I noticed some manifolds have them, some don't. What is their function?

Thanks,
Craig

Do you mean like on headers? If so to cool them.

Standard black merc manifolds have all the exhaust ports and water ports all globbed into one big mass of cast metal. Note the black hoses that connect to each manifold, they carry the water. The performance motors have headers which are not all encased, thus exposed and viewed as exhaust pipes along with the water rail as you call it. In either case, you have both exhaust and water exiting the motor. Now there are mass HP setups which may differ from this, but this will cover most boat engines out there.

I’m talking about in particular, the 496HO. My 496HO exhaust manifolds have water rails. My understanding is they come with or without water rails.

-Craig

I think that they were supposed to help the manifold cool evenly, but not sure. They phased them out pretty quick due to leakage problems.

Dave

My 496 also had the rails you are talking about though they are not as obvious as the ones on the CMIs I just put on. I am not sure about the problems that Dave referred to as I have not had any trouble with mine with 250 hrs on it.

Yeah, mine is an early 2001 and has them.