Slots in the Shroud or Flappers?
 

I am getting ready to swap my leaking stock radiator out with a new aluminum one I got.
I already have an electric fan with shroud and figure since I am making the swap I should consider adding some high speed flow through slots or flaps on to the shroud.
In case I have not given enough details about what I am referring to: these allow air to pass through the shroud when you are driving down the highway because there is not enough pass through with the fan running. They also eliminate turbalance from the fan fighting the air being pushed through the radiator from driving 60-80 MPH.
Thanks
Ray

When I swapped to an aluminum shroud (no flaps) I had issues with the water temp rising on the highway (60-70mph). I didn't have rising temps from stop light to stop light. Took me a while to figure it out and then I switched to Becool fans which have built in flaps. The problem went away.

IMO, this is one of the most important features to look for when shopping for shrouds or fan/shroud combos.

Flappers. The whole purpose of a shroud is to give the fans the ability to draw air from the entire core. Air is like water and electricity, it follows the path of least resistance. Open holes in your shroud will allow high pressure air from the grille opening to flow through the radiator when travelling at higher speeds without the restriction of the shroud creating an air pocket to slow it down. In low flow situations when you rely on the vacuum from the fans to pull the air through the radiator core it will draw air from the path of least resistance. If you have open holes in your shroud it will be easier to draw air from there than to pull through the radiator core. If your shroud sits 1/2" off of the radiator core it will be easier to draw air through that gap than through the radiator core (don't laugh... we have all seen it.) The flaps serve to close these vents when there is vacuum inside the shroud forcing the air to be drawn through the core thus increasing the efficiency of the cooling system.

Does anyone have pics of how they made these flaps?