Date:         Mon, 30 Sep 2002 02:13:39 EDT
Reply-To:     FrankGRUN@AOL.COM
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Frank Grunthaner <FrankGRUN@AOL.COM>
Subject:      On I4 and TIICO Exhaust Fixes
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There have been a number of posts recently about the problems of 1) exhaust
components breaking, 2) excessive vibration attributed to the exhaust system
and 3) excessive noise related to same. For the details of some of the
following arguments, observations and speculations, see the archives as I
have offered much of this material before.

Once again, I see brute force offered as the solution (weld everything up
tight). Again, the rubber isolators of the VW mount system are attacked as a
part of the problem. Gentlemen, you can weld it up solid and everything will
be fine until the metal reaches the yield point and some part will break!
There are several key factors at work here:

1. Different thermal expansion rates! The block is cast iron, the head is
Aluminum as is the oil pan, the exhaust manifold is cast iron (but a
different formulation than the block), the headers are probably mild steel
tubing as are most muffler and catalyst casings. Of course, a high class
system will have substantial stainless content which maximizes the expansion
rate difference with the rest of the components. Because of these basic
properties, the different components expand and contract as a function of
temperature. While the head/block/oil pan are generally within a few hundred
degrees of one another, the exhaust system components can easily exceed 1000
degrees. The temperature of exhaust components depends on engine load,
timing, fuel mixture and valve timing (and many other things) and can range
from 350 F when idling to more than 1200 F at full load.

2. Heat, heat and more heat. The typical automotive exhaust has a significant
length of tubing before the exhaust gas sees the outside world. Larger
cross-section tubing leads to lower gas velocities than smaller. Most I4
conversions use larger cross-section tubing than the standard GJ stock
approach. Lower exhaust velocities and higher header temperatures. Depending
on timing, mixture and valve overlap it is not uncommon to find stainless and
mild steel components severely thermally embrittled and full of stress
cracks. The rattling sounds associated with many of these conversions are
often internal baffles pulverized by heat stress and no longer connected to
the inside of the muffler case. With improper timing, one can readily see the
exhaust system (often at the entrance to the muffler) glowing read during
normal operation.

3. Sound dissipation requires length. Larger the cross-section, the longer
the length needed to smooth out the acoustic pressure waves. The muffler,
resonator and exhaust tube all interact to mix and scatter the pressure waves
coming from the exhaust. So to eliminate the noise, add length and scattering
chambers. The SA approach is correct. To quiet down the I4 pressure wave, run
it forward of the engine at least to the central area of the vanagon. Then
cross to the other side and run it out the back. The resonances will be below
300 Hz. This approach will cool and quench the exhaust gases giving much
improved muffler lifetime. If you insist on going tight and short, use long
mufflers with significant internal pathlength and use two or a resonator and
a muffler. But be prepared to burn the exhaust piping or the muffler packing
and partitions because of excessive gas temperature.

Now the engineering response to points one and two, heat and differential
thermal expansion, is to add compliance. This typically means a welded metal
bellows with wire mesh external protection. Now there are some danger points
here. All welded metal bellows are thinner than standard steel pipe. This
means fragility to puncture and to heat. They will burn through more readily.
The corrugated surface also provides more efficient thermal uptake from the
exhaust. These components can handle compression, expansion and deflection
perpendicular to the central axis of the bellows. They cannot handle
torsional forces. There are three types of welded metal or formed bellows
available to the general consumer - mild steel, stainless steel and inconel.
In VW's solution to this problem for the GJ series, they used either a slip
ball joint at the exhaust manifold flange, or a bellows assembly. Their
bellows assembly is made of inconel to handle the temperature stress as well
as residual vibrational stress. I venture that the local muffler shop only
supplied mild steel flex couplings. My advice - use flex couplings in the
system, preferably with the dual port manifold and the VW bellows.

The issue of vibration remains. The VW mount for the I4 diesel was
specifically engineered for compliance (muffler free to follow the expansion
forces) through rubber mounts. The rubber mounts are also passive vibration
isolators tuned to keep the muffler mass from resonating with the active
vibration modes of the engine. The mass, length and temperature of the I4
gasoline exhaust systems exceed the limits of the VW design. In my
conversion, I have reverted to heavy duty truck grade exhaust system hangers.
I use one on each side of the muffler/resonator (I use both muffler and
resonator until I substitute in the turbo Audi 3A motor). This provides
support, passive dampening and enables free muffler motion. The hanger
approach effectively supports the base weight of the system removing that
stress from the rest of the components.

Now the level of vibration coming from the I4 can be influenced by any and
all resonant amplifiers in the system. An improperly supported exhaust system
can add to the vibration power. A properly designed one can dampen out system
vibration modes. The support of the muffler and the support points are
important. You can establish if your system is inadequately supported or if
there are resonant nodes from poorly placed supports by holding a heavy
screwdriver against pipe, muffler or cat and noting the vibration change.
Moving the hanger clasping point to the center of mass of the exhaust and
keeping a second point a few inches from the end should resolve all exhaust
related issues.

In summary, let the exhaust system move, add one bellows with hanging muffler
supports and make the thing as long as possible. If high temperature exhaust
system paint burns off, your system is running hot and will fail. If you bolt
and weld it up tight (no rubber supports) you will fix the problem for 6
months before something else breaks, With Vanagonitis luck, we can only
assume it will be expensive. So, want long muffler life, less than 30 dB
muffler noise with quick engine response --- mount a turbo!

With apologies for length,

Frank Grunthaner