There is nothing wrong with looking for the balance between performance and efficiency.
The Road and Track take (you can find similar non Internet forum articles everywhere).
Engine-management computers rely on the as-designed coolant temperature (typically 193 degrees F) to better atomize fuel, maintain heat-exchanger efficiency, provide lower emissions, optimize fuel economy and other factors, so running the engine colder than designed can have multiple ill effects. These are the sort of effects that are not immediately apparent, so such issues are more long-term durability concerns.
Tuners often fit 180-degree F thermostats to give hot-rodded engines a larger cooling margin for when they’re making tons of heat (horsepower) at full throttle. This is a bona fide need on an engine that has sprouted a supercharger, but simply fitting a cooler thermostat to a stock engine will not lower the intake air temperature enough to matter and will not support a meaningful timing increase.
Cooler than 180-degree thermostats are not recommended on modern cars. Most domestic automakers’ engine-management systems go into open loop at 168 degrees F, meaning the engine is operating in its warm-up mode with perhaps an overly rich mixture and un-optimized ignition timing. Eventually the computer will consider such long warm-up times abnormal and could trip the Check Engine Light.
Furthermore, higher coolant temperatures promote engine efficiency, especially for fuel economy and emissions, and reduce cylinder wear (a cold block wears faster than a warm block), and the engine internal dimensions have been set for the designed operating temperature. If you’re really beating on your car in the summer and figure it’s overheating, then a 180-degree thermostat could be the answer to return your coolant temps to as-designed levels, but anything colder is counterproductive.
another related thought. If you are running an IC or going to, most rely on the radiator puller fan(s) to cool the FMHE. Typical fans settings with a 180* thermostat is 176 -182* whereas the stock fans settings with the 195* is 208 - 212*.
