I did my experiment to my own satisfaction on a 96 degree day.
It was done under jury-rigged conditions and thus is not really suitable for presentation here. If I have time later in the summer I will quantify things more formally, but for now I felt compelled to at least issue a warning in the interest of saving someone's motor.
Do NOT attempt to block the grill on a Suzuki/ Geo beyond a half block, vertically and open on the radiator side. Also do NOT atempt to accomplish the same thing horizontally by blocking one or the other horizonal vents.
Also, if you're contemplting clening up the undertray it would appear as if the exiting air from the stock arrangement is marginal, at least in combination with grill blocking. I'd recomend against blocking any part of the area immediately aft the suspension arms.
The results in oil, water and engine compartment temps are quite severe and exponential. Water temp alone is not a sufficient indicator of the other conditios as it appears that the cooling system is up to the task of keeping the water cool but for reasons not yet clear it has a lesser affect on oil temps. Essentially as long as your fan is turning you're good to go on water temp., it'll just run all the time.
My car has an automatic wih A/C and possibly has a larger radiator than standard. If it does the results may be worse on a manual trans, non-A/C. version.
I hope you'll trust me on this one. The results were immediate and obvious, even under informal testing.
In the spirit of keeping this section "pure," I'm outta here for now.
Everyone wants to live inTheory. Because everything works THERE.