2 pc manifolds

QSHIPQ at aol.com QSHIPQ at aol.com
Fri Oct 26 22:39:05 EDT 2001

My main focus is to dispell the myth that right side motor mounts is THE 
cause of cracking in manifolds.  The failure of both the motor mount AND the 
manifold is heat, specifically heat cycles.  Can a broken motor mount 
increase the failure of the manifold?  Sure.  Is it the cause, no.  The cause 
is the lack of support, and thermal expansion of a log style cast manifold.  
Please take a gander at Corky Bell's book on page 126 and 127.  Specifically 
look at what audi *didn't* do to handle the unequal temp distribution thru 
the manifold.  Further documentation on cracking manifolds can be found in 
Humphries Book:  Automotive Supercharging and turbocharging Manual pp 92
"A prime consideration for the manifold is its strength to carry the weight 
of the turbocharger when working at high temperartures, up to 1000C.  The 
frequent rapid expansion and contraction can cause internal scaling and 
corrosion leading to cracking."

Fact:  All 10 NA cars use EM support brackets.  With the additional weight of 
the turbo and wastgate hardware, *most* 10vt cars don't use EM support 
brackets.  Why not?

I would further purpose, that to reduce cracking, the motor mount is a 
secondary concern.  The motor mount failure increases the vertical movement 
of the engine and transmission only.  The key in EM: design, attachment and 
support in respective order.  

So first, if you can get the bux, do the 2pc, it's the best design.  Then the 
dialynx is second to that, then the single piece third.  All of them benefit 
from milling flat the head mating surface.  All of them benefit from the 
superceded attachment parts, and some 'receded' attachment parts.  This would 
include the old two piece or the new single piece EM/trans support bracket.  
This would include the updated studs, flat washers and nuts for attachment 
(allowing for some expansion as mentioned in 10-16 and in the "fastener" 
recommendation section).  This would include the downpipe support bracket at 
the RH trans wing (receded part numbers, found on pre 84 urq's, can be 
retrofitted to all 10vt cars).  This would include the transmission tail to 
catalyst support yolk as well.  All these are weight bearing supports for the 

The MOTOR mount on the other hand isn't a weight bearing support for the EM.  
The motor mount failures cause manifolds to pop studs, break the cast to 
turbo downpipe welds, and cause leaks in the EM/turbo system>which leads to 
increases in (inconsistent) heat at the manifold.  

The relationship of heat vs expansion changes at every measure of a log style 
manifold.  The heat in each collector is different from the head/EM port, and 
ultimately the collector at the turbo.  Which means the thermal expansion 
isn't consistent front to back. Any hiccup in the fueling system 
exponentially exacerbates this problem.  Result?  Warping manifolds, and 
rapid/inconsistent manifold temps>cracking.

I5 motors are susceptable to cracking just from length alone, the longer the 
manifold, the more expansion, and the more inconsistent the heat is across 

Bad motor mounts aren't the primary cause of cracking manifolds.  IME/O, bad 
motor mounts are what pop studs, and can warp manifolds.  Cracking, OTOH, is 
a heat cycling issue, possibly compounded by the results of bad motor mounts, 
but not the primary source.  YMMV, the documentation on cracking manifolds 
points to heat cycles, the more rapid and inconsistent, the bigger the 
problem.  Audis own revision to the 2pc manifold would "support" that 
documentation.  The "n" of known issues with the hydraulic motor mount 
failure is #1 EM stud pull causing exhaust leak.  Ask any audi tech, this 
applies to ALL 10v I5 motors.

My .02

Scott Justusson

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