To shroud or not to shroud
The main reason for using a shroud is to eliminate the dead spot. On a 120mm fan, about 15% of the actual fan area is the motor. Surprisingly, on some 80mm and 90mm fans the motor is not much smaller than the 120 mm fan and take up about 27% of the fan area!. This is a pretty significant dead spot. I've seen any between 1/2" and 1 1/2" suggested for the standoff between the fan and radiator (mine's at 1"). I'd like to see some real numbers on the differences between shrouded and unshrouded fans, as well as the difference between different standoff distances.
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