I've finally gotten around to doing some mods to my '97 Civic DX sedan )besides just maxing the tire pressure).
I removed the air tube, so now it gets all of it's air from the top of the engine. Ultragauge showed intake temp of 88F at 60MPH and 100-110F in city with outside temp at about 55F, no grille block. With the partial lower block, it got to about 120F in city, but outside temp was probably 60F at that time. Not sure if its helping efficiency at all. It does make the intake drone really loud at high throttle positions, but I shouldn't be doing that anyway
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I blocked off most of the upper grille using foam pipe insulation. I cut the foam to length, cut the foam in half (to use on the other side), and stuffed it in. It feels pretty solid and should last quite a while. The biggest danger to it will be someone pulling it out as they walk by. I blocked off half of the lower grille with cardboard. It is in front of the AC condenser, which never gets used. It survived the trip to Home Depot to get the pipe insulation, but a good rainstorm will be the end of it.
Future mods will be replacing the cardboard block with a plastic one, smooth wheel covers, probably rear wheel skirts, and maybe a bellypan. I just have to find a place to get coroplast/abs plastic from. The local bus company uses coroplast for the advertisements on the sides of their buses, and according to their website, they just dispose of it after the contract is up. I sent them an email the other day asking if I could get some, haven't had a reply yet. If I don't hear back from them or they say no, there are a few sign shops around here that I'll check.