do to smooth out the flow going into the exhaust manifold would reduce the temperature in that area.

I'd say the main concern is reduced wall thickness in that area where it's eroding and how far it is from the water jacket. If you had access to a scrap head that you could cut up, that would give you some idea of how much margin you have. Maybe Pelican or some other guys on the forum have cut up a head or have run one like this to failure and could give you a better idea of the ultimate failure mode. I suspect it would crack well before it actually eroded into the water jacket. If it 's a FWC engine, loss of coolant would be easy to detect, but if it's raw water cooled, a leak there would quickly kill the engine. If it just eroded through at the gasket surface to create an exhaust gas leak to atmosphere, that would be easy to spot . . . what us old turbine guys would call a "graceful" failure mode! Denny

Yeah what he said.