cast iron vs. aluminum manifolds

Don't use Aluminum. Mercruiser had the bright idea of going to aluminum manifolds and ss riser on the 8.1 Horzions. They had a huge issue and recall from them prematurely failing, as in 2-3 years and leaking into the motor. They went back to cast iron manifolds and still use the s/s risers. Do a search on the internet about them.

If we're talking exhaust manifolds here, then ditto.
J & T tried this in the mid-eighties on 8V92s with predictably sad results.

On what? Assuming its an engine you usually go to aluminum to save weight and allow for better flow as you can customize the passages much easier. Aluminum expands at a different rate than cast iron so this must be accounted for in the design.

On most boats saving a few pounds isn't going to make any difference. On the reverse side my buddy's 68 mustang (390 cid) had a 90 lb cast iron intake manifold. Changing to aluminum saved about 70 pounds with more hp and better handling to boot. So it all depends on the situation.

Loren, you can go one up from the old J&T cast elbows. We have replacements available in 316L stainless that not only reduce weight, but also last much longer.