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superceded attachment parts, and some 'receded' attachment parts.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.
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