HMMMMM.....

I wonder what the coolant temp is on the cold side of the radiator at op temp...underway....

So you're saying you wouldn't go with a separate heat exchanger?

It depends on ambiant temp, air flow through the radiator, cooling fan trigger temp etc. Its subject to the same variables as an air to air or air to water intercooler. The only other changes I would make is to go to a colder thermostat, more powerful electric cooling fan and colder fan temp switch.
Thats my plan for the OM603. Already ditched the clutch fan, swapped a 71c Tstat and running 75% water. In 90f LA traffic today it never touched 90c. I'm still waiting on the BMW 88c fan switch so for now I'm turning on the AUX fan manually. I haven't figured out how to turn the cross over pipe into a heat exchanger. An MB aux electric water pump is up to the task of running coolant through the heat exchanger.

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yeah but I cant help think it would be better off without the engine heat...it would be interesting to set it up "same loop" and then go "separate loop" later. Ford has a separate system with a refrigerant cooled coil in the tank.... keeps the chill water at a constant temp. I hear ya on running it colder via the therm and the switches...but op temp is op temp or is colder always better with engine coolant temps?

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-SpecialDelivery
1985 300CD Silver/Blue H&R Suspension (Sold, still cryin over that)
1982 300SD Silver/Blue '85 OM617 (Sold)
1982 300D - Blue/Blue (Sold)

Having recently replaced my radiator, thermostat, and then water pump, in that order, on my '85 300D, I can state with confidence that by far the biggest factor of the three in determining running temperature is the water pump. After replacing the first two, it was still hitting 110 C, even with a 71 degree C thermostat. After a new water pump (the old one had moderate resistance when turning the shaft by hand, with the pulley removed; the new one spun freely), I could not make it rise above 85. This was with air temperature around 65. The only difference in going to an 80 C thermostat is, with an 80 C 'stat the minimum op temp warm was 80. With the 71 C, around 78.

(I write of the car in the past tense because, on the night of September 28/29 I hit a deer in it, so, it is currently non-operational).

Tell me more about this BMW 88 C switch...

I don't know the specifics of the OM617 but on OM603 the 3 prong temp switch has a M14 x 1.5mm thread. You'll have to check if the your engine is the same. There are 2 BMW temperature switches with this same thread. BMW mounts them directly into the radiator hence the lower temp ranges.

ÜRO PARTS Part # 61318361787 = 80/88c

ÜRO PARTS Part # 61311378073 = 91/99c

Rock auto has them for $11 but they charge for shipping. It only makes sense to buy from them if ur getting other stuff shipped from same location. You will need to grab the BMW electric plug from the JY and splice it into your wiring harness.
http://doozaadoozaa.webs.com/immage/temp_sensor_pole.jpg
http://doozaadoozaa.webs.com/immage/temp_sensor_plug-circuit.jpg

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Thanks muchly.