A production SI engine originally designed at Fiat Auto to operate with unleaded gasoline was converted to run on natural gas. To that end, in addition to designing and building the CNG fuel plant, it was necessary to replace the multipoint electronic module for injection-duration and ignition-timing control with an ECM designed to obtain multipoint sequential injection. The engine was modified so as to work either with gasoline or natural gas. For the present investigation, however, the engine configuration was not optimized for running on methane, in order to compare the performance of the engine operated by the two different fuels with the same compression ratio. In fact, the engine is also interesting as a dual-fuel engine because of its relatively high compression ratio ≈10.5 that is almost suitable for CNG operation. The engine had the main features of being a multivalve, fast-burn pent-roof chamber engine with a variable intake-system geometry. These were particularly interesting due to the lack of data for innovative CNG engine design concepts.A test bench was built and instrumented in an engine hall for typical dynamometer experiments and a water-cooled piezometric transducer was set on the cylinder head for pressure data acquisition in the combustion chamber. A two-zone burn-rate analysis of in-cylinder pressure time-histories was performed by means of a new procedure including the unsteadiness effects of the convective heat transfer process. The combustion process in the CNG engine was thus investigated for different air-fuel ratios and spark advances at WOT and constant speed. The speed was selected to meet maximum bmep at stoichiometric operating condition. Operation with CNG showed the capability of substantial reduction of fuel consumption and pollutants emissions.