The Caterpillar 3406B air-to-air aftercooled heavy duty diesel truck engine has been developed for fuel economy. The air-to-air aftercooled system has been used by Caterpillar on earthmoving engine applications and by Europeans on heavy duty diesel truck engines to increase power. The application of air-to-air after-cooling on the 3406B engine not only allows improved fuel economy but also results in increased horsepower to 425 (317 kW) and lower gaseous emissions. The air-to-air aftercooled system offers even further benefits as future legislated gaseous emission levels are reduced. This paper addresses the development, performance, and application of the 3406B air-to-air aftercooled truck engine.