Usualy cast iron is near impossible to weld as it cracks when cooling after welding, so the weld seam just falls off. But great if it worked out for you, probably the metal used for the exhaust manifold is not the simple cast iron, due to the extreme thermal stresses it experiences.

I have done it many times with mig,but understand your thoughts Albert.

It should not work as they are different metals and should be welded by using the "buttering "method with special rods and MMA not mig,I have brazed it as well and that posed no problems,best results with MMA and pre-heat and peen after weld run.

if you want to weld cast iron with a mig clean it, pre heat it then weld with 316 or other high nickle stainless wire, then alow to cool as slowly as possible