[mr0077]

Mike, this is an involved subject, but a "simple" (hopefully accurate) synopsis is this:
A given cooling system can only cool to a certain temperature under a given set of conditions. If the system could only cool to 195 degrees on a given day/speed, etc., putting in a thermostat with a lower rating would only open the t-stat at the rated temperature, allowing full flow, and take longer to reach 195 degrees. If your system would cool to 165 under those same conditions and you put in a 195, you could watch the temp gage rise to 195, the tstat would open, the temp would try to drop to 165, the tstat would close, and the temperature would start rising again to 195, then keep rising and falling as the tstat opens and closes. The tstat won't lower the running temperature, it only helps warm the motor up to its opening temperature quicker, for efficiency and longevity. The system will then run or try to run at whatever its steady-state capacity will allow for the given conditions.

Running a tstat is strongly recommended by most...if you run without one, you should put a restrictor in its place, so the block is running under pressure flow, and to eliminate steam pockets and hot spots throughout the block. I've heard of some instances where taking the tstat out actually promoted boilover due to steam pockets and hot spots.