Depends on which book you're referring to. You want to run the engine as hot as is practicable. Running it too cool makes it wear quicker, make less power, and burn more fuel. Peter Burgess (MGB tuning guru) in his book "How to Power Tune the MGB 4-Cylinder Engine" says "The ideal 'stat is 88 degrees Celcius (190F) ." The late, great Smokey Yunick says:
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" It is easy to see how overheating can be a problem, but I think some racers overlook the fact that it is possible to 'overcool' the engine. Some guys go to great lengths to keep the engine temperature down to 180 degrees. And, though the engine doesn't overheat, they don't realize that they're putting energy (heat) into the cooling system that could be used to produce power at the crankshaft.
Running the engine at 180 degrees will drop the overall horsepower by 2%-3%. For max power the cooling temp should be at least 200 degrees...."
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With a 50/50 mix of coolant and water and a 16 psi radiator cap, you won't overheat until the other side of 265ºF....