how long will an aluminum intake and aluminum exh manifolds last on a raw water (salt water) cooled motor if flushed out after every use?.

i have a set of vortec heads to put on my 350mags and need a new intake and my manifolds need to be replaced anyway, wanted to see if i can loose a little weight with the manifolds

Take the intake and get it hard anodized, it should help get you much better service life.

probably going to cost 100-150, you will get a better deal if you get more parts done.

We just replaced the intake and manifolds on our Hp500's this year . They were new in 1996 with 976 hrs on them and did not leak. Sold the intakes and manifolds and they are still working. You need to flush, flushing is the key. Stainless Marine make a very nice set for SBC.

I have a buddy with a Marine Power SBC in a wake board boat and it has aluminum exhaust manifolds and risers stock. He uses it almost 100% in salt and they are about 3 yrs old and still OK, but think they last about same as cast iron in salt. (~5 yrs)

You should easily get 5 or more years out of them as long as you take care of it......meaning flush the hell out of it. You need to run it for at least 10 min. to get all of the salt out of it. I grew up and spend all of my life in salt water and ran 95% of the engines I built with aluminum heads and intake. Until a few years ago when the header prices became reasonable, they all had aluminum exhaust manifolds also. Having the intake hard coated is a very good idea. It's just a good insurance policy. You can also get the heads done, but it will cost a few bucks. Good luck.
Eddie