I read some where that pro stockers have to run iron, because its in the rules. Read some old threads on here and some say they went from iron to aluminum and seen nothing on their time slip. So looks like the horse power penalty for aluminum is a wash with the lighter weight. Gonna research these Callies iron blocks some more. All i got so far is the Callies iron is a lot heavier than the aluminum block and about $1000 less.