To obtain the most efficient fan, the highest profile, or thickest fan, is usually the fan to use. Different motor positions, as well as many other variables, produce different clearances between the radiator and motor, or radiator and grill. For this reason, different “profiles” are available. The lower the profile, the less CFM (efficiency) the fan will deliver. Lower profiles also deliver less amps, for lower amp alternators.
For max CFM, with max clearance issues, check out the Sidewinder Electric Fan.