Can some one school me on how a Mercruiser 120 Manifold works ?

[nearpar]

I have a suspected crack ( although I don't see it yet ) in my Mercruiser 120 hp exhaust manifold. I would like to know how this manifold actually works. I don't understand how water doesn't get into the engine when it is running through the exhaust manifold.
The raw water looks like it could go into the exhaust ports. I understand that the intake side is sealed off and the only way the water could get in would be through a cracked manifold on the intake side. SO....can someone show me a diagram of the flows in this manifold so I can better understand how it works ?

Does raw sea water go through the manifold around the intake ports, but directly into the exhaust ports, but when the engine runs the pressure keeps the sea water out of the cylinders ? then the sea water and the exhaust gases mix together and exit the riser and then where ?

Thank s in advance

 

[Don S]

Re: Can some one school me on how a Mercruiser 120 Manifold works ?

This is a picture of a V8 manifold, but it works the same basic way.

 

[Scott Danforth]

Re: Can some one school me on how a Mercruiser 120 Manifold works ?

i will assume that this is a raw water cooled boat.

on all raw water cooled exhaust manifolds, there is a water jacket around the actual exhaust section. the water keeps the manifold cool. the hot water is discharged from the manifold to the water jacket on the riser thru the slots in the gasket. from there it makes the turn and joins the exhaust gasses to exit the boat.

the riser's job is to make sure that the exhaust pipe goes higher than the water outside the boat before it goes down (think of a sink p-trap, however upside down and in reverse).

on the 2.5 and 3.0 blocks, the intake manifold and exhaust are just one big complex casting having intake ports, exhaust ports, and a water jacket.

now if your getting water in your engine, there are a few places to look. first, has the manifold rusted thru, second, has the manifold to riser gasket surface rusted and gone to heck. third, has the manifold riser gasket gone to heck. forth, was it not winterized properly , froze, and then cracked? 99 times out of 100, if the later is true, the block also has cracks somewhere

 

[Scott Danforth]

Re: Can some one school me on how a Mercruiser 120 Manifold works ?

dang Don, a picture is worth 1000 words.

 

[nearpar]

Re: Can some one school me on how a Mercruiser 120 Manifold works ?

Nice picture thank you very much...

 

[TilliamWe]

Re: Can some one school me on how a Mercruiser 120 Manifold works ?

Nice picture thank you very much...

I agree!