Years ago, I read an article by Smokey Eunick on engine cooling. If you have never heard of Smokey, you have missed one of the truly great innovators and personalities in racing.
Firstly, it was his belief that there is no better medium for heat transfer than water.
These are some of his takes on temperature and thermostats. Heat is transferred from engine metal to the radiator by water molecules. When a thermostat is removed, water flows faster and more violently over the transfer surfaces (engine and radiator). If the molecules move too rapidly over the heated or radiating surfaces, efficient transfer cannot take place. Consequence is that the engine may actually run hotter.
If an engine is set up to run most efficiently at, let's say, 200 degress, changing only the thermostat will be detrimental to performance or longevity of the engine. Consideration must also be given to fuel delivery/jetting and possibly timing. If the thermostat is cooler, the engine set up will be too rich and the engine will load up with carbon. If the thermostat is too hot, the mixture will be too lean and engine life decreases.
In Smokey's day, they had to take all things into consideration and do it by hand. Today, our new vehicles have computers which can compensate for small changes in temperature, elevation, humidity, air and fuel delivery, etc. However, a change in hardware can drastically alter performance. For instance, if a 205 degree thermostat is replaced with a 180 degree, the computer is fooled into thinking that the engine is not yet up to temperature. The result will be a "too fat" mixture. Some automatic transmissions, particularly overdrives, will not function as intended until the computer tells them that the engine is up to temperature.
Ain't nuttin' simple.
