This new family of aluminum cylinder heads for competition Chevrolet small-block 90 V6 engines offer a significant improvement over conventional Chevrolet designs. They feature an 18 valve angle vs. the older 23 angle design with 43cc chambers. They also feature relocated valve centerlines, shallow combustion chambers, centrally located spark plugs and high-flow exhaust ports. All heads with date codes of 9-1-97 have .080" added to the head face for 9.1:1 compression ratio. The relocation of valve centerlines (.275" toward the intake manifold) minimizes shrouding at high lift. It also allows the installation of larger valves (up to 2.20" diam. intake valves in some applications). These changes produce a substantial improvement in total airflow potential. Raised intake runners maintain the runner/port relationship with the revised 18 valve angle. This low-port version has excellent flow at low valve lifts due to their tall short-side radiuses and deep valve bowls. The shallow wedge-shaped combustion chambers allow builders to achieve high compression ratios with small piston domes. Spark plugs are .350" closer to the center bore than previous designs which improves flame travel across the chamber and reduces spark advance requirements. High-flow exhaust ports are .450" higher and .240" wider at exhaust manifold flange than previously. This minimizes turbulence in the runner and provides a smooth transition from the valve bowl to the header pipe for increased exhaust flow.Technical Notes: These heads are recommended for maximum effort competition engines. They are sold without valve seats and valve guides. Valves with .300" longer-than-stock stems are recommended for low-port V6/90 180 heads; .400" longer -than -stock valves are recommended for high-port 18 heads. Aftermarket shaft-mounted rocker-arm assemblies and pushrods are required. Piston domes and valve pockets must be matched to the revised combustion chamber design. Aftermarket head bolts with small diameter heads are recommended to provide wrench clearance with large diameter valve springs. A two-piece Bow-Tie cross-ram intake manifold (P/N 10051125-6) is recommended for Chevrolet V6/90 engines equipped with the 180 cylinder heads that require a four-barrel intake.