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Originally Posted by Salkcin
The fans are 92mm Enermax UC-9FAB - rated 1500-2500RPM and 28-47CFM. I found out that 1800-1900RPM are at the level of noise I can tolerate so calculating the airflow at 1800RPM it will be blowing 33,84CFM - a total of 67,68CFM with 2 of them. That's more than one 120mm @ 1300RPM can blow (~44CFM).
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That's figures in open air, not against a backpressure. The rad will create a static pressure that makes the flow drop. The pressure/flow relation is not linear, so without at least mfger data about the fans you're about to use, you won't get any idea. Also you'll certainly have a drop in performance/noise ratio, as 92mm fans make quite more noise to push as much air as a 120mm.
If you want a powerful & quiet 120mm try the Delta WFB1212M, i've been impressed by this one, it's up there with the best of them if you want quietness, and it flows 72 CFM at 2100rpm...
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Originally Posted by Salkcin
Enermax have a speciel RPM regulation (patented?) method that regulates the fans inner resistance so it's allways 12v. It should give a variable fan that allways have the best terms of overcoming air-resistance.
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Overcoming "air resistance" is more a factor of the blades geometry, and total surface. Most (if not all) fan motors have quite enough power to rotate at their full rated speed whatever the conditions are.
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Originally Posted by Salkcin
But my biggest concern is that the airflow through the radiator won't be balanced with 2x92mm fans since the radiator is so much wider than the fans. I think it will suck most air through the center of the radiator meaning that water passing through in the outer channels won't be cooled good enough.
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With proper ducting (large enough shroud) this is not really an issue.
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Originally Posted by Salkcin
Could a custom shroud with same design guideline as the original 120mm shroud (40% of the fan size from the radiator core) make the airflow about even through all of the fins?
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Keep the 120mm shroud, ans slap a 120mm fan on it.