I recommend electrolysis. All you need is a plastic garbage can, an old lawnmower blade or similar piece of scrap steel and a battery charger of at least 10 Amp capacity. Fill the garbage can with water enough to cover the item being cleaned.
Add Trisodium Phosphate (washing soda, as what most laundry detergents used to be) to get a solution that conducts enough current to be readable on the battery charger's meter. Connect the positive lead to the old lawnmower blade and the negative lead to the manifold. DO NOT LET THE CLIPS FOR THE BATTERY CHARGER TOUCH THE SOLUTION!
The current will eat the clips. Add the soda slowly until the current reaches a couple of amps.
Leave the manifold in the solution for a couple of hours and check it's level of "clean". Leave it longer if necessary. When you remove it from the solution, clean off as much of the black stuff that is on the surface as possible, rinse off all the solution, dry the manifold and paint it immediately to prevent rust. Stove blacking will also work quite well to prevent rust. If you want the manifold to look good after it's been used for a while, you will have to use high temperature paint or reblack it regularly. It's fast, it's cheap and it does not remove any metal from the manifold. How much better can it get?
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