4H-SiC ingots with diameter up to 30mm were grown by the Modified Lely Method. A combined approach of this sublimation technique, involving heat transfer modeling and experimental results is presented. Different geometric modifications of the reactor and process parameter are studied. Their relation with defect occurrence and polytype change are discussed. Calculations show that the modifications have an influence on the radial and axial temperature differences inside the growth cavity. It results fi om a combination of experiments and calculations that the material characteristics are sensitive to small parameter variations, and that they can be deteriorated by minor variations compared to the working temperatures.