Question for you inboard folks/or others if you have any idea's

[Miles Offshore]

I have a question concerning (inboard) exhaust manifold set-up on my seacraft.(I posted question on WMI board as well so sorry if you read this twice) I have center risers with a 3" or 4" extension between the risers and freshwater cooled manifolds. I am getting ready to install new risers and maybe extensions and am curious if I blanked off the extensions both top and bottom would the exhaust gases become too hot?? Reason Im asking is I'm thinking that without any fluid's running thru them ,fresh or salt, that they would last a lot longer. Currently I think they are blanked off to aloow freshwater to flow thru them so they should be ok but my manifolds themselfs look terrible after only 5 years of medium use but the extensions dont yet look bad cosmetically yet although I hav'nt had a chanch to inspect them internally yet. Thanx for any ideas in advance. Craig