| Subject: | Re: Watching Oil Temp (long, as usual) |
| From: | Jake de Villiers |
| Date: | Wed, 20 Jul 2011 15:38:19 -0700 |
When I reached out on a 98 degree F. afternoon and shot the black asphalt roadway with my IR temp gun it read around 140 degrees - if the air going past your sump is 98 and the roadway is 140, that's still a useful differential from your 220 degree oil, is it not? My point about the RPM is that a relaxed climb at 3200 won't be building heat in the engine like 3800 RPM will - that's all. I found that we could climb longer and cooler at 25 MPH in second gear in the '84 - it took longer but the van was much happier. On Wed, Jul 20, 2011 at 1:47 PM, Rocket J Squirrel < camping.elliott@gmail.com> wrote: > 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 > > > > -- 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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