PROCOMP BB CHEVY CYLINDER HEADS UP TO 748 HP OUT OF BOX!CALL SHANON AT 507 445 3235 WITH ANY QUESTIONS. PC3021 New set of BARE Aluminum Big Block Chevy Motorsport Heads (2 heads) Produce Up To 748 HP Out Of Box With Advanced Port Design Manufactured from 356 T6 aluminum Have Accessory holes to except factory accessories. 320cc Intake Runners 115cc Chambers Spring pads are machined to 1. 740 to except the largest roller springs Bronze Valve Guides Premium Hardened Seats Machined for 2. 250 intake and 1. 880