[Jackpine]

I'll chime in all right. Yep, it's hot in Arizona and I've kept the stock thermostat in, basically, for the reason Sburn gave you.

The whole point of the thermostat is somewhat misunderstood by many. First, it's there to warm your engine up quickly. Coolant won't circulate until it opens, so the block get warm fast. Then, when it DOES open, it does so near the optimum operating temperature of the engine. It opens part way at first, and becomes fully open a bit later. At this point, your engine's temperature is maintained by the balanced design of radiator area, fan velocity, coolant quantity and chemical properties. The engine will not really overheat if things are designed and maintained properly unless you run the engine at very high rpms, pulling a heavy load, and at a slow driving speed in very hot ambient temperatures. If you get into colder conditions, the thermostat will begin to close to keep the engine in the ideal heat range.

If you use a colder thermostat, it will open too soon. With a good cooling system, the engine will stay between 180 and 200 for too long. However, if it was going to overheat with the stock thermostat, it will do so with the colder one too (and probably just about as fast, once the coolant starts circulating). Once the thermostat is open, regardless of the temperature it opens, the rest of the cooling system controls temperature increase.

So really, what the thermostat is for is to put a "floor" on the operating temperature range. You want that floor to be around 195.

When I'm towing, I monitor the CHT (since there's no coolant temperature sensor). About the highest I've seen it get is 216 (under very heavy load). The failsafe CHT is 258, so you can see there's a lot of headroom. I should add that the 50-50 mix of antifreeze/water and the fact that your cooling system is pressurized, raises the boiling point of the coolant to somewhere close to the failsafe temperature too.

- Jack