Originally Posted by rado
The turbo on the N20 is oil-cooled (only), not water-cooled. Google "BMW N20 technical specification" and you can find the very detailed BMW documentation on this engine. The N20 does have an oil cooler (oil-to-coolant heat exchanger) that's separate from the turbo, so I suppose you could say that the turbo is *indirectly* water-cooled, but to call it "water cooled" is miss-leading. I don't know if the N55 turbo is oil-cooled, water-cooled, or both.
Please re-read what I posted. The TURBINE is WATER and OIL cooled. The oil cooler has nothing to do overall with the turbine. It is an oil cooler. That is all. It is used to cool the oil. Non-turbo engines also have these. Here is the link, page 103, look at the diagram. It shows the passageways of the turbine, and has the numbers below that explaining what each passageway is.
It shows in the picture #2 is the oil inlet, #6 is the oil passageway, and #5 is the cooling passage with #7 being the coolant return line. It does not show the oil return line or the water inlet line, but trust me, they are there. So there is no *indirectly* water cooling going on here, The Turbine is oil and water cooled, like most stock modern turbines are.
Go to your car, lift it up. Look under it. Look at the turbine and you will see the oil inlet at the top of the turbine, an a large outlet at the bottom, which usually goes back into the block or oil pan, depending on the design. Now look at the inside and outside directly to the left and right (but below) of the oil inlet. You will see water lines that go in and out of the turbine. These usually return somewere on the back of the engine, which then of course go through to the radiator to cool the coolant off.
I am currently on my ipad, so sorry I cannot upload the picture here, but like I said above, page 103 of the link has the picture of everything.
Edit: Evidently, this forum does not like outside links. So, Google BMW N20 technical specification, and click on the link labeled 21 Sep 2011 with the above technical training document, as you asked me to do, and scroll to page 103 of that document. You will see what I posted above to be true and correct.
BTW, I have raced turbo engine cars since the early 90's in Japan, and I helped rebuild them also. So yes, I do have a decent amount of experience in this field. I also have a 420HP RX-7 that is my play toy, hence I opted for a 328, not a 335. I stopped the power at 420 since anything above 450 makes the stock transmissions go KABOOM, and aftermarket would cost me in excess of $5-10G for a weekend play toy. I no longer race on a circuit, so there is no need for 500+ HP, although the engine is built to handle it.