To meet future CO₂ emissions limits and satisfy the bounds set by exhaust gas legislation reducing the engine displacement while maintaining the power output ("Downsizing") becomes of more and more importance in the SI engine development process. The total number of cylinders per engine has to be reduced to keep the thermodynamic disadvantages of a small combustion chamber layout as small as possible. Doing so new challenges arise concerning the mechanical design, the design of the combustion system concept as well as strategies maintaining a satisfying transient torque behavior. To address these challenges a turbocharged 2-cylinder SI engine was designed for research purposes by Weber Motor GmbH and Robert Bosch GmbH. The design process was described in detail in last year's paper SAE 2012-01-0832.Since the engine design is very modular it allows for several different engine layouts which can be examined and evaluated. This paper shows a comparison between three major combustion system concepts: - Gasoline Direct Injection, - Advanced Gasoline Port-fuel Injection, and - Compressed Natural Gas Port-fuel Injection.As a first step the engine behavior was examined using different turbochargers on the test bed to find the best match for a 2-cylinder engine application, especially with respect to low-end torque and transient behavior. Subsequently the 2-cylinder engine was equipped with three different combustion system concepts and tested on the engine test bed using the standard measurement equipment including high and low pressure indication. The concepts will be evaluated on the basis of their low-end torque and transient performance. Therefore a measurement method and a key performance index were defined. Additionally the influence of the combustion system concept choice on fuel consumption and CO₂ emissions will be evaluated using drive cycle simulations. For each concept the advantages and challenges will be highlighted and discussed.