The Edelbrock #60439 Performer RPM High-Compression O-Port cylinder heads are designed for Big-Block Chevy applications. The Edelbrock #60439 Performer RPM heads are perfect for street performance, daily drivers, street rods and muscle cars where seat-of-the-pants performance is needed. These cylinder heads deliver great throttle response and power from 1500 to 6500 rpm. They also feature the highest quality intake and exhaust valves and valve springs. For use on pre-pollution controlled vehicles only.INSTALLATION NOTE: • EQUIPPED WITH SPRINGS FOR USE IN HYDRAULIC FLAT TAPPET CAMSHAFT APPLICATIONS • 100cc semi-open chamber cylinder heads feature a 1-1/2° rolled over (angle milled) design that improves intake port alignment and provides a smaller combustion chamber without shrouding the valves • 9.2:1 compression with flat-top pistons for an outstanding high performance street head • Match with Edelbrock head gasket sets. • Recommended spark plugs 14mm x 3/4" reach with flat gasket seat (Champion RC12YC).