NetNews Usenet Archive 1992 #19/NN_1992_19.iso/spool/rec/autos/tech/12002

Internet Message Format | 1992-09-01 | 7.5 KB

Path: sparky!uunet!dove!cme!speckle!chris
From: chris@speckle (Chris Schanzle)
Newsgroups: rec.autos.tech
Subject: Re: Pre-luber summary & Oil Changes
Message-ID: <chris.715353658@speckle>
Date: 1 Sep 92 13:20:58 GMT
References: <[e-mail]> <[e-mail]>
Sender: [e-mail]
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Nntp-Posting-Host: speckle.ncsl.nist.gov

In <[e-mail]> [e-mail] (David Hinz Mfg 4-6987) writes:
>Paul Welch ([e-mail]) wrote:
>: Here's just a little more background on why I brought this up in the
>: first place:  I have two vehicles in particular that this might be
>: a good idea for.  First, there is my Saab 9000 Turbo.  Saab recommends
>: that you let the engine idle for 20 to 30 seconds after parking the
>: car before you shut the engine off, to allow the turbo to cool.
>: I'd be interested to know if this is a theory or if they have
>: ACTUAL DATA that shows some benefit here (I'm skeptical as to
>: how well one could cool a turbo with engine oil that is already plenty
>: hot).  But, if you could circulate that oil for more like 5 minutes,
>: especially if your not continuing to add heat with the engine running,
>: now it starts to make sense.  The benefit of having this system in
>: place for when you go to start the car is really secondary, in this
>: case.

>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?]

>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.

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.

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.

I don't know about your turbo, but even with a bypass valve (to bleed
off pressure back to the intake side of the turbo when decelerating) I
would suggest it takes about three seconds for the turbine to spin
down from full (10 PSI or more) to it's idle speed when you let off
the gas.  If you don't have a bypass valve, the pressure will feed
back into the turbine and slow it even more quickly.  Also, small
turbos that spool up quickly also spool down quickly due to their
lighter rotating mass.

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.

TIP1: let a turbo motor idle for at LEAST 30 seconds (I personally
never idle for less than a minute) if you were driving gently for at
least a few minutes before finding your parking space.  If you just
came off the highway (e.g., to fill up with gas), your turbo may be
glowing orange hot even if you were not in boost (verify this some
evening; engines that cruise at less than 3000 rpm shouldn't glow orange
if timing and air/fuel is optimum).  I watched mine take three minutes
to stop glowing one night (boy, was I freaking out too :-) so I'd say
a minimum of 5 minutes of idle time.  You can extrapolate between
these two extreme conditions.

I couldn't stand to wait in the car for the required time, so I bought
an HKS turbo timer.  Great little box with five buttons: 30 [seconds],
1, 3 and 5 minutes and "off".  Press the button you want for idle time
after ignition shutoff and presto!  The car keeps running even though
you're walking away.  They have a safety brake features that kills the
engine within two seconds if the safety brake is not engaged.

It's also great fun when people ask, "Did you leave your car on?"
(Note the "appliance" type of operating reference.)  I've come up with
replies than range from, "Oh, the battery (or starter) is dead so I
just let it run all day," to "It's more efficient to let it idle than
to turn it off."  Usually people just say, "Oh..." and continue on
their way...

Anyway, the turbo timers are about $125 and easy to install if you
have the wiring diagrams for your ignition system.  Easy-install
kits are available for some models.

TIP2: run Mobil 1 and change it about every three thousand miles.
Mobil 1 won't coke anywhere as easily as conventional oils in the
turbo, will gradually clean sludge out of your engine (my valvetrain
is much cleaner with Mobil 1 than Penzoil/Castrol used at the same
change interval: 3,300 miles (3 times per 10K miles)), has less
friction, great viscosity range, and is about $3.50 a quart.  Hell, I
even run 5W-30 in my lawnmower (all 2/3's of a quart).

I justify Mobil 1 by telling myself Jiffy Lube would charge about as
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.  :-)

TIP3:  if your turbo does go bad and you replace (or rebuild) it,
replace those oil lines too!  Often turbos go bad because of lack of
sufficient oil; sometimes it's restricted right at the turbo fitting.

My experience is drawn mostly from a pair of '87 Honda CRX Si's that
have been outfitted (and highly modified since) with HKS turbo kits.
The turbo is a AirResearch (sp?) T2 water cooled type.  More details,
including references, are available on request.
__________________

"We do not react, although we think at speeds    Chris Schanzle
 that seem to be reactions.  We revel in that    [e-mail]
 ability.  And the volume of our thought
 processes in those moments explain why a brief
 incident can require so much description.  If you cannot do this, you are
 no race driver.  That's why being one is such a big deal."