Turbo idle-down

[Al Powell]

Our Phriend Phil said about cooling off the turbo by idling.....

>EXACTLY the wrong thing to do.
>
>Despite all the crap about oil-cooled turbos and water-cooled turbos, all
Audi turbos are
>actually air-cooled.  Oil has almost no cooling effect, and serious water
flow through the
>later models' water jackets (only around the bearing) doesn't start until
some time after the
>ignition has been switched off.
>
>You need airflow to cool the turbo, and there is no airflow if the car is
stationary - except
>the paltry contribution of the alternator fan.

I must respectfully disagree that it is "EXACTLY" wrong, for these reasons:

1) Idling the turbo down for a few seconds assures that it's no longer
spinning at high revs when you kill the engine, just in case you did goose
the engine when pulling into your home. This is A Good Thing in itself, and
makes idling the car for a few seconds worthwhile.

2) If there is no other benefit, at least idling the car lets the hot turbo
housing have a few more seconds to dissipate a bit more of its heat into the
heat sink of the manifold.

3) As noted by another lister, both turbos and motor oil definitely cool
down after some minutes of idling. Therefore, although the cooling process
is surely slower than that experienced with more air flowing past the turbo,
it's not accurate to imply that NO cooling takes place. Whether 30-45
seconds of idling makes any particular difference in this process, I can't
say.

4) I have a 280ZX Turbo that has no water cooling. A logical extension of
Phil's argument would be that there is no virtue to idling it before
shutting it down.  However, if it's NOT going to be cooled by having oil
circulate through it, then air is the only thing left - and it's sitting
behind a shield and doesn't get much airflow.  My opinion is that the oil
DOES contribute to turbo cooling significantly. If it didn't, oil-cooled
turbos would not work.

5) Audi turbos are not *only* air cooled. Idling gives both the oil and
water passages at least some chance to have a beneficial effect. Again, A
Good Thing.

My observations and opinions only.

************************************
Al Powell
apowell at gocougs.wsu.edu
1958 Fiat 1200 Transformabile Spyder
1983 Datsun 280ZX Turbo
1993 Audi 90Q
1997 Chebby Blazer
1999 Chebby Blazer
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