This paper describes numerical simulations that compare the performance of two combustion CFD models against experimental data, and evaluates the effects of combustion and spray model constants on the predicted combustion and emissions under various operating conditions. The combustion models include a Characteristic Time Combustion (CTC) model and CHEMKIN with reduced chemistry models integrated in the KIVA-3Vr2 CFD code. The diesel spray process was modeled using an updated version of the KH-RT spray model that features a gas jet submodel to help reduce numerical grid dependencies, and the effects of both the spray and combustion model constants on combustion and emissions were evaluated. In addition, the performance of two soot models was compared, namely a two-step soot model, and a more detailed model that considers soot formation from PAH precursors. Experimental data from four different diesel engines under different operating conditions were used to establish and validate the computation cases. The results show that the simpler KIVA-CTC combustion model can provide acceptable results over a wide range of operating conditions and with much higher computation efficiency than the KIVA-CHEMKIN model. However, the soot predictions are not as good, and more model constant tuning is required. The investigation reveals which model constants are more important than others during the model calibration processes, and guidelines are provided to simplify the model calibration process. Based on the results, general ranges of the relevant model constants are presented and a procedure is recommended to help calibrate models against experimental data.