A variable air path on diesel engines offers further potentials to manage the challenges of engine development - such as reduction of emissions and fuel consumption, as well as performance increase. The Miller cycle is one of the possibilities, which is well known as an effective way to reduce process temperatures and so NOX emissions. The present paper discusses the potentials of this strategy for heavy duty diesel engines by identifying and analyzing the effects caused.The investigations were carried out in the upper load range. First the isolated effect of the Miller cycle was analyzed. The results show reduced NOX emissions, although increased PM and CO emissions were measured. Further, the Miller cycle caused a reduction in peak cylinder pressure. This pressure reserve can be used to combine the Miller cycle with further measures while maintaining the maximum cylinder pressure of the reference operation point. On the one hand, a performance increase of about 10% was achieved. On the other hand, the combination of Miller cycle and increased boost pressure showed great potential to optimize the NOX-PM trade-off and led to an efficiency rise. To understand the effects caused, the losses of the process were separated and compared with the reference operation point.The experimental tests were carried out on a single cylinder heavy duty test engine equipped with an in-house developed camless valve actuation system, and operated in a flexible test environment.