To properly size a fan to your radiator, we're looking for coverage on the actual radiator core. The actual tube to fin area of the core rule of thumb is that we want at least 70 percent of that core covered by an electric fan. So, if your goal is to eliminate your engine driven fan or your stock clutch fan and shroud and go to electric fans, you’re going to need to get a lot of coverage, so it's getting a steady draw of air through as much core as possible as opposed to one single fan in a concentrated spot on the core.
As far as your CFM ratings go, it's not as important as coverage. A lot of times we hear, "Well, it's not shrouded; it's not covering up the whole core." That's fine as long as we've got at least the 70 percent covered. A little bit on the sides isn't that bad and it's nice to have some flow-through of air as you're going down the road. But, when you come up to the stoplight, you've got dedicated coverage coming through the core that's really pulling and keeping the car cool.