[dataway]

Originally Posted by Formulajones

Correct. It's often misconstrued. The stat rating is where the engine temp will be at or higher. Depending on how efficient the cooling system is. As in my example above, it never runs 160, the original cooling system just isn't capable. I find most of my cars here with copper radiators, even the 4 cores, aren't capable of that (but they are slightly better). However I have 2 with Griffin radiators that can keep the temp closer to 160, even under high load conditions, still using the stock clutch fans and shrouds.

It's all going to depend on how well the rest of the system works.

Yep, what happens is that with a 160 degree thermostat, and an excellent cooling system, on a 90 degree day the Delta T between coolant and air is only about 70 degrees. Hard for a radiator to make use of such a low temperature differential. It's not that the system can't handle it ... it's that it's not ENOUGH for the system to handle efficiently, so the radiator will let the coolant increase in temp until the Delta T is sufficient to remove the same amount of heat that engine is adding in.
Same reason a household AC has a hard time making cool air on a hot day (not enough Delta T across the condenser), or a heat pump trying to pull heat out of 20 degree air.