Edelbrock's all new line of heads are designed for entry-level street performance engines and ideal for operating in the idle to 5500 rpm range. Manufactured in the USA, by Edelbrock, these heads feature A356 aluminum castings that are heat treated to T-6 spec for superior casting strength and quality. Their modern combustion chamber designs improve the combustion process and larger than stock intake and exhaust ports move more air through your engine. These features improve an engine's efficiency, resulting in more horsepower and torque and improved mileage potential. Additional qualities include hardened spring cups for long-term wear protection and Heli-coil» threaded inserts in the exhaust flanges and rocker stud bosses for increased strength; these are the details that most low-cost competitors leave out of their heads. E-Street big-block cylinder heads are sold as singles and Made in the USA..