When looking at radiators, radiator style (cross flow vs. down flow radiator), compatible fan type and material construction are generally the most important aspects to look at. As you delve into parts catalogs and repair shops however, the question of whether to go with a cross flow or a down flow style radiator is a common source of confusion. If you have an older car you may already have a down flow style radiator. Should you convert to a Cross flow or is Down flow fine? What are the pros and cons?
All things equal such as surface area, construction material and fin design, there is no difference in cooling performance between a cross flow and down flow radiator. Where the differences start to add up, depends on what kind of car you have and what kind of space you have to work with. Down flow radiators tend to be tall and somewhat thin. This makes them a good fit for trucks and vehicles where a lower profile hood line isn't really a important consideration. Cross flow radiators in contrast, are short and wide. This makes them perfect for sports cars and smaller compacts that have a lower hood line. On cars with a low hood line, a down flow radiator will have to be short in order to fit in the engine bay, which will reduce the amount of surface area you have available for cooling. Less surface area translates directly into lower cooling potential.
Another thing to think about is how you will be driving your car. Space considerations aside, how much time you spend driving hard or at higher rpm can also decide whether a down flow radiator is the proper choice. Because the end tanks sit on the top and bottom of a down flow radiator, the cap, which is also located on the top of the radiator, has no other choice but to sit on the end tank. The end tanks in a radiator are the highest points of pressure. What this means is that during extended high rpm where the water pump is moving the most fluid, and coolant is at its hottest, the spring loaded bypass valve in your radiator cap will open sooner, venting excess pressure and fluid into the overflow reservoir. This translates into lower cooling efficiency. The radiator cap on a cross flow radiator sits in a lower pressure area, the space between the two end tanks. This means the valve in the radiator cap will stay closed longer because pressure has to rise higher in order to make the pressure in this area rise high enough to open the bypass valve. This promotes higher overall coolant system pressure and keeps more fluid circulating in your coolant system, which improves efficiency.
Unless you do a lot of hard driving and track events, the hassle of converting your car from a pre-existing down flow style may not be worth the time and effort. If you're looking to upgrade, then the most important aspect may be the space available in your engine bay. Surface area makes the biggest impact on cooling, so whichever style gives you the biggest amount of space will generally serve you the best.