Serious street performance, mild bracket racing and oval track. Maximum torque and throttle response from 3,500 to 7,800 RPM. Best for 363-427 cubic inch engines. Works with most standard components. Every intake port, exhaust runner, valve bowl, and every combustion chamber is 100% CNC machined on special dedicated PRO1 castings. These heads are ideal for high compression, big cubic inch small blocks and are great for supercharged applications. Exhaust runners are raised .135 in. for improved flow. Manganese Bronze valve guides are used for long life, and hardened valve seats provide durability with unleaded fuels. Features 3.000 in. exhaust bolt pattern. Assemblies include Stainless Steel valves, premium springs, locks, retainers, studs, guide plates and seals.