Several recent posts on both the Vanagon and Subaru-vanagon lists have 
recently addressed the issue of exhaust pipe cracking, noise and vibration 
for vanagon engine conversion systems. While my comments really refer to the 
I4 conversion approach, there are clearly aspects of the problem that also 
pertain to the Subaru boxer motor as well. I have gone on at length about 
this problem in the archives, so go there for more info. 

For those of you who thought you were to be spared another long posting from 
this address, I can only note that I have been on travel, and not had ready 
access to post in a more timely fashion.

First, the problem of exhaust pipe cracking and breakage. There are three key 
issues here:

1. Proper suspension of the exhaust system
2. Exhaust gas temperature
3. Exhaust system length.

For the first issue: Proper suspension. The design of the diesel exhaust 
system mount is instructive. The engine mounts have a cast landing for an 
unusual muffler mount. I say unusual because it only allows a several mm. 
Movement laterally, and virtually no movement perpendicular to the engine’s 
long axis. Note also that there is only one rubber isolation mount on the 
passenger side while two orthogonal axis rubber mounts are used on the 
driver’s side. Closer inspection of the rubber mounts show that these are 
basically passive vibration isolators, in the tradition of motor mounts as 
opposed to the traditional exhaust mounting hardware used on American and 
Japanese  (and other VW) automobiles. These mounts are primarily designed to 
dampen resonant vibration modes. Because the mounts are fixing the diesel 
exhaust rigidly to the motor, they are only allowing for differential thermal 
expansion. I say rigidly, because all of the exhaust train is bolted 
together: head to exhaust manifold to exhaust pipe to muffler to tail pipe. 
Not a flex joint in the mass. Also, with no connection to the body other than 
through the engine and transmission mounts, there is no impact on vibration 
isolation from the body with this mounting system. The point is that the 
mount fixing points on either side of the muffler are primarily dampers to 
keep vibrations from amplifying and adding to the precarious stress profile 
seen from thermal stresses. 

When these mount points are “improved on” by the DIY installer or the 
uninformed engine kit entrepreneur, or the local muffler jockey, the engine 
mount points are changed, the mount material is changed and the 
muffler/exhaust system is changed. What changes? Well, stiffness, mass, 
central moment (like center of gravity) pipe length and pipe diameter. Effect 
of change? System has a new set of resonances. Buzz is a big part. Lord knows 
the original VW solution was a compromise as every vanagon N/A can readily 
attest. Add these resonances to heat and the result is cracked exhaust, just 
ask when!

But note, the exhaust on the original VW (or Subaru) donor car was quite 
superior to the conversion product. Why? Well, the phalanx of number 
crunching snuffies at their disposal for one thing, the number of units they 
made for another. Nothing like building a s**wad of anything to eventually 
get it right. Two big differences in the exhaust systems for the Golf and 
Jetta:

A. They are longer
B. Flexible segments are built in. In the single port manifold, the clamped 
swivel joint provides stress relief. In the dual port arrangement, a full 
flex joint is provided. 

Second problem: Exhaust temperature.

The details of the combustion process are complex but not counter-intuitive. 
In even modern engine designs like the 8V watercooled I4 group, the 
waterboxer and the Subie knock-off, the fuel has not been completely consumed 
before exiting into the exhaust manifold. How much unburnt fuel remains in 
the gas stream in the exhaust manifold depends on the air/fuel ratio, the 
ignition timing, the camshaft timing, the exhaust system flow and to a lesser 
extent the intake manifold flow. Now many amateurs and professionals change 
several of these parameters simultaneously with any engine conversion. First, 
the air/fuel ratio: effected by the location of the O2 sensor, leaks in the 
exhaust system, intake air leaks, air flow pattern between the air flow 
sensor and the throttle, etc, etc. Rich, open loop or full throttle 
operation, higher exhaust temps and the peak burn point further away from the 
combustion chamber. Second, timing and chipping. The issue here is both idle 
specs and the advance curve. Retarded timing, more gas burning in exhaust. 
Camshaft timing: often off +/- half a camshaft sprocket tooth. Could be 
worse. Different cam from stock. Net effect more unburnt mixture out the 
exhaust valve.

But the biggest issue after timing mistakes is that of flow. The standard gas 
I4 install has three times the linear footage after the exhaust manifold as 
does the typical vanagon application. Consequently, the peak afterburn front 
moves out from the head significantly. Use a larger exhaust pipe diameter 
than the diesel for serious flow, bring the high temperature exhaust gases 
further down into the exhaust pipe. The gases are cooled by scattering from 
the walls and conduction by thermal transfer to the pipe walls. Higher flow 
also moves the peak flow out from the head.

The net effect: the exhaust pipe and muffler run far hotter that the 
materials are designed for. When I first stumbled into this problem, my 
stainless steel RV muffler was rattling. Took it back to my TIC (means 
tongue-in-cheek) muffler shop. Diagnosis … muffler baffler came loose. 
Replaced defective muffler. Back again. Hmmm, baffles loose again. Well we 
sectioned the muffler and found the interior had crystallized! Hot, Hot. FMD 
(friendly muffler dude) lectures me. Says it will go faster if I burn the 
fuel in the combustion chamber, not the muffler. OK, Brute force. Larger 
stronger (largest, strongest) muffler installed. Then exhaust pipe cracks. 
Always at welded seams, always crystallized. 

Too hot plus vibration. Will do it every time.

BTW, vibration means noise after pumping the spectrum into the air and the 
body.

So, the third problem, system length. The plain fact is that it is easy to 
put a high flow exhaust on an I4 powered vanagon, it’s hard to generate one 
with the back pressure of the stock Jetta or Golf. Any exhaust (possible 
exception is the original diesel version bolted on to a gas engine) will have 
a significantly higher flow that the G/J group. Most aftermarket mufflers and 
cats have significantly higher flow than stock. Also the tube length is 
nearly 50% less, so conversion exhaust is likely to flow twice the stock G/J 
value. 

Finally, noise! The length of the exhaust strongly dampened acoustic pulses. 
As for resonances, lengthen the tube, lower the resonant frequency. Three 
times the length, about one third the frequency for resonance. 

So what are the answers:

1. Proper suspension .. I use stiff hangers at stock locations. Details in 
the archives. Must have two point mount (at least). Both sides of muffler 
must be supported. Support to motor, not body. Add flexible element between 
cat/muffler and manifold. This will allow differential thermal expansion and 
will impede flow substantially, thereby reducing temperature at cat/muffler. 
Frankly, I use the dual port manifold and the dual port flex factory mount 
before going to collector, then cat/muffler, then out.

2. Add EGT monitor (exhaust gas temperature) gauge.  Works well. Or use 
handheld IR pyrometer to map temp along pipe circuit. Them change ignition 
timing to move burn front back.

3. Don’t wrap system with insulating wrap. Will lead to even higher wall 
temperatures and faster failure. 

4. Add turbocharger to end of exhaust. This will stop the flame front and 
nearly eliminate the racket. More power too if you connect the compressor 
output to the throttle body inlet. Assumes you connected compressor inlet to 
AFM, etc, etc.

5. Stop being so impatient! Run the exhaust gas circuit forward towards the 
driver, past the trans, across the body center, back to the rear on the pass 
side, into muffler and then out on the driver’s side rear. 

For more details on the “best” exhaust (defined as quiet, good power, quiet, 
good economy, quiet), I sent the following excerpt to Alison Bell about the 
long exhaust and turbo solution: (This was to be sent several weeks before 
the current traffic)

Alistair,

I wanted to propose  a dialog about exhaust system routing. I think several 
things are clear:

1. Longer exhaust will send cooler gases to the muffler, reducing internal 
muffler failure from embrittlement and crystallization. The inlet pipes to my 
muffler (stainless) are discoloring because of high temperatures and high 
temperature exhaust manifold paint quickly burns off.

2. Longer exhaust will cause some backpressure which is probably good. Right 
now, the short closely coupled systems (mine in particular) show high EGT 
values suggesting that the burning charge wave is moving near or into the 
exhaust manifold. The O2 ratios and cycling are fine, but the sensor is 
really hot. Next weekend I may use the pyrometer to map the exhaust 
temperatures, but this is only easily done at idle. Some backpressure will be 
consistent with the Air/Fuel/Timing maps of the ECU. Should reduce 
temperatures in the chamber (peak) and help quench the hot exhaust at the 
manifold by reducing flow.

3. The longer exhaust will also cut down on the exhaust noise characteristic 
on the I4. Now, the muffler and resonator hold down the racket in my 
installation, but at 4 to 5000 rpm it is still obnoxious in the rear 
passenger area.

4. Lower muffler temperatures will reduce the heat soaking the engine 
compartment probably extending the lifetime of many components and reducing 
oil temperatures.

So, ..... This past weekend, I was studying the engine layout possibilities. 
Following your advice, I am using a SAAB 900 Turbo intercooler (air to air) 
which will cool the compressed air from the turbo. It mounts cleanly in the 
drivers side area behind the rear wheel. Currently, I plan to open the bottom 
of the engine air scoop area (where I have the air cleaner mounted, I will 
add a small radiator fan at the base of the opened cavity; then do an 
aluminum interface between the body, fan and intercooler. Finally, I will 
mount a rear-facing duct, sealed towards the tire, to eject air to the rear 
of the van.  I'm thinking this will eliminate the possibility of waterborne 
mud from the rear tire spray clogging the intercooler fins. As far as size 
goes, it looks like the intercooler was designed for this application. 

My second major issue is that of a special oil sump for the turbo and a pump 
to clear it. I have settled on an aluminum extra capacity oil sump from the 
Beetle air-cooled applications. By using a sump cover on the top and bottom, 
I can drain the sump at will, use a pipe through the top cover into the VW 
screen arrangement to feed deareated oil to the external hydraulic pump, weld 
a short flange into the top for the turbo oil return outlet to dump into the 
sump. A vent line in the top cover will go to the crankcase vent system. The 
pump (I have chosen a gear pump from Barnes flowing 3 gallons per minute) 
will be mounted on a regular VW power steering pump bracket, driven by a 
v-belt. Actually looks just like the vane power steering pump. Oil sucked 
from the turbo sump will go from the pump to the external Mazda RX-7 oil 
cooler and then into the main oil sump. It looks like the VW sump extension 
(1.5 quart capacity) will fit between the turbo and the block. There is a 
clear space to the rear of the block of up to 5 inches(before interference 
with the trans and the CV joints. 

The SAAB turbo has a clean 2.5 inch diameter exhaust pipe outlet with a 3 
point conical mount. The muffler shop could easily take the exhaust straight 
back from the turbo outlet bringing it up to clear the CV joint area and then 
... .

Looking at the undercarriage, the problem is the large cross member just 
forward of the transmission. There are holes for the former heater air tubes. 
One could see a three piece exhaust pipe going from the turbo to a junction 
near the heater hole, then a cross piece going from that junction to the 
junction on the other side, then passenger side junction to the muffler. The 
muffler could still run widthwise across the rear, with the outlet on the 
drivers side rear.

Any other ideas? Heat shields can be used to buffer any heat to the floor, so 
I don't think floor heating is an issue. I can see going forward to the van 
midpoint level with the crossover pipe. Perhaps welding in a resonator in the 
center crossover section. Of course, the turbo will really reduce the sound 
level to begin with. But ... ideas, comments,

Thinking,

Frank

Well, as always, too long. But I wanted to say it. All comments eagerly 
accepted (well positively anticipated, … well solicited at least!)

Hope this helps someone.

Frank Grunthaner