In Feb./03 I did a test to determin the time needed to warm a cold engine from an engine block temperature of minus 15 F (ambient was
-22F) to the +32F temp. suggested (by engineers and environmentalists) as a minimum temperature for engine starting. The lower the temp at start-up the greater the engine wear and pollution. The car was a Ford Crown Victoria w/4.6L engine, the engine heater was an "in-line" type made by Pinedale Products of Chalk River ON. It is rated at 600 watts and circulates coolant at about 1 gal./min.
When the heater was plugged in the ambient temp was -22 F, engine temp (sensed by a thermistor mounted near the thermostat housing) was -15F. After 30 minutes, eng. temp was +14 " 60 " " " " +34F " 90 " " " " +50F " 120 " " " " +57F
This lack of linearity can be explained by ambient temp and supply voltage fluctuations but mostly due to the Stefan-Boltzmannn law re heat loss. A standard block heater(mounted in the water jacket) will not deliver similar results because of the time required to raise the coolant temperature enough to create convection witin the block.