From: NAME: Alex McLellan
FUNC: MDA/CAATS
TEL: (604) 231-3054 <AMCLE AT A1 AT CATM01>
To:
"british-cars@autox.team.net"@mdavcr@galaxy@hobbes@psi%mda.2@mrgate@catm01
Some inconsequential remarks about SAAB turbos, plus a little of the
obligatory LBC content.
Alex
(ajm@mda.ca)
From: NAME: SAAB@NETWORK.MHS.CompuServe.COM
<"SAAB@NETWORK.MHS.CompuServe.COM"@MDAVCR@GALAXY@HOBBES@PSI%MDA.2@MRGATE@CATM01>
Date: 11-May-1994
Posted-date: 12-May-1994
Precedence: 1
Subject: [#2165] (S) Turbo Cooling
To: AMCLE@A1
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Date: 11 May 94 15:00:44 EDT
From: The Saab Network <SAAB@NETWORK.MHS.CompuServe.COM>
To: BlindCopyReceiver:;@compuserve.com
Subject: [#2165] (S) Turbo Cooling
Message-Id: <940511190044_555063.0_DHL26-35@CompuServe.COM>
#2165
5/11/94
Message Summary
Subject: Re: Turbo Cooling
==================
Message from vanwyck@geology.wisc.edu
------------------
Don't worry about being "flamed" by me, I quite agree with you. Turbo
paranoia probably came with the status of having the first production
turbocharged cars on the market. Inproved manufacturing and better oils
probably removed any further need for pampering but the legend lives on.
==================
Message from dcc@dcs.ed.ac.uk
------------------
In a turbo, the turbine-compressor arrangement spins at very high
speeds (up to 200,000 rpm in tuned setups), and it is also in the
presence of very high temperatures due to the turbine (you know the
way a football pump heats up - I think it's called "adiabatic warming"
?). The Garrett turbocharger used in older Saabs is lubricated *and
cooled* by the engine oil, which also forms a liquid bearing for the
compressor. If the turbo isn't "warmed down" after hard use, the built
up heat can raise the oil sitting in the turbo to dangerous
temperatures, destroying its chemical properties. A little oil flowing
through the turbo for a few moments is sufficient to cool it down -
Saab recommend 20-30 seconds running with no engine load prior to
switching off.
If I have been driving gently through town (i.e. with no boost) for a
few minutes prior to parking the car I don't bother, as this will have
cooled the turbo anyway. On the other hand, I also use my car ('84 900
turbo 8) for road rallies, and I always let it idle a few minutes at
the last time control and drive very gently to the finish venue to
give it a thorough cooling off period.
The effect of repeated overheating of the oil in the turbo will be to
reduce the lubricating properties of the oil, causing increased wear
on all mechanical components, but especially the compressor bearing.
An extreme example of this is found in an (admittedly badly designed)
Austin/Rover car from the early 80's, the MG Metro Turbo - this is a
small town car with an A-series 1275cc 4 cyl pushrod engine (as found
in the MG Midget, Morris Minor, and the Mini) which is mildly
turbocharged. The oil return pipe from the turbo drains into an engine
oilway which lubricates the main bearings. Switching off the engine
without proper warm-down, even on a single occasion, can turn the oil
in the turbo to a fine, crystalline *powder*. The next time the car is
started, this of course passes into the main bearings, destroying the
engine.
Although modern multigrade oils are getting better every day, they
work best when the engine is at working temperature. So, drive the car
rather than letting it warm up at idle, but keep off the turbo until
it is at full working temperature.
The moral of the story......
1. A turbocharger is a piece of precision machinery - treat it with
care.
2. The universal adage for looking after turbos is true - use the best
quality oil you can get, and change it frequently.
Dave
David Crooke, Department of Computer Science, University of Edinburgh
Janet dcc@ed.dcs : Internet dcc@dcs.ed.ac.uk : IP talk
dcc@129.215.160.2
Work: JCMB Rm 1408, King's Bldgs, W Mains Rd., Edinburgh EH9 3JZ. 031 650
5164
Home: 12 (GFR) West Savile Tr, Edinburgh, SCOTLAND EH9 3DZ. 031 667 4854
==================
EOF
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Date: 11-May-1994
Posted-date: 11-May-1994
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