I run a 14" Hayden 'S' blade, 1100 cfm, along with the 2 original 6" fans that run when A/C is on. I live in the Mojave desert in Calif. I always run below half way on the gage.
If I were to need another fan, I would use one of these .... $80......
http://www.jegs.com/cgi-bin/ncommerc...enbr=361....it pulls 1800cfm for the same amperage draw.