Crossflow cylinder head

A crossflow cylinder head is a cylinder head that features the intake and exhaust ports on opposite sides. The gases can be thought to flow across the head. This is in contrast to reverse-flow cylinder head designs that have the ports on the same side.

A crossflow head gives better performance, but the popular explanation put forward for this — that the gases don't have to change direction and hence are moved into and out of the cylinder more efficiently — is a simplification since there is no continuous flow because of valve opening and closing. But since there is overlap between the intake and exhaust profiles there is a point in which both valves are open. At that point the inertia of the exhaust gases leaving the cylinder helps to aspirate the intake gases into the cylinder. The other main reason for a crossflow's performance is that the ports and valves can be larger and its physical separation of the hot exhaust manifold keeps the air in the intake manifold cooler. Most modern engines are of a crossflow design.

"Crossflow" is often used to refer specifically to Ford Motor Company's Kent Crossflow 4-cylinder OHV engine. This unit has been used in cars from the 1960s up to the present day, albeit with the addition of fuel injection and a modern engine management system.

This term is used for engines which have only one intake and one exhaust valve per cylinder; four (or five) valves per cylinder engines get their superior performance from total port size, not the relative location of the ports, which doesn't have to be mentioned.