is there a reason as too why the thermostat of the newer cars are rated at 170deg as compared to the older subarus rated at 190-195deg.

Because it sits at the radiator outlet.

Let me take a crack at this.

 

This is specific to subaru boxer only. Most modern cars run higher at 190-195.

 

The subaru thermostat is located at the radiator outlet, but the spring is located on the hot engine side.

 

Actually in older subaru cars its 180-185 with 190 being the upper end. Older cars took longer to warm up then newer cars for several reasons. The cars run leaner, and therefor hotter. The cooling system pressures have steadily crept over over the years, along with what is considered "hot" and "over heating".

The thermostate is still a mechanical item, and is slow to open. If you ever watched one in a pot of boiling water its a slow process. By having the thermostat open at a slightly lower temperiture, it keeps a more even temperiture in the engine. It also avoids the engine getting too hot. Engine Temps that swing widely and unevenly will cause the engine to run dirty, and fail emissions. One of the keys to emissions is an even heating or cooling of the engine, as much as possible.

By openeing the thermostat sooner reduces any spike in water temp, and therefor keeps an even temp in the engine.

Blu, my 97 OBW in 5 degree (F) weather chowed a coolant temp of 173 degrees with the heat full on. The coolant was probably 175 at the thermostat.

 

Other engines from other mfg's run hotter.

 

The name of the game is meeting emissions.

 

nipper

When thermostat is fully closed, the temperature it senses is indeed close to the engine temperature, even if it sits at the rad. outlet. But an important consideration is what happens when the engine is fully warmed up. Then the thermostat remains partially open at all times, increasing or decreasing the flow to maintain the designed operating temperature. Assuming the target temperature is fixed, a thremostat located at the rad. outlet should have a lower opening temperature than one located at the inlet.