A/C used the smaller pulley/6 vane so the fan would turn faster for more airflow at idle for additional cooling yet the 6 vane would keep it from cavitating at higher RPM's with the higher vane speed from the smaller pulley since the 6 vane flowed less coolant. For the whole scoup from the Master go to www.moparchat.com then scroll down to & click on "circle track chat" then read thru Sanborns' cooling mods that are in a sticky at the top of the page