A process to adjust the ignition timing of an air-fuel mixture in a combustion chamber of an internal combustion engine, the process comprising the steps of a. determining a first quantity indicative of a pressure of the mixture for a cycle of the engine, b. determining a second quantity indicative of a speed of the engine, c. determining a third quantity indicative of a first temperature of a conditioning fluid used to thermally condition the combustion chamber at a zone of heat exchange between the engine and the conditioning fluid, d. providing a heat exchange mathematical model for the combustion chamber, the model mapping the set of three quantities, consisting of the first, the second and the third quantity, onto a fourth quantity indicative of a second temperature of a wall portion (3, 5) of the engine around the combustion chamber, e. estimating the fourth quantity by means of the three determined quantities and by means of the mathematical model, and f. adjusting the ignition timing as a function of the fourth estimated quantity, characterized in that the mathematical model maps said three quantities onto the fourth quantity by means of a numerical integration of a heat flow flowing through the wall portion (3, 5), the heat flow being modelled as a flow resulting from the presence of a first and a second heat flow modelled as respective products of a first and a second heat exchange coefficient, with a first and a second temperature difference respectively, wherein - the first temperature difference is between the second temperature and a third temperature of the mixture corresponding to said pressure; and - the second temperature difference is between the first temperature and the second temperature, and/or in that step f comprises the steps of f1. determining the second temperature by means of the fourth quantity, f2. determining a fifth quantity indicative of a relationship between the second temperature and a fifth reference or calibration temperature for the wall portion (3, 5), and f3. adjusting the ignition timing depending on the fifth quantity. a. determining a first quantity indicative of a pressure of the mixture for a cycle of the engine, b. determining a second quantity indicative of a speed of the engine, c. determining a third quantity indicative of a first temperature of a conditioning fluid used to thermally condition the combustion chamber at a zone of heat exchange between the engine and the conditioning fluid, d. providing a heat exchange mathematical model for the combustion chamber, the model mapping the set of three quantities, consisting of the first, the second and the third quantity, onto a fourth quantity indicative of a second temperature of a wall portion (3, 5) of the engine around the combustion chamber, e. estimating the fourth quantity by means of the three determined quantities and by means of the mathematical model, and f. adjusting the ignition timing as a function of the fourth estimated quantity, - the first temperature difference is between the second temperature and a third temperature of the mixture corresponding to said pressure; and - the second temperature difference is between the first temperature and the second temperature, and/or in that step f comprises the steps of f1. determining the second temperature by means of the fourth quantity, f2. determining a fifth quantity indicative of a relationship between the second temperature and a fifth reference or calibration temperature for the wall portion (3, 5), and f3. adjusting the ignition timing depending on the fifth quantity. and/or in that step f comprises the steps of f1. determining the second temperature by means of the fourth quantity, f2. determining a fifth quantity indicative of a relationship between the second temperature and a fifth reference or calibration temperature for the wall portion (3, 5), and f3. adjusting the ignition timing depending on the fifth quantity.