At 12:47 AM 6/4/2011, Rocket J Squirrel wrote:
>Winter or summer, the thermostat will pretty much bring the engine up to
>80C (or 87C, depending its setpoint), yes?
>
>Seems to me that regardless of whether there is snow on the ground or
>molten asphalt, the engine will just get hotter and hotter until the
>thermostat cracks open, and it's the thermostat's job to modulate the
>amount of water going to the rad to keep the coolant -- at the point
>where the thermostat senses it -- at the setpoint temperature.

It isn't a setpoint so much as a balance - the 'stat opens over a
range of about 35F, so while you're in the control range of the
system the temperature at the thermostat will be between ~185F and
220F.  The fan will be running at high speed ~ten degrees before the
thermostat is fully open.

Once you hit 220F you're now dependent on the ability of the
radiator/fan to reject heat so you've got a new balance between 220F
and the system boiling point, which should be close to 260F if you're
running 50/50 ethylene-glycol coolant and your pressure cap is in
good shape (265F at 15 psi, I think our caps are 13 psi).  It's still
a balance because heat transfer goes up with the square of
temperature difference.*

Somewhere at the top of that range you'll notice a little red light
blinking on your temp gauge, and VW specifically says (owners manual,
page 37 in the '89 book) that elevated gauge temps are "not serious"
until the light starts blinking.  According to "some antifreeze mfr"
260F is a common setpoint for engine overheat alarms.

*this margin between 220F and boiling point decreases at altitude,
since it's absolute pressure and not gauge pressure that really matters...

"Open-deck" engines like the Subaru (according to the Subaruvanagon
list folks) are not so tolerant of temperature excursions, so the
Subaru cooling system is regulated a good deal more tightly than the
WBX  in normal operation.  It still has the same margins at the
extremes, but the thermostat system works more crisply.

Yrs,
d