Casting alloy is tbh, fairly easy. The only reason I didn't get round to doing any myself, is because nobody wanted my furnace blazing away in their garden
If you don't mind digging a hole in your garden, a furnace can be made in about an hour. Dig a hole, about 1.5 foot deep, with a bit of steel pipe of about 3" diameter feeding the very bottom of it, at as shallow an angle as possible. Grab an old hairdryer to push air into the pipe, and therefore feed air to the coals that will fill the hole. Your simple furnace is now finished
A crucible for small loads can be made from a disposable welding gas bottle, with two bolts welded to the top. A suitable handle for lifting, can be as simple as a length of box-section, with a bit of bent steel to support the two bolt stubs. A length of rod can be used to tilt the crucible.
Make your desired shape you wish to cast, out of styrofoam (polystyerene's actually a bit hard to use for this purpose) Add a few risers out of styrofoam to the pattern, and dunk the lot in plaster. A 6mm-1cm coating would be PLENTY (leave the tops of the risers/etc uncovered of course)
Stick your pattern in a metal bucket, cover it with sand, until just the risers are uncovered. Make a set of simple funnels, out of baked-bean tins, by chopping off BOTH ends of the tin. Bury the tin about 1" under the sand, to make sure no aluminium can leak out from under it.
Now, to use it all!!
Fill your crucible with large chunks of CAST aluminium - the shiny stuff won't work as well, or be strong enough, and large chunks will keep the surface area to aminimum, reducing the amount of aluminium oxide (slag). Make a layer of charcoal on the bottom of your furnace, and get it lit.
Once it's lit, hold your crucible in the centre, and fill in around with with more charcoal. Turn on the hairdryer, and leave it to heat up,and melt the alloy.
Get a mild steel tube (rusty is actually preferable, and NOT stainless IIRC) and use this to blow Argon gas through the molten alloy (pure welding Argon, not the Argon/CO2 mixes) this will help degass the alloy, which makes it less porous
Skim off the slag from the top of the molten alloy (a rusty steel ladle works rather well) as this will be aluminium oxide, which will NOT cast.
Pour your molten alloy into your prepared mould, keep away from the fumes - they're nasty! Then leave it for a while to cool and solidify.
Once it's cooled, break off the plaster, including internal pathways if you've made any - which you will have, as it's a manifold! Clean up the end casting, machine the flanges flat, drill+ tap any parts that need it, and voilla!
BUT... you're almost gauranteed to cockup the first couple of castings, as you find out that your risers didn't work too well, or you made parts too thin, and alloy didn't flow - or you've made them too thick, and they've cracked as they cooled/etc. The outlay for making ONE manifold, is actually very high!
TBH, just make one out of steel pipe/flat stock. The performance benifits aren't going to be noticed anywhere but on a dyno, and it can be done and finished in a singular afternoon. I learnt the hard way....
