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A head is attached to the engine block atop a row of cylinders. The head contains the valves, valve springs, rocker arms, and also has ports or tunnels for the intake air and exhaust gas (yes, this means the intake and exhaust manifolds are attached to the heads). Heads can help make more power because they can contain larger valves and have larger ports, allowing more fuel in and gas out of the combustion chambers. Heads can also alter the compression ratio of the motor, because the head contains the top of the combustion chamber. Because the head is so critical with regards to how well the engine breathes, heads and cam should be carefully matched for optimum performance.