Re: Oil Cooler for a 97 VW VR6

[Eric Schumacher <e.schumacher@postoffice.worldnet.att.net>]

OILCOOLER

The article that follows outlines how to fabricate a very efficient oil
cooling system for the VW VR6 engine.  The system described, while in the
strictest sense a bolt-on, needs to be fabricated.  I have described sources
for the key parts and the modifications required to make them all work
together.  The result is very low cost and gives results that cannot be
bettered at any price.

If you believe even a tenth of the on-line posts on the subject, the VR6
engine needs all the help in the cooling department it can get. Like the
diesels and 16V engines, the VR6 has oil squirters that direct a spray of
oil over the bottom side of the piston crowns to maximize the cooling effect
of the oil.  VW choose to transfer this heat to the radiator via a oil to
water heat exchanger.  This is a cost effective solution on their part in
that an oil radiator is eliminated with the additional benefit of fast
warm-up of the oil.  However it also provides hotrodders who live in hot
climates an opportunity to easily increase system cooling capacity.  A well
designed cooling system has excess capacity and uses a thermostat to set the
operating temperature. 
One of the best VW oil cooler systems around was the original  factory
system used on the European GTIs.  This system replaced the US style oil
filter mount with one containing connections for an external oil cooler and
a thermostat with a large diameter hot-wax actuated piston.  Like most OEM
VW parts this is a very high quality piece not to be compared with some of
the after market stuff produced by obscure manufacturers.  This thermostat
is of the same design as used in 911 & 935 Porsches with front mounted oil
radiators. In fact the pistons are interchangeable with the VW ones except
that the Porsche thermostat opens 15 degree C lower in temperature than the
one VW specified for the Euro GTI.  

Lucky for us the Audi 5000 turbos use the same thermostat with the added
advantage that, unmodified, it will bolt up to the VR6 block in place of the
oil/water heat exchanger.  Turbo Audis are plentiful in junkyards and I only
had to pay $3.50 for this fine item at my local Pick-A-Part. The earlier
turbo Audis had two filters on this mount and turbo diesels have a cover
where the second filter would go.   I am not sure but it looks like one
filter is for the engine and the other for the turbo.  The mount I obtained
( VW PN 034 115 417) was off of a 86 Audi turbo and it differs from the
earlier cars in that only one oil filter is used and the oil lines to the
cooler are 1/2 inch while the earlier cars,  like the Euro GTI have 3/8 inch
lines.  The bad news is that there is an interference with the upper of the
two ignition knock sensors on the VR6.  Knock sensors, as implemented by VW,
are notoriously finicky about their mounting.  Because of the foregoing I
was reluctant to alter the sensor mounting in any way.  The are numerous
ways to make this accommodation and the path chosen will depend upon the
nature of the resources available.  A simple way would be to add a 1/4 inch
thick disk as a spacer between the adapter and the VR6 block.  This would
work well with the older two-filter adapter but would require fabrication of
a new longer mounting bolt to use the 86 filter mount.  I chose to lop off
the filter mounting flange and weld a cover over the hole.  This allows the
Audi adapter to mount on the VR6 with the thermostat portion of the adapter
pointing straight down with plenty of clearance all around. The oil line
hole for the turbocharger must also be closed either by welding it over or
by plugging it with a 3/8-18 setscrew tapped into the hole in the casting.

On the VR6,  the oil inlet and outlet end up facing straight forward where
there is plenty of hose clearance if 45 degreeree end fittings are used.
Both VR6 water hose connections, that formerly went to the oil/water heat
exchanger were closed with 5/8 inch water hose caps caps. These are
available at Trak.

It is tempting to consider bypassing the canister filter that comes with the
VR6 and using a conventional spin on filter on the Audi mount however I do
not think that this is feasible.  I am guessing, but I'll bet that VW
engineers considered the maximum VR6 oil pressure excessive for a spin-on
type filter.

The oil cooler used by the earliest Audies is the same as the one used on
the latest Bug engines with the dog house oil cooler.  Later Audies use a
similar shaped cooler but with banjo fittings right on the cooler.  I don't
believe the Bug/Audi shaped cooler is optimum in form factor for most VW
applications since it has a rather thick core that is better suited to a
location with a high air pressure and on the Golf it is hard to find such a
location.  An aftermarket cooler that is thinner but with much more core
area,  mounted below and behind the bumper, should be much more effective.  

The cooler I selected came from a rotary powered Mazda.  It is a monster
thing 5x19.5 inches with large I/O fittings.  Conveniently the threads into
the aluminum fittings on the Mazda cooler are the same as the threads on the
older Audi oil filter mounts.  Unfortunately they come out on the side and I
had to relocate the fittings to the right end.  In my installation in a 85
Golf the oil lines exit the thermostatic mount from 45 degreeree fittings
and go horizontally to the right, past the A/C compressor and around the end
of the radiator.  The two oil lines continue forward out of the engine
compartment through the 5 inch space between the right end of the radiator
and the frame and directly into the cavity in the backside of the bumper
where the oil cooler is mounted vertically by bolting its mounting tabs to
brackets that extend rearward from the top inside of the steel bumper
insert. The 3/4 inch thick aluminum brackets are secured by bolting to PEM
nuts (nutserts) installed in the top of the insert.  The cooler is
positioned toward the drivers side of the car and protrudes slightly below
the bumper.  The hoses lay in one continuous arc directly into the right end
of the Mazda cooler.  

The Mazda oil cooler has a built in thermostat that I did not want to use so
I removed it and its associated spring and replaced them both with a 2 inch
length of 1/2 inch hose. When compressed by the end plug, the hose is just
the right diameter to seal the thermostat cavity.  As an aside the Mazda oil
cooler with thermostat might be used with a simple non-thermostatic sandwich
plate type oil cooler pickup.  The major problem with this approach is that
the hoses are subject to very high pressures when the oil is cold.  I
rejected using the Mazda thermostat to avoid the added flow restriction of
the hoses to cold oil.

The plumbing for an oil cooler installation should be low restriction, light
weight, tolerate high pressures and be very reliable.  Personally I also add
low cost to the list.  To meet these requirements I choose size 8 A/C hose
fittings, the kind that use an O ring to make the seal.  These fittings are
available in either aluminum or steel in a wide variety of configurations.
They seal well and have very low flow restriction. If you are serious about
an OEM look you can have hoses made up at any aA/C shop, other wise use
clamps and aA/C fittings scrounged from your local junk yard. The
aftermarket fittings designed to work with clamps are the easiest to work
with but the OEM crimp-on fittings can also be used by cutting off the
crimp-on ferule. To connect the hose fittings to, on the right end of the
cooler, I welded on two female fittings removed from an junk A/C condenser.

I used the hose ends from the Audi that match the fittings on the
thermostatic filter mount. The hose end fittings are odd metric things that
use a tapered ferule and O ring for sealing but they have nice low
restriction  bends and will work on fractional-inch sized hose if modified.
The Audi oil hoses are rather nice steel braid covered Teflon lined types
but it is unlikely that a suitable cooler location can be found on the VW
that the hoses will reach 

 The hose ends on the late Audi are designed to be used with steel braid
covered Teflon lines however they are a little contorted for use in a
typical VR6 installation. I chose to use 300 PSI rubber covered lines so I
made fittings by  silver soldering hose barbs on to the cutoff ends of the
Audi fittings. The contorted fittings can easily be reduced to a simple 45
degree bend by cutting them off just past the first bend and brazing on a
hose barb cut from the end of a steel A/C hose fitting. If you choose to use
the early Audi oil filter mount, the hose ends from the cooler end of the
Audi hoses are just right unmodified for use on the engine end of the hoses
in a VR6 installation.  They have the additional advantage that they are
designed for use with rubber hose and require no modification. Also the
threads on the early oil filter mount are close enough to 5/8 -18 to use
standard AN fittings if so desired.

Attached are three pictures showing the components prior to welding
(VR6OIL3.JPG), the thermostatic oil pick off mounted on a VR6 (VR6OIL1.JPG)
and the complete system including hoses and Mazda based oil cooler
(VR6OIL2.JPG).

 I have tried to provide some background leading to my decisions so that my
junk yard parts research could help other fabricators that have unique
installation problems in other vehicles using the VR6 engine.


Lotsa Luck Eric
85 GTI with VR6 power