Jericho
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Not at all. the largest rotating mass, the inducer and exducer have more central mass than the surrounding cases, they are also encapsulated in the cases and hence shed less heat, The cases are exposed to the atmosphere and there is some cooling to the atmosphere. But the majoriety of cooling for the rotating assemblies is done buy intake air coming into the inducer and exhaust air passing thru to the exhaust pipe. The biggest concern is uneven expansion /cooling , MY turbo generally sets right about 190 degrees at initial start , to about a min at idle, if you introduce more fuel, as one comes off idle, EGT goes up proportionaly. fuel is HEAT. smacking the throttle down too fast if the two masses aren't in sync results in a decrease in internal clearances , generally its a tightness of the case to the rotor. Some wear is NORMAL , but tip squeal or interference of the rotor to the case shroud is not. Repeated insufficient warm up or cool down will accelerate wear. A complete seizure will be evident and generally is a precursor to eventual failure . this turbo is totally AIR COOLED, ( read "no intercooler") weather its external cooling or engine exhaust gas cooling. The oil is maintained at roughly the same temperature as the BLOCK , relative to the mass of the block ( radiant cooling) and the cooler. Undoubtedly some cooling for the turbo is gained as the lubricating oil is passed through, but it is proportional to the size and velocity of the supply and return line , one cant cool more than the pipe can flow, Oil temperature has to be some what close to the Engine block , hence , radiated water temperature. ( pressurized liquid generaly can run hotter than non pressurized liquid) COKING, the hot oil partialy degrading in the bearing cavity or lines is an issue if the turbo is run hard for a long time, say 800 plus and then shut right down with no scavage cooling time, BUT I seriously doubt shutting down at 300 degrees is an issue, The rebounding of the turbo temperature immediately after shut down to a higher level than indicated at shut down is known as "soak back" its common in all turbines. I have observed soak back in my turbo after a post 300 degree shut down and have never seen with a lazer any more than 35 degrees. OIl still in the bearing cavity then should have come in at any where from 160 to perhaps 210 degrees indicated rad temp ( seldom see 210 on mine unless I work it real hard and long) the drain tube in the turbo ensures the majority of the oil in the bearing sump gravity feeds back down to the sump in the oil pan, sic, most of it is not there to sizzle, THe heat removed, (fuel cut off), the turbo down to 300 at shut down and the oil draining away and you have two co equal synced masses cooling cases, rotor , I have see some coking in aircraft roots vane compressors and Jet engine turbine bearing sumps but no real comparable coking in turbo;s on M 35s,If you want more cooling run an intercooler, bigger oil cooler ( but the engine needs OIL temp to be sufficient for proper operation as well), oil additive to increase lubricity and shed heat , all out there in the market. or a good high temp oil, I use Rotella its grade certified and has the heat range no problem , SO to simplify, run easy until up to op temp, and cool sufficient to hit 300 degrees before shut down, EASY PEASY The seals in the turbo ARE rated higher then the projected maximum op temp for safe operation, Most important to them is AGE, ( some 40 years old) and brittleness or shrinkage,swelling cycles with use and misuse l If one is leaking check rotor play with a good micrometer and reseal kit it, they are easy ! monkeys rebuild them for chump change!
