Pro gears are made from 9310 and then heat treated. It is a softer alloy than the 8620 Street gears. The softer 9310 alloy allows the gear to absorb higher impact loads that are generated in drag racing without developing cracks. A harder 8620 street gear could shatter under the same loads. As a side effect the Pro gears are not the best choice for street use as they will wear faster.