Post by
C-Kwik »
https://forums.nicoclub.com/c-kwik-u426.html
Thu Apr 08, 2004 5:31 pm
Well, I don't consider running at a track or 140 mph to be most situations. My turbo has probably seen that less than 1% of the miles on it. And if you are coming off a road course with no cool down lap, you are asking for warped rotors as well. A bit off topic, but I use a full cool down lap to get everything down to reasonable temperatures anyways. And at a drag race, you typically get a long drive back down the pit lane at like 5 mph.
And no, I in fact meant that oil cooled bearings only typically need about 30 seconds.
I am quite aware of the different parts of the turbo transferring heat into bearing after shutdown. That is exactly why I stated, "You only need to cool the overall components of the turbo to a point where the hottest part of the turbo will not transfer enough heat into the bearing after shutdown to cause the oil to coke onto the bearings." But also keep in mind that additional heat goes also soaks into the manifold and the downpipe.
As far as oil and water flow, consider that oil pressure is never really all that high in a turbo. Or at least it should not be. Typically, you should somewhere between 5-25 psi. Too much pressure and oil will leak past the shaft seals. That means there is not a whole lot of oil to take away the heat. The water connections are larger and can handle more pressure as the water jacket is isolated from the bearing. In any case with so much more flow and the fact that the water is a better medium, the bearing section will stay significantly cooler. In fact it stays cool enough that the oil temps don't really reach temperatures where oil breakdown occurs. Of course oil still breaks down faster than in an NA motor, but an oil cooled turbo can actually operate at temps where the oil can breakdown.
And just from my own experience, I've used an oil cooled turbo for 2 years. I have my turbo timer set at 30 seconds. I had to replace my turbo recently. But there were no signs of coking. The bearing was just starved of oil for a small amount of time. In any case, if 30 seconds was not sufficient, then I'm sure I would have seen some problems.