Date: Wed, 20 Jul 2011 13:47:47 -0700
Reply-To: Rocket J Squirrel <camping.elliott@GMAIL.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Rocket J Squirrel <camping.elliott@GMAIL.COM>
Subject: Re: Watching Oil Temp (long, as usual)
In-Reply-To: <CACGkSd2=szM=dWUxfveBgvJZHLw9dSUL_3=1gPFysG1ijH2BsQ@mail.gmail.com>
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Drat. I /knew/ someone would question my off-the-cuff "20mph in 1st gear
statement."
I just tossed out those numbers to illustrate the fairly slow
groundspeed I find myself climbing desert grades in. Last summer it was
baking outside as I worked over several steep grades driving south on
395 from Pendleton, Ore., to John Day, Ore. -- towing my little camping
gear trailer.
I ran the engine around 3800 rpm. I posted about this when I got home,
hoping to sort out a weird issue where the engine died for a few seconds
several times up each grade, which was pretty nerve-wracking. The exact
rpm-to-speed numbers are in those posts. I don't normally track the
relationship.
3200 rpm would certainly result in a somewhat cooler engine. But that's
beside the point of my little "airstream" joke. The pavement, just
inches below the sump, must have been hot enough to bake a pizza on.
Doesn't seem to me (though maybe someone has some numbers to show
otherwise) that at 20mph (or whatever you get at 3800 rpm in first gear
in an AT Vanagon) as much air is slipping past the sump as one would
hope for in such a situation.
-- RJS
On Wed, 2011-07-20 at 13:03 -0700, Jake de Villiers wrote:
> When I stick my hand out the window at 20 mph I certainly feel the air
> going by...
>
> What kind of RPM are you using at that speed Mike? I do long steep
> bits at about 3200 RPM.
>
> On Wed, Jul 20, 2011 at 10:27 AM, Rocket J Squirrel
> <camping.elliott@gmail.com> wrote:
> "Airstream," he said.
>
> Like crawling up a desert grade in 1st gear (AT) at 20 mph in
> 100 degree F weather.
>
>
> On Wed, 2011-07-20 at 10:13 -0700, Jake de Villiers wrote:
> > There's some almost information in your post Mike.
> >
> > The oil pump moves the oil at a fixed rate whether cold or
> > hot. Just like your water pump, it increases the rate with
> > RPMs.
> >
> > The sump, whether your finned aluminium VW one or my black
> > painted steel Subaru one is hanging down in the airstream
> > for a reason. The cooling air pulls heat from the metal
> > surface, allowing the heat from the oil to take its place.
> >
> > And yeah, the volume of coolant vs the volume of oil makes
> > the temp gauge cycles very different from one another.
> >
> > Happy motoring!
> >
> >
> >
> >
> > On Wed, Jul 20, 2011 at 9:10 AM, Rocket J Squirrel
> > <camping.elliott@gmail.com> wrote:
> > On Wed, 2011-07-20 at 08:47 -0500, Dave Mcneely
> > wrote:
> >
> > > Bottom line, I think the oil has a major heat
> > source, and only if an
> > > external cooler has been added does it have a
> > real functional way to
> > > dissipate heat other than to the coolant through
> > metals that both
> > > contact and through such things as the sump. The
> > sump is like a
> > > reservoir of liquid, it cools slowly compared to
> > a radiator. The
> > > coolant has a large heat sink, and thus has a
> > temperature gradient.
> > > That gradient includes the oil.
> >
> >
> > So what we're saying is that the oil is in contact
> > with some really hot
> > bits, and because there is high thermal resistance
> > between the oil and
> > the coolant, the oil can climb to much higher
> > temperatures than the
> > coolant.
> >
> > It doesn't help matters that the hot oil drains down
> > to the sump which
> > isn't cooled by anything except the hot pavement
> > under the engine!
> >
> > Others have also explained that while the water pump
> > can move a lot of
> > coolant up to the heat sink in the front, the little
> > oil pump just chugs
> > along pushing the oil more slowly and in lower
> > volume so that even if
> > there was an external oil cooler the size of the
> > radiator, the stuff
> > ain't moving fast enough or in enough volume to cool
> > the really hot bits
> > it's in contact with like the coolant can for the
> > hot bits it is in
> > contact with.
> >
> > I think I've arrived at point where I have a better
> > understanding of why
> > the oil temp gauge shows higher peak temps during
> > high engine demand
> > than the coolant gauge shows -- which is the answer
> > to my original
> > question! Many thanks to all who have taken the time
> > to respond to my
> > inquiry!
> >
> > -- RJS
> >
> >
> >
> > --
> > Jake
> >
> > 1984 Vanagon GL 1.9 WBX - 'The Grey Van'
> > 1986 Westy Weekender/2.5 SOHC Subie - 'Dixie'
> >
> > Crescent Beach, BC
> >
> > www.thebassspa.com
> > www.crescentbeachguitar.com
> > http://subyjake.googlepages.com/mydixiedarlin%27
> >
>
>
>
> --
> Jake
>
> 1984 Vanagon GL 1.9 WBX - 'The Grey Van'
> 1986 Westy Weekender/2.5 SOHC Subie - 'Dixie'
>
> Crescent Beach, BC
>
> www.thebassspa.com
> www.crescentbeachguitar.com
> http://subyjake.googlepages.com/mydixiedarlin%27
>
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