Cooling System Model Shift

[Scott Fisher <sefisher@xxxxxxxxx>]

So while everyone here has been debating the woes of yanking the
thermostat from their cars, I've been learning about a very different
cooling system -- the one in my daily-driver '83 Audi GT.  Not to get
into too much detail about non-Alfas on this list, but I need to mention
a few unique things about it.

Most unique, at least in my experience, is that the thermostat is
connected to the *bottom* hose -- not to the *top* hose.

In all other cars I've ever played with, the thermostat sits on top of
the cylinder head (or in the intake manifold between the heads, in the
case of Detroit V8s).  This seems to make sense -- it's high in the
engine so hot water will rise, it's connected to the head where temps
are the highest, and it basically controls flow out of the engine and
into the radiator.  We're all familiar with this, I presume.

Not so the Audi.  In this car, the thermostat is connected to the water
pump inlet, meaning that heated coolant inside the engine constantly
circulates on one side of it, and cooled coolant in the radiator
circulates on the other side.  There's no restriction from the water
outlet (high in the block, NOT in the head) to the top of the radiator,
but there is a vent hose going from the top of the radiator to the
coolant expansion tank, where there's also a drain hose going from the
bottom of the expansion tank to the bottom of the radiator.  Filling the
radiator (such as after draining) is accomplished by filling the
expansion tank.  Finishing it off, the electrical switch for the fan
relay is mounted at the bottom of the radiator, set to come on at about
190 degrees F, roughly the same temperature as the thermostat.

This seems so backwards from the cooling systems I've worked on, such as
the '67 Alfa's, and also cooling systems in the various British sports
cars I've owned.  It's almost as if the entire purpose of the system was
not to *cool* the engine, but rather *to make sure the engine ran at a
predetermined, high temperature* -- as though its point was not to
remove heat, but rather to keep in *just enough* heat.

Looked at in this way, a "cooling system" takes on a different
coloration.  If the intention of a cooling system is not to drop the
operating temperature as much as possible, but rather to keep the
operating temperature within a narrow band (and a band for which the
lower range was fairly high, say 190-200 degrees F), then things shift. 
In such a model, the purposes of the components take on a very different
look.  And the Audi system doesn't seem quite as weird, backwards and
stupid as it did when I was looking at a cooling system from the other
perspective; it's actually a fairly clever way of keeping the engine
"hot enough."  (This is tied to emissions controls, as I recall; modern
EFI/catalyst vehicles like to be warm enough for certain kinds of
combustion, as well as catalyzation, to occur, in the interest of
driveability and cleanliness.)

Just something to think about as I wait for the funds to build up for me
to put the hot-rod motor in my Alfa...

- --Scott Fisher

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