It may be no differences in efficiency. But let me explain my case:
I build a little mini-PC with the air outlet side, directly from the processor fan. Testing the same layout (same board, same box, same processor) 9 blades fan (Foxconn) usually rotates between 1170 to 1200 RPM, the 11 blades fan rotate at 980 to 1100 RPM, and the sound difference is very noticeable. I have done extensive testing and I'm sure this is so.
I would add that the sound is not the fan noise when turning, but the "wind." The Foxconn 9 blades fan "blows" a lot when it is confined in a small space.
Perhaps the 11 blades fan move more air is ejected laterally and better (in my design). I do not know, I have no sufficiently precise measuring instruments. But the difference in noise is very noticeable, so I'm pretty sure.
As per the Engineering Department, you may want to make sure the fan is plugged into the correct fan header. For confirmation purposes, they have tested both processor fan types and the results were that, when plugging processor fan into a chassis fan header they spin faster and had a high pitched noise whine at boot. When plugging the processor fan on the processor’s fan header, they both ran quieter and slower.
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