The Edelbrock #60947 Performer RPM Bare NHRA Stock/Super Stock cylinder head is designed for 1955-86 Small-Block Chevy applications. This cylinder head delivers great throttle response and power from 1500 to 6500 rpm. Cast and heat treated at the Edelbrock Foundry and machined at Edelbrock's manufacturing facility in the USA, the Edelbrock head is the best in quality and performance. All Edelbrock cylinder head is 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 head offers 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!