[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.