Exhaust gaskets

[sogood]

Why are there two different types of exhaust manifold to elbow gasket available and are they interchangeable. I'm specifically referring to the square type gasket, one of which has four slots, one along each side between the fixing bolts, whereas the other type has only two slots, opposite each other, with two small holes opposite each other along the other sides. This despite the fact that my manifold and elbow have four slots in them. 5.7 Mercruiser, V8 circa 1990.

​I'm replacing my gaskets and the original ones were aluminium, with the slot/hole combination, but the hole looks like its a blockage waiting to happen. Why have slots in the manifold, just to close off 95% of it with such a gasket?

Thanks in advance.

 

[Bondo]

Ayuh,.... It depends on the system used,... If yer old gaskets are slots, 'n holes, use the same again,....

If ya switch, yer gonna cause other problems,...

 

[sogood]

Thanks for that but could you elaborate please? The system is raw water in salt. What problems might I encounter by minimising the risk of blockages and by improving water flow? As mentioned the fittings are four slot and I don't know if a PO had replaced them with the slot and hole variety, possibly fitting the wrong gaskets in the process.
Thanks again.

 

[alldodge]

The 2 slot gasket is used to help keep some restriction above the manifold to keep them full of water. Water flows the path of least resistance and if 4 slots were used one side of the manifold would get more cooling water then the other. The 4 slot gaskets are only used when extension risers are used. The extensions are used when the motors exhaust is not at the minimum height above the water line. The 4 slot gasket is place between the manifold and extension, and then the riser (actually elbow) is installed using the 2 slot gasket. This keeps the manifold and extension full of water

 

[sogood]

Many thanks for that. I like to know the "why" as well as the "what". It gives me a fuller understanding of how things work. So, in a nutshell, as my exhaust doesn't have risers then I should stay with the slot and hole variety of gasket. Thanks again to the gurus on here!