Originally Posted By: brianl703
Originally Posted By: BuickGN
Water cooled is great if you have an aux pump that circulates water after the motor is shut down. If not, they do nothing for you. The water that is left in the turbo will vaporize and leave it as if it were a regular non-water cooled turbo.
First, it ought to be noted that one of the laws of physics says that if the coolant is vaporizing, it is removing heat from the turbo.
Second, let me quote what Saab has to say about the subject:
"The turbocharger is water-cooled, which lowers the temperature in the bearing housing considerably. Temperature reduction reduces the risk of the oil boiling and the damages that can arise from such. Coolant is taken via a pipe from the cylinder head. After passing the bearing housing, coolant is led further via pipes to the thermostat housing. When the engine has been switched off and the coolant pump has stopped, coolant in the system self-circulates through thermosiphoning."
Incidentally, Saab recommends Mobil 1 0W40 or Castrol Syntec 0W30 for the car that this service manual information is quoted from. (The GM LL A 025 spec)
The theory is great and I do understand what you're saying. But in the real world, our members that had coking problems with traditional turbos had the same problems with water cooled. There are a few ounces of water left in the bearing housing at the most. The 10lbs of 600 degree cast iron in the turbine housing is going to boil it quickly. Thermosiphoning may occur in some setups depending on how the turbo is mounted and how the coolant hoses are routed. For the cars I deal with, the turbo is higher than the coolant level so it's doubtful the coolant will flow uphill with the engine off.
I still stand by my theory that proper cool down is more effective than a water cooled center section but not to say that the water cooling doesn't help a little.