What is the maximum air temperature in which I should consider using a radiator block? Will it overheat my engine on 100-degree days? How can I determine if it is overheating my engine?
The first part depends on too many factors to give any answer. As for the second part: Measure the temperature of the water LEAVING the radiator (tape a thermocouple to the outlet hose, tape some fiberglass insulation over the thermocouple to keep air from blowing on it. Have the tip end of the thermocouple under an insulated area perhaps 2 hose diameters in lenght ). Measure ambient air temperature (outside the car). You will need a thermocouple meter with two channels, like the Fluke 52 or the Fluke 54 if you want datalogger capability so you don't need to scribble down numbers. Block up the radiator some random amount. Drive the car in a variety of conditions (high speed especially) and monitor the engine temperature guage. Change the amount of radiator block and repeat until the engine is as hot as you are comfortable with UNDER ALL DRIVING CONDITIONS.
At this point, you have data that says: When the outside temp is (A) and the radiator outlet temp is (B) then my engine is as hot as I dare.
Now take the difference between 100F and (A), call this number (diff). Remove radiator blockage until (B) is reduced by the amount ((diff)+(safetyfactor)) where (safetyfactor) is your comfort level with the accuracy of your measurements. Also, make a part of your grill block easily removable in case of unexpected problems.
I should add, that long ago I used to do radiator blocks on a 1975 toyota corrola in the wintertime in Boston, MA. At that time I was not taking any data on what I was doing, other than "I am freezing in this car and I need it warmer".
When spring rolled around I was too stupid to remove the radiator block. The thermostat in the car tried to open so much that it fractured that little sheetmetal bridge the little pusher rests on. The thermostat then snapped closed and stayed that way all the time. I removed the thermostat entirely and threw it away. I was too late, the overheating caused the head gasket to blow at 10 pm on the mass pike in framingham MA. Fortunately in those days the mass pike people let me park the car there at the toll both overnight. And so the engine rebuild project began...
carrying an engine block upstairs to a second floor aparrtment...
