The performance of a spark ignition engine strongly depends on the phase of the combustion process with respect to piston motion, and hence on the spark advance; this fundamental parameter is actually controlled in open-loop by means of maps drawn up on the test bench and stored in the Electronic Control Unit (ECU). Bi-fuel engines (e.g. running either on gasoline or on natural gas) require a double mapping process in order to obtain a spark timing map for each of the fuels. This map based open-loop control however does not assure to run the engine always with the best spark timing, which can be influenced by many factors, like ambient condition of pressure, temperature and humidity, fuel properties, engine wear. A feedback control instead can maintain the spark advance at its optimal value apart from operative and boundary conditions, so as to gain the best performance (or minimum fuel consumption). Such a control can be realized using as pilot variable a combustion phase indicator, i.e. a parameter which depends exclusively on the phase of the heat release process and assumes a fixed value for optimal spark timing. The purpose of the present work is to compare the behaviour of the most used combustion phase indicators using two different fuels one after the other (common gasoline and Compressed Natural Gas, CNG) on the same engine, in order to assess the influence of different heat release progress and to verify the possibility to feedback control the spark timing apart from the fuel used. The comparison has been carried on by means of experimental test on the engine test bench, analysing in-cylinder pressure acquired with varying spark advance for different operative conditions of engine speed, load and air-to-fuel ratio.