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Yet one more opinion
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Posted by Ari [Email] (#2847) [Profile/Gallery] (more from Ari) on Tue, 16 Mar 2004 05:26:11 Share Post by Email
In Reply to: Turbo Cool-Down time?, PGAero [Profile/Gallery] , Mon, 15 Mar 2004 16:21:59
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I'm probably repeating others here, but I'm too lazy to review what they've written. My bad.

Two issues, and they're both oil related. The turbo spins at very high speed under boost; much lower speed at idle. Obviously, it takes a few seconds to drop down in speed. The bearings in the turbo use oil under pressure for lubrication. When you stop the engine, the oil pressure drops to zero, and the oil flow stops.

First - if the turbo is spinning very fast with no oil pressure, the bearings will wear some. The slower the turbo is spinning, the less wear. So letting the turbo spin down before shutting the engine off is good. That said, this is a pretty minor effect. The turbo spins down FAST - in just a few seconds.
The second one is more important. The turbo is hot, and hotter under boost. The oil is being pumped through fast, and pulls away heat. When you stop the engine and the oil flow stops, the oil just sits there, baking. Heat oil too much, and it turns to coke, which is a pretty abrasive form of carbon. It also clogs the passages and keeps oil from flowing. So 'coking up' is bad.
If you let the car idle some, it allows the turbo to cool. This is mostly from the nice water jacket Saab provides on the turbo, and from oil flow pulling heat away. The reduced turbo temps help prevent oil coking.

OK, so what's a reasonable cool down? I depends on what happened just before. Quite frankly, with the nice water jacket, you don't need much. If you were driving down the road at full boost, and power-slid into your parking space, I'd give the engine 30 seconds or so to cool down before shutting it off. I'd also consult a professional about anger issues.

If you drive 'normally' - cruised along at 1500 or 2000 rpm for a minute or two before pulling into the parking space, then I wouldn't bother to let it idle more than a few seconds. Driving off boost at low rpm is probably better than idle - more oil/water flow, and very little load.

This was much more of a 'problem' with the non-water jacketed turbos found on the earlier 900. And even then, I never had a turbo failure. With the water jacketed turbo on the 9000, and reasonable oil changes, you really don't need a cool down period unless you've run the car hard just before bringing it home.

What's more important is start-up. The greatest wear is starting a cold engine. Clearances are off, and cold, thick oil isn't where it should be. I like letting a car idle for 30 seconds or so before I set off - it gives me enough time to get my seatbelt on, re-adjust the seat and mirrors, and adjust the radio. I stay out of the turbo until the temp needle is up off the peg. This is true of any car, Saab turbo or not. The number of folks I see that get into the car and drive off on the starter motor, and then try and keep it at max revs once it catches, never ceases to amaze me. They keep engine rebuilders in business.

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