If you believe even a tenth of the online 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 deg C lower in temperature than the one VW specified for the GTI. Typically the opening temperature is stamped on one end. Be sure to test it before use, these things can wear out.
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. 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 are1/2 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 turbocharger oil line hole 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 clearance, if 45 degree hose-end fittings are used. Both VR6 water hose connections, that formerly went to the oil/water intercooler, were closed with caps.
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 Imaximum 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 later Bug engines with the dog house oil cooler. Later cars use a similar shaped cooler but with banjo fittings right on the cooler. I donis 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 degree 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 thru 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 horizontally by bolting it to the inside-front of the bumper. The hoses lay in one continuos 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 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. The 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 a/c shop, other wise use clamps and a/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 old 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 braidcovered 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.
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.
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