We were playing around with a buddy’s ’65 Chevelle a while back, installing a different fan shroud to help his big-block car run cooler. Once the shroud was installed, my buddy Kris produced a small slip of paper about the size of a postcard, held it in front of upper lefthand corner of the radiator, and let go. The fan and shroud created enough airflow at idle to hold the paper tight against the radiator core. This is a no-cost trick you can use to judge how well your fan and shroud assembly are working. If the system is properly designed, it should pull air across the radiator’s entire core surface. If not, then you’re leaving some efficiency on the table. Even electric fans need some way to pull air across the radiator’s entire core surface. The other half of this idea is that I’ve seen solid aluminum shrouds for electric fans that don’t allow air to escape from between the radiator and the shroud at high vehicle speed. In this case, the shroud can act like a door, allowing pressure to build up behind the radiator, preventing cooling air from flowing through the radiator. The solution is to create several small flap doors that easily swing out to relieve pressure. This creates a system that optimizes low- and high-speed cooling.