as said the AC app has a faster pulley ratio for more airflow but that higher ratio can make an 8 blader cavitate at higher RPM's so they dropped back to 6 blades & with the higher ratio 6 blades is enough to cool. You want the best of both worlds? get an 8 blade & weld on an anticav plate & now you have the most flow & no cav possibility. I cut a round plate out of the side of an old washing machine with the OD slightly less than the tips of the vanes so you can tack several vanes and drill a hole in the center to tack there also. to go further you can press the vanes closer to the case (.030" is ideal) & support the shaft on the other side where the cross is when doing this & put some feeler gauges under the vanes (.030") as it may "jump" when pressing then there's a big problem. then between the cav plate and the flat on the timing cover you want .030" also (keep gasket thickness in mind) & a thicker cav plate or bolt a flat piece on the flat (flat head bolts & "the right stuff" Permatex gasket maker which is the best