The size of passages in the head can affect engine output. Smaller intake and exhaust passages and ports allow more torque at low engine speeds. This is because smaller passages improve mixing of air and fuel at low speeds, which causes more efficient combustion.
At high speeds however, these smaller passages restrict airflow. To reduce the effect of this, this engine has two inlet valves. One opens at low speed and the other operates at higher engine speeds. Larger passages produce greater power at high engine speeds.
Each intake and exhaust passage can be formed separately in the head. Intake passages for adjacent passages may have a common, thin wall between them. This is called siamesed. Exhaust ports in the same head can also be siamesed.
When all intake and exhaust ports are on one side, it is called a counter-flow head. They can be cast separately or siamesed.
When all of the intake ports are on one side and exhaust ports are on the other, it is called a cross-flow head. This allows for straighter passageways and higher efficiency.