Different cars are effected in different ways. Many camaro grills for example are below the bumper. The radiator air enters below the bumper flows upward throuh the radiator and into the engine compartment. If placing an air dam on the later generation camaros, place the dam directly underneath the radiator, not under the front. I work for a Chevy dealer and we have had mystery overheating problems on lowered camaros due to a misplaced or misinstalled air dam. I have seen the same effect on other vehicles with an air dam lower in front than the one below the radiator. It all depends on where the front end flow is designed to go, over or under the front bumper.
Keep working on the experiments. In '85 before Nascar started installing governors, Tim Richmond hit 240 MPH at Michigan during testing by completely blocking off the front end, (he could only run 2 laps before overheating)
for the winter, I put 2 pieces of AL scrap I had laying around in my grill blocking about 80-90% and my temp gauge starts creeping when it's 45 outside in just highway cruising @ 60 mph. yes my cooling system is fully functional and clean I even removed the AC condenser in front of the radiator, the car just runs that warm.
1991 Toyota Pickup 22R-E 2.4 I4/5 speed
1990 Toyota Cressida 7M-GE 3.0 I6/5-speed manual
mechanic, carpenter, stagehand, rigger, and know-it-all smartass
"You don't get to judge me for how I fix what you break"
I saw no temperature changes with either my temp or permanent grill blocks on my Cavalier. This is due, no doubt, to the huge opening between my bumper and radiator. Closing that hole is the next aero mod on my list.
Speaking of warmer engine temps, I finally got fed up with low temps this winter, and took action this morning; I covered about 2/3 of the radiator with cardboard, leaving it open in front of the fan. I had been running 130-140*F on the highway, but I ran at thermostat temps of 180* both ways today. Yea! And I'm running at cooling fan temps of about 204* when I get off the highway. Now I really want to get my 195* 'stat installed...