A comparative investigation of both fuel consumption and exhaust emissions has been carried out on a production SI engine operated by either gasoline or compressed natural gas (CNG). The engine had the main features of being a multivalve, fast-burn pent-roof chamber engine with a variable intake-system geometry. It was originally designed at Fiat Auto to operate with unleaded gasoline and was then converted at Politecnico di Torino to run on CNG. To that end, in addition to designing and building the CNG fuel plant, the multipoint Bosch Motronic M1.7 electronic module for injection-duration and spark-timing control was replaced with a Magneti-Marelli IAW ECM designed to obtain a multipoint sequential injection. With this new ECM, the engine was modified so as to work with either natural gas or gasoline. For the present study, the engine was not optimized to run on CNG in order to compare the performance of the same compression-ratio engine under different working conditions with the two different fuels. The engine was also interesting as a dual-fuel propeller because of its relatively high compression ratio, ≈ 10.5, which is almost suitable for CNG operation. Besides, the engine features are of particular interest for the use of natural gas as an alternative fuel, due to the lack of data for innovative CNG engine design concepts.A test bench, previously installed and instrumented in an engine hall for typical dynamometer experiments, was equipped with an upgraded multipurpose exhaust gas analyzer including a device to separate methane from non-methane HC pollutants. The engine performance analysis was carried out in terms of fuel consumption, torque and exhaust emissions, although pressure time-histories were also acquired in the combustion chamber for heat-release studies. The engine tests with the two different fuels were conducted as “spark sweeps” at fixed engine speed and air-fuel ratio. These tests covered a range of different engine speeds, air-fuel ratios around stoichiometric conditions, as well as different loads. Each “sweep” consisted of several spark advances centered around the MBT timing. However, only the results of a demonstrative engine-test sample are here presented and discussed. Operation with CNG basically showed the capability of a substantial reduction in both specific fuel consumption and pollutant emissions.