In-cylinder pressure measurements and indicating diagrams have proven to be a valuable research tool for the analysis of combustion in spark-ignition or compression-ignition engines. With the use of thermodynamic models, the rate of heat release and mass fraction burned curves are calculated, and from the latter the CA50 parameter (crank angle fifty), which is the angle in which 50% of the total fuel has been burned. The empirical process of obtaining the optimum start of combustion typically leads to a value of CA50 from 8° to 10° after top dead center. This paper attempts to numerically investigate which properties have an influence on this optimum CA50. A simple thermodynamic model was implemented which used the Wiebe function for the rate heat release. The CA50 was then evaluated for combustion duration in the base configuration and in a theoretical adiabatic engine. Results showed that the CA50 is mildly sensitive to combustion duration and highly sensitive to the wall heat transfer coefficient, reaching very low values (still always located after top dead center). These lower values provided higher work output in the model. The main conclusion is that the optimal combustion timing of 8° to 10° is due to a compromise between power, wall heat transfer and exhaust gas energy, which prevents lower values.