How far do fans have to be from a rad or from a mesh grill to perform optimally (i.e comming close to within 5% of their performance in open air). My rad and fans are in a cramped case and I need to know the optimum distance. If you put a fan directly on a rad it will do pretty poorly (i.e seen this in a review somewhere). The GTX360 rad has about 1/8th of space from the rad to the fan surface. The same goes with mounting fans onto case grills. Right now I have exactly 1 inch between a fan and a given surface (i.e 1 inch radiator shroud).
I agree that 5 is kind of big. I would assume this is based on how long it takes the turbulence at the fan blades takes to "spread" to the deadspot, so the entire air column in front of the fan is moving...I have no idea if the guy who posted this had any proof or made those up...but hey, it's a sticky on SPCR that's gotta be worth something haha :P
I'm sure the graph for performance vs distance is non-linear, you could go from 0.125 to 0.25 and get a huge improvement, and then go from 1.5 to 2.5 and barely see an improvement (maybe even a decrease)
Hmmm... Can't find the links anymore for the testing done at XS, but one of the users at Overclocking.net seems to think it makes a 2-3C difference. That's more than any other results I've seen, but like I said, I can't find the other results.