An internal combustion engine is provided with a roughly concave cylinder head, wherein a ridge descends into the combustion chamber. A first wall is formed from the ridge to a cylinder periphery to form a seat for an air intake valve. A second wall is formed from the ridge to the opposite cylinder periphery to form a seat for one or more exhaust valves. The valves must criss-cross to open/close. The result is a more efficient flow of intake air and exhaust air to and from the combustion chamber which creates more horsepower. A plug compatible cylinder head can be installed on an engine to get added horsepower without a supercharger.