The MGA With An Attitude
What Is "NORMAL" Operating Temperature? - CO-101A

A common question: "What is considered normal operating temperature for an MG"?

An engine will never run at a precisely constant temperature. The thermostat is used to enforce a minimum running temperature for efficient operation. If the radiator is marginally on the small side (very common), then in warm weather the operating temperature will run higher up to a point of natural balance between heat generation and heat expulsion. MGs are usually like this, running at notably higher temperature in hot weather. There is good reason for this character.

The only way to keep maximum coolant temperature always under 190dF would be to install a much larger radiator so it is nearly always over cooled. For this the radiator needs to have enough thermal transfer capacity to expel all excess heat in the worst conditions. Consider that you might use a radiator large enough to keep operating temperature at 190 with ambient temperature of 120 while towing a trailer up a mountain. Another way to look at this is running the engine constantly at full throttle in the hottest weather, and using a radiator large enough to handle this maximum heat flow with the smallest difference in temperature between coolant and ambient air. Coolant at 190d and ambient at 120d provides only 70d differential to drive heat from the radiator into the air.

When ambient temperature is 75d and coolant is 190d, temperature differential driving heat out of the radiator into the air is 115d. For this you can use a radiator 40% smaller than for conditions described in the prior paragraph. MGs are sports cars built to be as light as possible (within economic reason). The radiators are intentionally small to reduce weight (and production cost). For a sports car they may have been pushing this concept to the limit, resulting in coolant temperatures higher than 190 on any moderately warm day. If coolant temperature runs 195d with it's 75d ambient (120d delta), then when ambient rises to 100d you might expect coolant to rise to 220d (still 120d delta). Pulling a long uphill grade generates more heat and may drive coolant temperature higher in order to expel more heat from the same radiator. This is nothing to cause worry.

When operating temperature rises above 190 with the full open thermostat, it is then running under condition of equilibrium between heat generation and heat expulsion. Running with heavier throttle at faster speed, going up a long hill, or pulling a heavy load (trailer for instance), the engine will generate more heat and coolant temperature will run higher. It is very common for an MG to run up to 220dF in hot weather (and sometimes higher).

So the answer to the question is, normal operating temperature is a range with minimum determined by the thermostat. Maximum it determined by work load of engine, size of radiator, and ambient temperature (and requiring some minimal air flow to keep it working). If you want hard numbers, then use a 180d thermostat and expect 190d on a moderately warm day. For hot weather driving don't be surprised if it runs up to 220d. This is why the car has a pressure cap on the radiator. Plain water boils at 212dF at sea level (lower temperature at higher altitude). Boiling temperature increases 3.5dF with each 1-psi increase of pressure, so the 7-psi pressure cap raises boiling temperature of water to about 236dF (beyond end of temperature gauge scale). Using 50/50 glycol and water for coolant pushes boiling temperature about 20dF higher, so it can run higher than 250dF before it would boil.

I would recommend a thermostat rating of 180dF. This will guarantee the engine will run at 180dF minimum in very cold weather (after it warms up), so it can run efficiently (without excessive constant choke) and the heater will work. In moderate ambient temperatures you might expect it to run at 185-190 minimum, as there is as much as 10 degrees range required for full opening of the thermostat. Occasionally I snap a picture of my temperature gauge pointing at 85-90 psi in the oil pressure range (about 250dF), but my engine has never suffered noticeably from overheating. Under hot running conditions the first problem to arise is more likely to be boiling fuel in the carburetors (when standing still or creeping) causing it to run lean and stumble. Immediate cure for that is to pull out the choke.

Now stop worrying and go drive it.

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