During low speed operation of a turbocharged engine the transient response time of the engine from low speed and low loads to high speed and high loads is slow because of the slow response time to raise the boost pressure level from the turbocharger. The present invention provides an apparatus to improve the transient response time of the engine by causing at least a selected subset of the combustion chambers to be operated in a two-stroke pumping mode of operation while maintaining the others in a four-stroke mode, thus, increasing the airflow through the turbine of the turbocharger during low speed and/or low loads operation of the engine maintaining a high boost pressure level.