The Edelbrock #60447 Performer NHRA Stock/Super Stock Cylinder Heads are designed for Big-Block Chevy Oval Port V8 applications. Cast and heat treated at the Edelbrock Foundry and machined at Edelbrock's manufacturing facility in the USA, Edelbrock heads are the best in quality and performance. All Edelbrock Cylinder Heads are designed from scratch using the latest in computer technology by our own highly skilled engineer's. Using highly efficient ports and state-of-the-art combustion chamber shapes, Edelbrock cylinder heads offer improved performance throughout the rpm range for great throttle response and top-end horsepower. Top-quality features include threaded inserts in rocker stud and exhaust bolt holes for superior strength and durability, and manganese-bronze valve guides with ductile iron valve seats. These are features that some other companies don't include!INSTALLATION NOTE: • Bare heads will have valve guides and seats installed, but will require final sizing and a valve job to match the valves you will be using. • These oval port heads will support over 540 hp with 9.5:1 compression when used as part of the Total Power Package® • Unique oval intake port shape produces flow similar to rectangular ports, but the smaller port size provides excellent seat-of-the-pants performance and crisp throttle response • Semi-open chamber design maximizes efficiency with streetable compression ratios • Smaller-than-stock 11/32" valve stems promote excellent flow and lighter valve weight • Performer heads include heat crossover passage for emissions related engines • Match with Edelbrock head gasket sets. • Recommended spark plugs are Champion RC12YC or equivalent 14mm x 3/4" reach with flat gasket seat.