up to 740 cubic inches (12.1 L). Pro Stock  (NHRA, MMPSA) "Doorslammers" must maintain a relatively stock appearance. NHRA engines can be no more than 500-cubic-inch (8.2 L)  displacement  while MMPSA cars can run a maximum of 820 cubic inches (13.4 L) (called "Mountain Motors"). Both classes require the motors to be naturally aspirated. Pro Stock