all nissans/garret turbo's after the first T2 ET/exa have been water cooled.

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I'm pretty sure it's to cool the turbo.....

But seriously you increase the turbo life by keeping the core cooler. Water cooled turbos don't require as much "turbo timing" as non water cooled as the cores aren't as hot. The real reason you have a turbo timers is so that you don't get carbon build up on your bearings by shutting the motor down when the turbo's too hot and burning the oil in the core.