Are new aluminum radiators better?

Dana67Dart said

When I was running a 7lb cap I boiled over a lot I put the stock (for my 67 273) 16lb cap no more boiling over.

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Every pound of cooling system pressure raises the boiling point 3 degrees. A 15# cap adds 45° to the 212° or so boiling point of water. A 50/50 blend of coolant adds a few more degrees of protection.