25 years ago I had an GM AC class instructor state 30 degree minimum temp drop, TD is proper term, WITH PROPER AIR FLOW, he stated for condensers, evaporators, radiators, I have relied on these specs, and found them true on many makes, please test TD on a few radiators on vehicles with rad fans on, AT TEMP, usually 40 degrees TD, all day long, put a pressure gauge on rad, drive it to see what pressure is when you see temp rise too much, and report back.

IF you're getting combustion gases in cooling system, you will never cool it with radiator.

Ken Meads
Technician
Lone Star Auto Service
Houston, Texas, USA