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Re: 2-piece EM: even more questions...
> However, to me it seems the thing hangs onto the heat too long and that is
a major factor in the cracking and
> warping of the unit.
The major factor here is that Audi chose to build an in-line 5-cylinder
engine ... because of this, the manifold has to be quite long and convincing
it to expand equally across its entire length is next to impossible,
especially when there is a turbo hanging off one end of it. By re-designing
it as a two-piece manifold -- two cylinders on one side, three cylinders on
the other -- and connecting them together with a flexible stainless-steel
bellows, Audi solved the problem as best it can be solved while still using a
cast manifold instead of a fabricated header with five individual pipes.
> Maybe the unit should have been cast in a material more suitable to getting
rid of any absorbed heat.
This is one of those chicken-and-egg things -- is the problem the design or
the material its cast from? -- but according to my neighbor across the
street, who's a metallurgist for Boeing, the material Audi used should work
just fine for this application. (He tested a piece from a broken manifold
for me in order to determine whether it was worth welding it up as well as
the correct rod and pre-heat procedure to use ... nice guy, eh?)
The bottom line here is that you shouldn't have any problems with the stock
two-piece manifold, coated or otherwise.