I was tasked with testing about every plastic molded fan on the market, back about 17 years ago, (now). The best most efficient fan was the Ford Taurus fan used from introduction on up to I believe 2004. Designed by a company in Massachusetts, these guys knew their stuff. Lots of hot rodders used them. The next best were Spal, efficient and compact. There have been lots of knockoff fans but none matched the performance of the Spal. The short chord, narrow blades, have too really be designed for the application, too work properly. They may quote 1750 cfm, but that is at zero back pressure. When you put a radiator, in there, the fans will usually "stall" , and provide insufficient airflow, for cooling. That was the real reason the Taurus fan worked in so many applications, the blades were a longer chord length ( wider ), and the curve of the blade kept it out of "stall" over a wider range. Many of the knock off fans would go into stall when combined with a 2 or 3 row radiator, with a moderate finish count of 15 fins per inch, basically doing very little air movement. The reason a steel clutch fan, or flexible steel fan, works is that wide long blade chord length, which provides higher pressure, to work with multi row radiators.
