Originally Posted By: Jeffs2006EvoIX
Originally Posted By: 440Magnum
Originally Posted By: Jeffs2006EvoIX
The Idleing down part really isn't necessary any longer since most turbos of the day are water and oil cooled, so the temp of the turbo stays consistant. Its not like the old days when a turbo was pushed you can actually see it glow red.
I gotta disargee here...there's nothing new under the sun about combination water and oil-cooled turbo bearings, and the temp of the turbo DOES NOT "stay consistent." When you demand boost, the turbine wheels get hot. Demand it long enough, and they'll still glow red. Even running the turbo "moderately," the turbine wheel gets up to exhaust-gas temperature. If you just shut it down, the water stops flowing through the turbo cooling jacket (if present) and the oil stops flowing through the bearing. The wheel is sitting there nearly red-hot, and that heat will migrate from the wheel, down the shaft, into the bearing and housing, and will subject the oil remaining in the bearing to extreme high temperatures. That doesn't matter if its your EVO turbo, the Garrett on a 1988 Chrysler Turbo II, or the Holset on a Cummins N-14 in a Kenworth (actually diesel turbos cool even quicker at idle since diesels have a very cool and high-flow exhaust stream at idle compared to gasoline engines).
The idea is to get the EXHAUST GAS temp down low and keep it that way for a couple of minutes before shut down, that way the exhaust stream (even though it may be 500 degrees F) will cool down the 1100 degree F turbine wheel, rather than letting all that heat migrate down into the bearing housing. The way to get the absolute coolest EGT is to idle the engine with no load, which is why some turbo cars (RX-8, for one IIRC) have an idle cool-down system built into the engine controller. You can turn the key off, lock the car, and walk away and it will idle until the engine controller thinks the turbine is sufficiently cool.
All I know is this. The Turbo is Water Cooled and Oil cooled. 3 Hrs on the dyno at WOT 90% of that time, the turbo housing never got over 300F by thermo temp. Which is well within the ability of most synthetics. I never saw my turbo glow red, and my tuner who builds Evo race engines does not idle the car down after the dyno runs. When asked why? He said the turbo is water cooled and never gets hot enough watch.
The water DOES NOT continue circulate strongly (maybe not at all depending on turbo placement relative to the rest of the cooling system) after shutdown. The turbine wheel itself, not the housing, is what gets thermally soaked up to the full EGT, and subsequently conducts heat to the bearing and oil, enough heat to boil all the coolant sitting in the turbo away. Its your engine, do what you want. It will probably survive- but then most turbos have always "survived" too. If you want it to last the longest it possibly can, idle it or get a turbo timer. Most people here are about going the extra mile, not just doing the minimum necessary.
