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I haven't solved it yet but I've been implementing many of the suggestions. My next move is to the try a 7-blade fan, and maybe a shorter clutch to optimize fan position.
If that fails to make an improvement, I'll try a new thermostat or a gutted thermostat. I put that low on the list of suspects since I thought a poorly flowing thermostat would still cause high temps at highway speed, which is the opposite of my problem. Do I have that wrong/could it be partially opening?
I did have had a thermostat failure before - my high flow NAPA thermostat failed in the closed position.
To a previous question about the ignition - the motor has a GM HEI setup, not sure what year.
WearnWrals:
Have you solved your overheating issue? Many great suggestions here to dial that system in. I also battled overheating issues under various operating conditions in a 455 powered Vista Cruiser. One cheap and relatively easy test is to completely remove the thermostat and run without it for a while. Your summer warm up time in Arizona heat will hardly be extended but you can eliminate the thermostat as a cause if your problems persist.
If it turns out the thermostat was the problem, I suggest replacing it with the conventional OEM style thermostat shown in the top sketch below. The so called "high flow" design below it was just the opposite in my engine. The bottom balanced sleeve design turned out to be incompatible with the thick casting below the thermostat mounting flange on my aluminum intake, which made it actually flow less coolant than the OEM style design.
I actually had the same thing with my 69 442, I put in a cheapo $4 poppet Autozone one like the one above and it cooled better then balanced sleeve one that was in it. I heard one theory on this is that it keeps more pressure in the block and heads and less bubbles form on the cast iron, ie. more coolant surface contact. This worked for me although it does NOT seem to be correct in most cases ie. more coolant flow is usually better..
Also I think the original cast iron empellers work better and don't cavitate like the pressed fan blade empellers depending on the front timing cover as a backing..
Your timing is at least 5 degrees retarded, it may be more like 12 degrees retarded with the most common Olds advance curve. Bump up the timing! I noticed no difference in thermostats other than that Mr Gasket balanced sleeve that stuck shut at speed. The closed impeller doesn't necessarily move more coolant, that is pulley ratio related but it should have less cavitation. Again, no noticeable difference but going from aftermarket underdrive pulleys to factory A/C overdrive pulleys, quite a few degrees less. Same with the 5 vs 7 blade fan, noticeable drop. Going to Dodge Stratus electric fans does about the same as the 7 blade clutch fan. Bump timing, go 7 blade, fill in the rubber strips and bump the timing!
