Tracy,
<... There is less mass being accelerated (energy) at lower flow rates.  In the extreme example  (zero flow) the same water in the pump housing is being spun around at a constant velocity which requires no energy...>
I'm not at all sure, but an explanation for larger absorbsion of power even though thermostat is closed is that the water trapped in the pump may not be behaving as you describe.  What if when the EGWP is being over driven (too much rpm for the plumbing to pass at "optimum to cool the engine" head pressure) any water that can't pass through the pump into the system recirculates in the pump.  Suppose it just squirts from the high side of the impeller to the low side and gets pumped around in circles.  Since the clearance between the impeller and the housing isn't that large, you would be pumping a lot of water through a small orifice - a phenomenon that absorbs a lot of power.  I can't see the impeller moving water relatively efficiently when all the valves are open and suddenly "spinning free" when they close.  We have a pretty good idea what's happening at a very local level in the pump when the thermostat is open.  I am much less sure what, exactly, it's doing when the valve is closed.

One could experiment.  One could take out the thermostat and drive the car water pump with an electric motor measuring amps to the motor.  Gradually close a valve in the system to replicate the thermostat and see what happens.  A largish DC motor would enable us to check things at various pump speeds and various restrictions.  I would hazard a guess that it takes as nearly as much power to pump water against a closed thermostat as against a partially open one.

Just a theory ... Jim S.

PS  Exactly when is the Shady Bend Fly In?  Does it start of Friday?  I'm hoping it's the weekend of the 24th.  I will probably be available then.
 

Tracy Crook wrote:

  Assumeing a constant pump speed, when the thermostat closes and head pressure goes up,  power required to drive the waterpump does NOT go up.  It actually goes DOWN.  Reason:  There is less mass being accelerated (energy) at lower flow rates.  In the extream example  (zero flow) the same water in the pump housing is being spun around at a constant velocity which requires no energy.  Of course there are losses in the pump so the energy consumed is not zero. This argument applies ONLY to centrifugal pumps (of which automotive waterpumps are an example) and not positive displacement types (like oil pumps). OK, somebody shoot this argument down and educate me. Tracy