after running the car hard (i.e. auto-x), come to a stop (keep the car running) and open the hood. wait til the fan comes on. i figured that the air would get pulled through the radiator/fans and then straight up the engine, so it would make more sense to have EVO style extractors, but this is not the case. the hot air goes to the sides of the engine and then up, right where the vents are coveinently located

another fun test is in the summer when its hot, you can actually see heat ripples coming out of the vents or in the winter, after a light snow, once the car is warm, the snow melts around the vents first. basically, they definitly help in keeping engine bay temps down. which is another reason i put them back on in the winter. i find that without them, the engine gets cold very quickly after being shut off and then is a PITA to start. with the vent covers in place, however, it takes a couple of hours to get the engine really cold again.