Wingit- taking your Q at face value, here is an example of a core that would be similar in concept, but simpler, to what you would need to develop. If you are planning to do it yourself, I say go for it. You could probably have the first iteration done in a week or so of full time work. The foundry itself will supply (& charge you for) the "rigging" which is how the iron gets into the mould and flows smoothly for a complete, void free fill. Given your intimation that this is probably a hobby or collector application, I suspect Emmanual King/Cattail Foundry could do it with something close.
But again, one step at a time.
First, you need to make a pattern on a matchplate that would form the outside of the form you want to duplicate. For CI, it also needs to be 1/8" per foot larger overall since the hot iron will shrink. Look at this simple pattern for an aircraft stick grip - notice that there are some extra nubbins on the plate. The keystone projects on the actual patters are called "core prints". they will make a hollow shape in the mould that extra-long ends of the cores will be place in, to support them. The gray shape in the middle is part of the "rigging" which the foundry installed on my pattern, to form a "pouring basin" to act as a reservoir and flow guide for the metal.
This is the other side (bottom, or drag side) showing how the foundry added runners as part of the rigging, to get the metal smoothly into the backside of the handles. Visualize how the pouring basin and runners connect to provide a void for moving the molten metal.
This is a shell core box, to make the cores. Your manifold would not need complex keystone shapes because it has enough protusions through the open ends of the manifold to support it from twisting. Round core prints added onto the ends of the patter runners would be fine. Notice the little round projections on the handle at the top and back - these are more core prints that support the core in the mold and keep it from twisting or floating when the metal is poured. You might have to come up with a few locations on the manifold to include something similar, and then maybe put pipe plugs in or weld shut on the finished. If that is objectionable and you can't find any "accessorY" holes in the original, there are other ways around this. Also, some old manifold shapes were such that just supporting the core at all the openings was more than adequate. You should have a talk with the foundry and ask.
The blue part is what the actual core will look like.
I use Freeman's (foundry supply) repro products to duplicate parts and patterns, and to make test prints.
This is a test taken from the pattern, and from the core box, to make sure they match up and don't have any crush when the foundry makes the real deal.
Everything blue represents sand (or no-bake). The voids will all be metal, including some protrusions from the runners, which the foundry will bandsaw and rough grind off (process known as "snagging") before dumping my parts in a crate.
2 versions of the product after boring. You can see how I positioned the small core prints to take advantage of holes that would need to be added anyway.
If you have to pay to have the work done, it is probably not cost effective.
If you wanted to do it yourself as a project, it is very accessible.
Read a bunch of books, then take a sample to the foundry and ask them how they want it mounted on what size matchplate.
Also ask them what process they prefer to use for cores, so you can make the appropriate core boxes. (You have to read ahead so you understand what they are telling you when you ask for advice. They won't actually tell you how to build things except maybe a few pointers. But they will define what process they intend to use, which will define how you make the patterns a core boxes.)
Go to the Foreman's site and watch all the videos about their products. You could fill and smooth an original and about build it up long enough just to duplicate the exterior with their products (not cheap, though, you can't cheap out and be successful). However, if your manifold is over a foot long, you probably have to bite the bullet and carve a new oversize pattern. At some length building up an original won't work because the port centers will shrink out of line.
Your core box would not need to be metal, these days no-bake products work as well as the old shell cores. Shell cores are cheaper for production multiples, though. Paste up sand cores are still viable at small foundries, but will cost several multiples in price compared to more modern processes.
I have more photos of using Reprothance for other casting products, but enough for now, you may decide it does not interest you.
smt
