Outside temperatures here are between 30-50 degrees these days (early December in New England), but when I get into the car the engine is anywhere between 70-90 degrees thanks to my newly installed "circulating type" engine heater. I'd call the project a success, though I'm still working to increase the system's efficiency so I can achieve higher temperatures before startup.
This thread in the "Success Stories" subforum gave some information on my recent block heater project. I promised to write it up fully sooner or later so here goes. I don't expect to get it all in here in one sitting so please be patient.
Background - what didn't work for me:
I've been wanting a block heater for this car for a few years but needed some determination to get past several hurdles that blocked my first attempts.
Heaters that install into a freeze plug opening are probably the most efficient of all. But I'm not that much of a mechanic and don't really want to open up things like exhaust gaskets, not to mention freeze plugs that have been in place for 15 years/270,000 miles. So that route was out for me.
Another solution that might have worked for an engine that had a recent overhaul, aka clean threads and surfaces all around: I found that ZeroStart makes a very nice heater that installs in the D16 series engine block drain, located just to the driver's side of the oil filter mount location. Part number 3100006. Perfect, I thought. Ordered and found my socket to fit the drain plug hex. As I recall it took a 17 mm. socket. However the drain plug's hex had only about 1/4" of "reach" into the open socket, not much to grip on. And even with a breaker bar, tapping on it with a hammer after spraying PB Blaster in the general area, I could not budge the plug. After 15 years and 270,000 miles the plug was in solidly.
What I did use:
I decided on a circulating type heater, one that uses a small tank with a heater element inside. These heaters circulate coolant by convection: heated antifreeze rises in a hose, causing a suction effect in the heater's input hose. This puts requirements on the hose layout: the output hose needs a consistent rise leading to where it enters the head. Any situation where the hose forms an upside down "U" will likely create an air pocket at the top of the bend, which defeats the siphoning action.
I used ZeroStart circulating type heater #330-8001.
It is rated 750 watts. Having seen it in action I think I'd rather be using the next more powerful model, #330-8002, at 1000 watts.
This series has a built in thermostat cutoff at 245ºF, and a backflow prevention valve. Depending on your chosen plumbing path, you could create a "short circuit" that would bypass or disable the normal intended coolant flow in your car. The backflow preventer ensures that coolant will only flow through the heater in one direction.
The drawing below shows two red "Tees" where I connected to the existing hose layout. The tee into the lower radiator hose is model 28113 from Kats Heaters. The tee into the heater hose was from an old-school NAPA parts store, made by Gates (the hose people).
Here's the layout I used:
I'll write more later...