In this study, a dual sequential spark ignition engine is separately tested either with gasoline or CNG at low and high loads. In addition, numerical engine analyses are performed by constructing a 1-D engine model in Ricardo-Wave software. Engine performance parameters in catalogue are generally given at full load conditions. However, during engine lifetime, vehicle engines rarely run at full load (wide open throttle) while engines work especially at the partial throttle openings. Engine characteristics (engine performance and exhaust emissions) are strong functions of throttle opening level. For this reason, determining engine characteristics at partial throttle openings at which engine mostly runs provides valuable information. In this study, partial throttle openings of 25% and 75% defined as low and high load conditions are examined for gasoline and CNG, as well. For this aim, the Honda L13A4 i-DSI (intelligent dual sequential ignition) engine was tested and engine characteristics were measured. This engine has unique features of dual sequential ignition with variable timing, asymmetrical combustion chamber, and diagonally positioned spark-plugs. Tests and numerical analyses were performed at specified low and high load conditions for gasoline and CNG by varying the engine speed from 1500 rpm to 4000 rpm with an increment of 500 rpm without excepting 2800 rpm. Engine characteristics were determined for the investigated parameters. Tests and 1-D model results are fairly matching each other. The average deviation between them is about 5.4%. Results show that the maximum torque for gasoline at 2800 rpm and 100% throttle opening reduced 12.6% and 26.3% for throttle openings of 75% and 25%, respectively. Compared to gasoline, CNG reduced the torque 15.6% and 19.6% for throttle openings of 75% and 25%, respectively. In general, CNG usage decreases all engine performance parameters (torque, power, volumetric efficiency, specific fuel consumption) and emissions (CO2, HC), except NOx formation.