Classic/Antique Car Repair: V-8s, thermostats, and gas mileage, gas mileage, warm climate

Question
Dick,

Just wondering if an AMC V-8, 390 ci, supposed to have a 195 deg. thermostat, was replaced with a thermostat of 180 deg. Would I notice a significant difference in gas mileage, say more than 3-5 mpg drop? If I live in a warm climate, would the 180 therm. compensate for the warmer climate, in terms of gas mileage?

The reason I'm asking this is because my engine was rebuilt recently and the machinist decided to run a 180 instead of the 195. My mileage has dropped 2-3 mpg. since the rebuild, but I understand that the rebuild may also contribute to this situation.

Thanks,

Steve

Answer
I don't know why your mechanic would advise using the lower temp thermostat - generally, engines run cleaner and stay cleaner inside if they are run at higher temperatures. Of course the radiator and the rest of the cooling system should be in good shape with the proper cap and coolant mixutre, so that boiling point is up around 250 degrees. This is the way all engines are set up these days, and there is a good reason for it.

However, I don't think it would make that much difference to the gas mileage - more likely, the engine is not now tuned optimally for best mileage and the mechanic may have done that on purpose for the break-in period.  I'd ask him about that, and also quiz him as to how long he wants you to run the 180 thermostat.   Of course, if the engine has been rebored, and if all new bearings have been installed, the engine will need some time on the road to loosen up before the performance and mileage comes back to what you are used to.  In other words, it's hard to know what the "rebuild" involved, but it you could be just going through normal break-in operation.  It is also possible that something was done to the carburetor to affect the mileage, either on purpose or because of an error in the rebuilding process on the carburetor, (if it was rebuilt).

Dick