i use delta fans. They are used for servers usually - most powerful 120mm fan i can find - flows 190cfm - which is a lot for something that small - it really is powerful. I have 4 of those, which is 800cfm of air directly onto the radiator with out an intercooler in the way. I also use a 6inch motorbike fan on the back of the radiator - it JUST fits. A fan that sucks is 20%more efficient than one that blows - but u should be ok with the 4 delta's.

Bottom one. This really does piss on any other "computer fan"

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