John has it exactly right. I might add that, in addition to fully closed or
fully open, the thermostat has an intermediate range and this is where it
performs its thermostat control function. Under the conditions Scott
describes, the thermostat is open only a crack and just a trickle of water
is flowing, but this is enough to maintain the design point temperature of
180 degrees. As the ambient temperature rises or the engine load increases,
the thermostat opens up more to increase the water flow and maintain its
design temperature. The transition from closed to fully open happens over
about 5 degrees. BTW, I would bet money that the Scott's thermostat is right
and the Smith gauge is reading 10 degrees high.
Regarding Nathan's question, it's my conclusion from reading the Ford wear
versus temperature data that 180 degrees is perfectly fine, and probably
better than 190-200 degrees for a Hi-Po or modified engine that's going to
be run hard. It brings down oil temperature, intake manifold temperature,
and provides some margin of safety.
My $0.02 - Bob
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