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RE: 2-piece EM: even more questions...
Gerard - I can't answer the question directly, or even authoritatively, but
I can offer the following "information" I was given while looking at
alternative turbo chargers and housings for an Audi. I believe the turbo
housing is often "cast" from the same type of iron as the manifolds. One
manufacture I spoke with is having their own housings cast in Mexico to
their specifications - the other manufacturer said that they had looked at
this alternative but had decided against it, questioning the process on the
grounds the metal used (and there was a name for the type of iron used in
the casting of high temperature parts) was NOT of the same temperature
tolerances as the OEM housings, or for that matter of the Audi RS/2 exhaust
manifold - they said you should be able to find a casting mark on the Audi
part (I think it was "N...something) indicating a high "nitride (or
something, I'm sure another lister will know) " content part.
When I asked the other manufacturer about the metal content/type for the
cast part, their response was that the temperature tolerance difference
between the two types of iron castings was something @ 50 degrees (i.e. in a
material tolerance range of something like 1,475 vs. 1,425 Fahrenheit, I
think) and therefore not relevant to, or significant for the application.
On the other hand, I see State Farm has recently lost this type of assertion
in the US courts with respect to its auto accident repair policies.
On the other hand, I don't think I've heard of any of the S4 exhaust
manifolds, stock or otherwise having any of these problems. If you haven't
seen them, the S4 are both one piece designs. Its my understanding that the
design change between the stock S4 manifold and the RS/2 was either partly,
or entirely to help move the hot exhaust gases away from the two outside
cylinders (Nos 1 & 5) by shortening the runners to those positions. So
could one cause be uneven thermal expansion / contraction?
The other thought would be that going to the two-piece design may relieve
some kind of support stress on the part?
Anyway, see if you can find a metal content casting code on the parts.
From: email@example.com [mailto:firstname.lastname@example.org]On
Behalf Of Gerard
Sent: Sunday, October 10, 1999 3:15 AM
Subject: 2-piece EM: even more questions...
Me again going on about 2-piece EM for the
I am definitely forced to get one of these
things for the 200 and that means agent's
only prices. What has to be done has to be
done, I guess.
I'm still curious as to why this whole thing
fails the way it does. Hence the following
1) what metal or alloy would one
use for the 2-piece EM to give
it better heat resistant properties
and make it stand up to heat for
longer, make it resist excessive
warping and make it tough enough
to resist cracking?
Maybe the same material the head uses? I think,
from what I've heard about the Dialynx headers,
is that casting the unit in the same material
but making it thicker, did not help the longevity
of the unit. It resists cracking, but apparently
it warps like crazy and is a b*tch to work with
(right Phil?). So maybe use something that does
sit with the heat very long or that takes a lot
of heat energy to heat up.
2) what in the design of the 2-piece
do more knowledgeable listers think
the engineers should have changed
to make it a stronger unit?
3) this one is for the turbo gurus on
the list: what would you change on
the design of the 2-piece EM to make
it work better in this turbo
application? I've seen some cars running
tube headers for turbos (none Audi
so far), so how does a cast unit
compare to a mandrel bent tube
header for the same turbo app?
If I get the 2-piece I am coating it, but there
is indication to leave it as is to do its job of
heat sinking. 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.
Maybe the unit should have been cast in a material
more suitable to getting rid of any absorbed
Sorry for the WOB. It's sunday and I've feeling