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From personal experience on my 1.5L aluminum blocked 94 Civic VX:
After 2 hours on time from a 400W freeze plug style heater, the heater exterior casing was showing 120°F, the exterior of the aluminum block was showing 92°F within a 2" radius from the block heater. The further away from the frost plug heater the cooler it got, down to 75°F at the opposite end of the block. Air temperature indicator showed 65°F under the hood of the car just inside the WAI opening (approx. 12" from the frost plug heater). Temp gauge that I have that is in the head of the motor only reads down to 100°F, so I'm guestimating that the acual water temp inside the block was closer to 80-85°F.
The thing about the setup from Honda is that the frost plug for the heater is not simply a threaded open hole in the block. It actually only goes in an inch and then is encapsulated with a vent hole on the top of the capsule and the bottom. Think of a milk cap recessed in the block with a hole at the 12 and 6 o'clock positions. This leads me to believe that the frost plug heater heats the coolant more indirectly by warming that aluminum block and allowing the warm block to warm the coolant rather than heating the coolant directly. I believe Honda has done that to reduce thermal shock of the coolant and temperature differential in the coolant itself, since the frost plug location is not in the center of the block.
I do know that on the Ford and Chevy V8 motors, once you knocked out the frost plug, it was a direct shot (unrestricted) into the water jacket.
So in your case, the frost plug may be a direct port into the water jacket allowing for faster convectional circulation of the coolant. Allowing for a shorter (2 hr) warmup time for your block.
Metro (Dan), Do you remember if your frost plug allowed direct access (unrestricted) into the water jacket or was it something similar to what I described in my Honda motor?
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