I'm trying to repair a pair of exhaust manifolds off of an inboard 350 c.i. I'm using a nickel-ferrous rod that I have used with great success on other castings and manifolds. However, on these particular casting, I'm having porosity problems and cracks around the weld (I am preheating the whole thing). Any of you know what these may be cast out of? I've heard white castings will give these kinds of problems, but I wouldn't think that would be suited for a manifold application. The metal is very grainy in appearance when I attempt to weld it. Thanks for any ideas!