I guess the smartarse response to that is "so what is this guy actually a specialist in?"

The principle behind this is simple enough, the hottest water wants to be at the top of the radiator, as it cools it will fall to the bottom. Indeed if the rad is tall enough, and the cooling system large enough, the cooling system will thermo syphon, as it does on many a vintage car, without using a waterpump.

The object of the exercise is not to keep the fan from coming on, but to keep the water at a teperature where it cools the engine to an acceptable degree.

It still seems sensible to me to react to the highest temperature, that of the water coming out of the head. It seems to me that if your fan reacts to the water temperature after it has cooled, potentially you could have excessive temperatures in the head.

At the end of the day it's your engine and your money. But I know what I'd do based on building many an historic rally car and sometimes learning the hard and expensive way.

Regards


Pete