From: hinz@picard.med.ge.com (David Hinz Mfg 4-6987)
Subject: Re: Pre-luber summary & Oil Changes
Organization: GE Medical Systems, Magnetic Resonance
X-Newsreader: Tin 1.1 PL5
Date: Wed, 2 Sep 92 07:49:04 GMT

Chris Schanzle (chris@speckle) wrote:
: In <1992Aug31.065716.27753@mr.med.ge.com> hinz@picard.med.ge.com (David Hinz Mfg 4-6987) writes:
: 
: >OK, it's not so much the cooling of the turbo as the lubrication of
: >the bearings.  At full boost, the SAAB turbo (and others, I assume)
: >is going up to 300,000 RPM(!)  If you just stomped it before you park,
: 
: [ I've always heard numbers in the 100,000 range; who was your src?]

Sorry, I don't memorize my sources.  Perhaps it was 100,000 RPM, but
really, the point is it's going BLOODY FAST.

: >and the thing is spinning away and you take away the lubrication, the 
: >shaft, which rides on a rather thick film of oil, will just sink to
: >the bottom (with no oil pressure) and start wearing the shaft and
: >the bearings.  The idea behind letting the turbo spool down isn't a 
: >SAAB things specifically, but they are the only ones I've seen who
: >mention it in the owner's manuals, perhaps because SAAB owners 
: >expect their cars to last forever.
: 
: People keep saying to let turbo motors run for a while before shutting
: them off "to let the turbo spin down."  It makes no sense and I can't
: sit back quietly today (I'm in a bad mood :-).  Also, I don't know
: anyone who is a good enough driver to pull into a parking space or
: parallel park while hitting 10 PSI of boost.  Get a grip.

Yeesh.  This is the last topic I would expect to see a flame on.  First
(here) you say it's not necessary, later you say that 4 minutes is better.
Which do you mean?

BTW, some of us DO maneuver in parking lots at 0.8 Bar of boost, thank YOU.

: If you have a turbo, you know by experience that if you are coasting,
: (minimal airflow and heat to the turbo) then mash the gas, you will
: definitely have more turbo lag than if you were cruising or even
: *slightly* accelerating and then mash the gas.  On my turbo, if you
: run at 5 inches of Hg or less (atmospheric pressure), you can actually
: hear the turbo begin to spool up.  It steadies its speed after a
: second or two.  I believe Saabs are VERY quick to spool up, so they
: should spool sooner.

Depends.  The ones which use the Garret (AirResearch) turbo are a bit slower,
some use a Mitsubishi (yuk) turbo, and spool up quicker.  I've got the H2O-
cooled Garret.  Turbo-lag is minimal for the Garret, I haven't driven the Mits
turbo hard (I don't own one, and don't like stomping other peoples' cars).

: Ok, so now let's try to get a feel for how long it takes for the turbo
: to spin down after making boost.  One way is to accelerate in boost,
: let off the gas (maybe even upshift), then put your foot to the carpet
: again.  Try to keep the RPM's the same (third or higher gear helps).
: Change the delay; you should feel more turbo lag the longer the delay,
: as the turbine slows more.  Given the same application, turbo lagp
: simply depends at what speed the turbo is spinning when you mash the
: gas.

Could be an interesting experiment.

: Turbo magazine also noted that the highest turbo temperatures are
: achieved about two minutes *after* shutting off the engine.
: 
: Bottom line: Logic tells me that letting the engine idle doesn't let
: the turbine spool down; it already is "idling."  Engine idling allows
: oil (and sometimes water, depending on the turbo) to cool the bearings
: around the turbine shaft to avoid coking (hardening, solidifying) the
: engine oil, that will reduce oil flow, and then bye-bye bearings.

OK, so what difference does it make, being good so that it lets the turbo
spool down, or so that it cools it off better, or both?  Who
really gives a shit?  If it's good for it, DO IT.  If you're a fool,
and prefer to rebuild your turbo every 100,000 miles, don't.

: much for an oil change (my cost is 3.5 qts * $3.5 + $3 FRAM filter =
: $15.25), and I sure wouldn't be getting Mobil 1!  And I won't
: overtighten my drain plug or oil filter, I can crank the motor to get
: oil pressure back before starting, and so on.  Plus, I get warm
: fuzzies knowing Mobil 1 is protecting my engine.  YMMV.  :-)

Right on about the Mobil 1, but Fram filters are crap.  Cut one open
some day, and see just how little is in there.  A member of the local 
SAAB club bought 5 or 6 different brands of filters, & cut them open.
The OEM filter (made by MANN of Germany) was by far the best constructed.
If you're spending 20 bucks on oil, why not pay 5 bucks instead of 3 to get
a good filter.

--

Dave Hinz  -  Opinions expressed are mine, not my employer's.  Obviously.
SAAB - Because you get what you pay for.
hinzd@picard.med.ge.com
Newsgroups: rec.autos.tech