Date:         Sat, 3 May 1997 14:52:55 -0700 (PDT)
Sender:       Vanagon Mailing List <vanagon@vanagon.com>
From:         mcleans@earthlink.net (Ed McLean)
Subject:      

In reference to my posting on "Overheating? Try these ideas. (long)" on
5/3/97, Victor Wong asked:

"Why do you think the engine runs cooler with lower oil level?"

I believe the engine runs cooler with lower oil level because of the
reduced amount of oil subjected to the "windmilling" effect of oil on the
outside of the rotating and reciprocating parts of the engine. The sheering
effects of moving parts in a viscous liquid or heavy mist causes heating of
that liquid.  Racing engines (both wet and dry sump engines) are fitted
with a windage tray to isolate the crankshaft from oil in the crankcase to
prevent this effect.  Also, in most automotive engines there is nothing
rotating in the oil in the pan except the oil pump.  In VW boxer engines I
believe the camshaft gear is partially covered in crankcase oil.  This
pulls a tremendous amount of oil up out of the bottom of the crancase.  The
sheering effect of this gear in the oil is probably related to the depth of
emersion.  Also, the amount of oil flying around in the crankcase and up on
the crankshaft gear is probably also related to the depth of coverage on
this gear.

He also asked:
"You'd think that our little engines could use the cooling and lubricating
benefits of every last drop of oil possible."

The amount of oil in an engine's crankcase (providing there is enough to
keep the oil pump from starving and enough to fill the crankcase to the
level of the cooling fins on the outside of the pan, or crankcase in a
boxer) generally does not have any effect on the cooling and lubrication of
an engine.  This is why racing engines use a dry sump setup.  Even in the
case of a dry sump engine, a large resevoir of oil will not reduce the oil
temperature, although it might take an insignificantly longer time to reach
a specific temperature.  Now, if the cam gear is spinning in the oil, it is
heating up the oil with some of the enerqy taken from the spinning of the
rotating parts.

Now, I'm not an engineer and I've not done any exhaustive testing but I did
measure the oil and water temperature at various locations on the engine by
attaching thermocouples.  I measured the temperature at:

       1)  the oil filter (to measure the temperature after the oil cooler
and just before it enters the engine lubrication galleys;
       2)  the large coolant pipe feeding the water pump (water feeding the
oil cooler);
       3)  the small coolant pipe leaving the oil cooler (water leaving the
oil cooler; and,
       4)  I measured the oil in the crankcase using a VDO drain plug
sensor and a VDO electric gauge.

It seems that with the oil level at the top of the dipstick range, the oil
temperature directly related to engine speed once highway speeds are
reached.  When the oil level is near the bottom of the operating range on
the dipstick, there is less correlation to engine speed and the temperature
is much lower.  It may be that the capacity of the oil cooler is able to
handle the thermal load at the lower oil level.  I did this experiment last
summer but if anyone is interested I will try to repeat the measurments
this summer to get some more accurate data.

Anyway, this is just what I think.
Does anyone have any other ideas?

Ed McLean