Tom: I'm not sure if it's an Al or Fe block, but I suspect it's Fe.
More data!
This time I plugged in both heaters at once (1050w @ 120v, according to the kill-a-watt). And... blew the breaker! (I was also running a hair dryer at the time inside the ForkenSwift to defrost the windshield
).
So I unplugged the hair dryer, and this is what I got:
(EDIT : note the ambient temp was highest for this test)
I find the plateau @ 172F interesting. I guess the engine/cooling system had reached equilibrium with energy in & heat loss. My other thought was perhaps the local temp @ the thermostat caused it to open and permit cold coolant in.