Testing OEM block heater: how hot after how much time plugged in?

[MetroMPG]

Mirage #2, The Even Fancier Metro, came with the OEM cartridge style block heater.

I'll have to confirm this, but I think it's a 400 watt unit. (EDIT: confirmed below by klroger - it's 400w.)

I was curious how fast it warmed things up, so plugged it in and checked the coolant temp every 10 minutes (via a ScanGauge).

Elapsed time (minutes)	Coolant temp (F / C)	Temp change (F) from previous
0	40 / 4 (ambient, cold soak)	0
10	42 / 6	2
20	49 / 9	7
30	57 / 14	8
40	62 / 17	5
50	67 / 19	5
60	71 / 22	4
70	75 / 24	4
80	78 / 26	3
90	80 / 27	2
100	82 / 28	2
100.5*	100 / 38	18

* That last reading was after starting the engine and letting it idle for 30 seconds. I figured the water jacket / block wouldn't be heated uniformly by the block heater, so circulating the coolant past the temp sensor would be a good idea to get a final reading. (I don't think that 30 seconds of idling actually added 18 degrees F worth of heat to the coolant).

It looks like around the hour-and-a-half mark the amount of heat being added has slowed down a lot. And at some point it's going to reach equilibrium, losing heat as fast as it's added. Though we weren't quite there yet.

Edit... graph!