Most radiators were downflow until the mid-60s, when OEMs began using crossflow designs, though many trucks kept the downflow design into the 1970s. A downflow design has tanks on the top and bottom of the radiator. Water flows from the pump to the top of the radiator, filling the tubes and flowing downward to the bottom tank, where it is pulled back into the engine. When comparing apples to apples, if the cores are the same size and design, along with the tanks, then there is not much difference in efficiency. Crossflow radiators have several benefits that downflow units don’t. Cooling systems are pressurized, and the radiator cap is there to manage the pressure. Downflow radiators can only have the cap on the hot side (where hot coolant exits the engine), so this is the highest-pressure area, allowing the cap to vent prematurely. This is a small benefit, but worth mentioning.
In many cases, you can order a new radiator that looks like a typical radiator but is divided internally into two or three sections. A typical radiator flows all the coolant through the entire core in one pass. A dual or triple flow core splits the core so that the coolant flows through each section of the core, giving the coolant more time to transfer heat, making it far more efficient. The U.S. Radiator Tripleflow option can drop engine temps up to 20%, which is a huge gain. If you have a hot-running engine, this could be the difference you need.
If you have ever been to a parts store, you probably noticed about a million radiator caps. While some of this is due to OEMs not using a standard cap size (Asian cars have very small caps for example), there is also a difference in the pressure relief. Heat means pressure, and your cooling system has a limit on how much pressure it can contain; this is where the cap relief valve vents to purge that excess pressure. When you get steam pouring out from under the hood, that is the cap venting.
