Block heater

   / Block heater #2  
Internal or external?
 
   / Block heater #3  
My preference is freeze plug type heaters.
 
   / Block heater #4  
Years ago, I had a 68 Plymouth which had an external propane engine heater. As I recall it was built by Phillips Industries in Minneapolis.
There was a small propane tank in the trunk, a heating tank inserted into the lower radiator hose, which was fired by a glow plug. No extension cords needed, it used the 12 volts from the battery to fire off using a glow plug, then would run with no electric usage. The on off controls were also under the hood.
 
   / Block heater #5  
Most efficient is frost plug block heater since all the heat goes directly into the block. Also usually lowest wattage so uses the least electricity
 

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