Reverse-flow cylinder head

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A reverse-flow cylinder head is a cylinder head that locates the intake and exhaust ports on the same side of the engine. The gasses can be thought to enter the cylinder-head and then change direction in order to exit the head. This is in contrast to the cross-flow cylinder-head design. This term is used for engines which have only one intake and one exhaust valve per cylinder.

The reverse-flow design is accepted to be inferior to a cross-flow design in terms of ultimate engineering potential, however, the reverse-flow design has been shown to be a more practical and economical manufacturing proposition and has similar potential in forced induction applications (where overly-large valves and "through flow" of gasses on cam overlap are not as desirable as under normally-aspirated conditions).

The real problem is that of temperature. With the exhaust ports on the same side as the intake ports, the intake air gets some of the heat, which reduces efficiency.