[mr0077]

Mike, you're right, in that the 195 system will try to keep a system no cooler than 195...if it wants to run at 175, it will cycle near 195; if you put a 160 tstat in the same system, same conditions, the 160 tstat would open at 160, and the system would keep slowly warming up and stabilize at 175, the temp it wants to run at under the assumed conditions. The 160 is trying to keep it at no less than 160, the 195 is trying to keep it at no less than 195...and a cooler tstat can affect the running conditions, but only if it replaces a tstat rated at a higher temp than the motor wants to run at. If the motor wanted to run at 140, both the 160 and the 195 would keep the motor running at a higher temp than the temperature it wanted to run at.

On your other question, I'm not sure if there are different i.d. openings; I know for sure there are some tstats with a larger o.d. than others...had to get a BIG'un for my FE. I'm guessing there would be some slightly higher running pressure in the block with a smaller tstat opening, but not sure if it would make a great difference in the running temp.

As an interesting aside, (for the colder temps in the winter), BMW always had the standard tstat on the motor outlet, but a few years ago Car and Driver reported that they started putting another tstat on the radiator outlet on cars sold in Germany. Seems that the coolant got very chilled at higher speeds (autobahn, etc.) and cycled into the hot block, causing a few cracked blocks. Not sure how putting the extra tstat at that point would allow the radiator water to ever warm up, though!