In addition to the two port sizes, the 335 cylinder heads used two style of combustion chambers, an open or a closed "quench" chamber. Both combustion chambers are very shallow, due to the shallow valve angles. The combustion chambers are almost a very shallow hemispherical chamber, rather than a wedge shaped. The closed chamber heads enclose the valves more closely, reducing combustion chamber volume, to increase the compression ratio. However, both designs have the same thermal efficiency and resistance to detonation. The closed combustion chamber promotes better swirling of incoming air fuel mixture, giving it a low-rpm torque advantage, and requires less machining to obtain high compression ratios. However, the open chamber heads valves are less shrouded, which improves low lift airflow, and they exhibit better emissions characteristics.