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It is not unusual with any 10-year old stock radiator for the temperature to rise during summer. If this occurs with your Datsun, don't panic. The engine can run all day close to the "H" mark. Just don't let it go over that.
Normal Temperature
If this worries you, fit a slightly larger radiator, like the B210 radiator.
Accoring to Owner's Manual: Water Temperature Gauge, this is the normal range.
TIP: Use the stock engine-drive fan. Electric fans are notorious for failing to cool the radiator in stop/go traffic. Of course an electric fan can work well, but many don't.
What is the correct temperature the engine should run at? Primarily, this depends on the thermostat. The engine can run fine at a variety of temperatures. You can get different temp thermostats, and the coolant should stay steady at that temperature.
Datsun factory options were:
* THERMOSTAT 170 F (75.5c optional, for tropical use) * THERMOSTAT 180 F (82 c, for standard use) * THERMOSTAT 190 C (88 c, optional for winter)
These three thermostats were mainly because of the heater. 170 for hot climes so the room heater won't put out much heat, 190 for cold climes so the heater puts out more heat.
180 is recommended for all street cars, it will keep the oil at optimum temperature (oil runs about 20 degrees hotter than the coolant). A guy at the oil company told me 180 is ideal for the oil working the best. He went through training for Mobile I think it was and they were formulating oil for 200 degrees, which works out to be 20 degrees warmer than the coolant for most engines. Maybe it's changed now but it won't be colder as engine now run even hottter than ever. If anyone has documenation for this please add it here.
160-degree is favored by some for drag racing as they tune the carbs for cold dense air, or for use with low octane fuel. But EFI computers don't run well at cold temps nor does the oil, and cold temps mean more engine wear. Cold temps are not recommended.
But won't I make more HP with a colder thermostat? Not if your carburetor jetting is stock -- it expects warmer less dense air. And when you floor the accelerator pedal, the air cleaner hot air duct opens up letting only cooler air in, and the stock Hitachi compensates via the power valve. Those clever Datsun engineers had it covered.
Many cars use a 205 degree along 14 lb. radiator cap to prevent boiling. A radiator is more efficient at higher temperatures, because the temperature differential to the air will be greater, thus shedding heat faster. In turn airflow can be reduced which increases aerodynamics.
After the thermostat, the radiator is the next factor. If your temp. stays steady except on hot summer days (which is normal), you can use a bit larger replacement radiator and it will stay steady even during temperature peaks. Use either a new unit, rebuilt unit, or a good used unit from a B210 or B310, which are larger than the 1200 radiator.