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Over this past weekend I had a bit of time to work on something, so I finished ducting the upper grill of my 4Runner to the radiator. I used cardboard to make an accurate template, then copied it in black coroplast. The bottom is secured with fasteners; the sides are jammed into corners of the core support and held by the grill; and the top is held by tabs to the grill and down by the hood support bar. The bottom of the radiator was already sealed/ducted from when I did my belly pan work in this area.
Currently the upper grill opening is about 1/4 the size of the area of the radiator, so that is a bit more than the 1/6 rule of thumb, but this seems appropriate as the duct is not 100% air tight.
Temperatures are still running fine, but I did notice a marked increase in handling. I assume most of the air going through the upper grill before this mod was bypassing the radiator and running under the vehicle as it has with all my past vehicles. I am getting ready to do a "grill block" on the bottom grill opening, which will also include mounting fog lights in the grill location, having a removable cover/plug to access the front receiver hitch, and extending my belly pan/skid plate all the way to the nose of the vehicle.
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