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Posted by Ari [Email] (#2847) [Profile/Gallery] (more from Ari) on Fri, 16 Feb 2001 09:22:45 Share Post by Email
In Reply to: Little need to idle your turbo, Ross, Thu, 15 Feb 2001 17:47:09
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There are two issues to deal with - heat, and turbine spool speed.

If you haven't been driving the car hard up to the point where you come to a stop, the turbine speed is low, and the heat has had a chance to dissipate. So under those conditions, idling isn't that important. Essentially, you've been near idle for a few minutes.

If you've been driving the car hard, the turbine still takes some time to slow down. I disagree that at idle the turbine speed is like a ceiling fan - not unless that fan is pretty fast. But even if the turbine spool is spinning fast, it has little load, so even a thin layer of oil in the bearings is fine. That's not the problem.

As others have said, heat soak is the worst part. Turbos get VERY hot, and when you shut off the car, the oil flow stops. Oil will turn to coke in the hot part of the turbo, and coke is an abrasive, chewing up bearings. Unless the turbo has had a chance to shed some of that heat, there will be some oil coking. A water jacketed turbo helps, but there is still a BIG temperature gradient between the inner part of the turbo and the water jacket, so coking is still possible.

Solutions - use a synthetic oil to help reduce coking. Synthetic doesn't prevent it, just reduce it some. Second, if you've been driving the car HARD, give it some time to cool down. This can be sedate driving, or if you've just come off the highway after racing that Porsche, let it idle.

What's just as important as cool down? Warm up. Never jump on the turbo until the oil temp is at least warm. Spinning the turbo up with no oil in the bearings, or cold oil that flows like margerine, will kill the bearings fast.

I usually warm the car up for a minute or two before driving off in the morning, and stay light on the gas until the temp needle moves up a bit. Even a warm engine I give a minute or so to get the oil back to where it needs to be.
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