The difference between 5V and 12V with my two 120mm Sanyo Denki fans on a 3/4" thick transmission cooler is about 1-2C at most and at least 6dba. I'm running a 3100+ Sempron overclocked from 1.8Ghz to 2.5Ghz at 1.85Vcore. At 12V they push 102cfm each, at a significantly higher static pressure, but temperatures remain nearly unchanged so I run 'em at 5V 24/7. Very quiet. On the other hand, if you use a thicker radiator like a heater core, you might see a greater benefit from running them at higher rpm due to the increased static pressure allowing use to make use of the additional thickness of the heatercore compared to the transmission cooler I use. I hope that made sense. There's no benefit for me to turn up my fans, but because you have a thicker radiator, it may be worth it, though it'd be quieter and more effective to just buy a second heatercore, run it in parallel and strap a couple more fans on it.
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