Here is something I recently discovered. The new aftermarket radiator I installed in my car a couple years ago started leaking, which turned out to be due to a loose side bracket that punctured a tube. Anyway, I was considering installing a junkyard Behr radiator that started leaking shortly after I brought it home (didn't at the junkyard, still full of anti-freeze). Took the tank off to fix the leak and found out the Behr had an aluminum core. Took the tank off the new aftermarket and found a copper core. From the study of heat transfer, the heat transfer coefficient is almost twice as great for copper as it is for aluminum. This means copper can transfer heat much more efficiently and quicker than aluminum. So if your car has an aluminum cored radiator, it will not keep the car as cool as a copper cored one. Might try scrapping some paint off of the core tubes and seeing what it is made of.

Ended up soldering the new copper radiator and reinstalled. Hasn't leaked yet.