Posted for Jim Sower <canarder@starband.net>:
<... do you plug this hole or not ...>
Just off the top of my head, I would say yes.  EWP doesn't need a thermostat
since the motor doesn't start pumping until the water gets hot.  No kind of
bypass is needed except to vent air from any high spots in the water jacket.

As for relative efficiency, I have a big BIG problem with the power that some
folks claimed the engine driven water pump (EDWP) absorbed, i.e. 6000 watts.
Like 8 hp.
Like over FOUR HUNDRED amps (Ed correctly pointed out that it was the equivalent
of starter current draw on a BIG engine.
I tried to think this through:
Could I start my engine with just a water pump belt connecting the starter with
the crank??  Indefinitely??
Where's a belt that could drive a 6000 watt (450 amp) alternator?
How long would my pisant 1/2" water pump belt last, driving that alternator?
That 6000 watts figure sounds more preposterous the more I think about it.

That said, I have no doubt that the EWP is enormously more efficient overall
than the EDWPs that we're all used to.  Nor do I doubt that it's efficient
enough to operate off of a normal sized alternator that most of us use.

Just that somebody's overstating their case.  Like by a @($^$% ORDER OF
MAGNITUDE.  Let's get real, girls.

Just tryin' to do the math  .... :o))  Jim S.