The Edelbrock #60539 Performer RPM CNC Bare Cylinder Head has the same specifications as the popular Performer RPM cylinder head for 1962-79 389-455 c.i.d. Pontiac engines, but feature a CNC machined combustion chamber. The CNC machined chamber results in a consistent and precise chamber for improved flow performance. Performer RPM CNC cylinder head is available in two chamber sizes. Performer RPM CNC cylinder head with 72cc chambers are best for higher compression engines. The Edelbrock #60539 Performer RPM CNC cylinder head is patterned after the same port flange configuration as the 1969-1970 Ram-Air IV Pontiac Cylinder Head.