The Edelbrock #60539 Performer RPM CNC Bare Cylinder Heads have the same specifications as our popular Performer RPM cylinder heads 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 heads are available in two chamber sizes. Performer RPM CNC cylinder heads with 72cc chambers are best for higher compression engines. The Edelbrock #60539 Performer RPM CNC Cylinder Heads are patterned after the same port flange configuration as the 1969-1970 Ram-Air IV Pontiac Cylinder Head.