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Have read alot of the threads on Grill Blocks. Does the following Summary of Grill Block Priorities sound right? (I'm going on memory here.)

1. If you have 2 grills, Block the upper first. This will generally cause you to induct slightly hotter air in the summer as it is closer to the road. However it will keep the blocked air rising over the vehicle undisturbed. For the same reason, if you have a single grill, block from the top, not the bottom.

2. Block from the sides inward instead of from the center outward so that blocked air flowing around the sides of the vehicle is undisturbed.

3. Duct the air from the remaining opening to the radiator. (Should this duct be wider at the radiator than at the grill, narrower, or the same size???) If you don't duct the air from the grill, at least seal the opening between the grill and the radiatior so that you get the best bang for the buck from the air you do allow to enter by not letting it escape before it goes thru the radiator.

4. If you are able to, duct the air from the back of the radiator out of the engine compartment via shallow openings in the hood not far from the front of the hood. Failing that, duct it out the wheel wells or sides of the vehicle.

5. Some cars may not need any opening in cool climates.

6. To determine the size of the opening, attach a wire to the fan or fan relay and route it back to a light in you can see. Determine how often your fan comes on w/o the grill block and with the grill block under similar conditions. The idea is to not have your fan cycling on much more than with no grill block. A 12 volt bulb or an led with about a 550 Ohm resistor wired in series with it will both work - but the 12 volt bulb will be of higher wattage and will get hot.

7. In the process of creating your grill block, you may also be able to, with little extra effort, create a more streamlined front end that better directs air over and/or around the sides of your vehicle. For example on my Subie, the front bumper protrudes out further than the upper grill. So I plan to slope the grill block from the top of the front bumper to the top of the upper grill opening.