Oh I see. so you were sending a huge volume of gas out and the small outlet couldn't keep up. Crazy that could happen even with a 3 month old OEM radiator cap. When I was going up the grapevine I was fairly heavily loaded for moving, but I made sure to keep the speed up above 55 (I had the pedal to the floor at maybe 3600 rpm). Vehicle speed will do far more than the fan could ever hope to do.I posted a video on these forums of what happened immediately after the AC stopped working (so I was above 227F). The coolant was boiling in the reservoir bottle and the the entire top of the radiator was steaming and boiling coolant was coming out of the top seams. The radiator cap did its job too well--it never gave way. Not great videos, but here they are: Cooling Situation...Is This Normal?
I wonder if people who really work their vehicles hard should go for the pure glycol coolant. It doesn't boil until 310F or something like that, so it will literally never boil over and lose cooling performance.
Edit: Evans Waterless Coolant, Prevent Engine Overheating