Flapper valves are added to the shrouds when needed to allow excess air from high speed driving to exit fan shroud. Without flapper valves, too much air enters into the shroud causing the air to go around the radiator instead of through it, resulting in the car overheating. If a pusher fan is the only fan used, cover as much of the radiator core as possible. Remember, shrouds cannot be used on pushers. Fan shrouds need a way to mount to the radiator. The radiator is needed in order to make mounts for the fan shroud. Fax a drawing of your radiator to Mattson’s Inc. at (714) 893-4933 if you want a quote from us to build you a custom shroud made that will fit your mounts.