Originally posted by: gchildX
We're not talking about the exhaust air. The problem is that the Asus A8N Premium motherboard has a heatsink in the top left corner of the board that cools the chipset (via heatpipe) and the mosfets. This heatsink has to have air moving over it or the chip will overheat (or so says Asus anyway. They recommend mounting a special fan to hit that heatsink if you're watercooling the CPU.)
The XP-90 with the fins mounted vertically stops any airflow from hitting that heatsink which causes the chipset to overheat. The XP-120 from what I can tell overhangs that small heatsink, so the air from the fan hits it no matter which way the fins are facing.
Does that help to clear up the problem?