Downsizing of SI engines in combination with turbocharging is state of the art to reach future CO2 emission limits. A single stage turbocharged engine has a conflict of objectives between high rated power and high low-end-torque. To expand the stable map area of the compressor a variable compressor is investigated on an engine test bench supported by the use of engine process simulation. The measurements were carried out on a radial compressor with high trim compressor wheel. The limiting factor of the feasible low end torque is the surge line. Different inlet cross sections are investigated to shift the surge line to lower mass flow. The compressor maps are measured simultaneously on a hot gas and an engine test bench. A combination of both maps provides the input data for a 1D-simulation model of the test engine, which is presented in this paper. A predictive combustion model is validated for full and part load operating points up to 5000 rpm based on the serial production engine. Because a diesel VTG is used with the variable cross section compressor the measuring range is limited to 4000 rpm due to limitations in exhaust temperature. The potential of operating at higher temperatures before turbine is evaluated using the simulation model. The investigations show high potential using variable inlet cross section to expand the stable operating area of the compressor. The peak efficiency of the compressor can be shifted depending on the operating point of the engine using the variable inlet cross section. Further the test engine is using scavenging to provide high engine torque at low engine speeds, which has a negative impact on CO2 emissions. The variable cross section compressor enables the engine to provide the same peak torque at the same speed without valve overlap.