The current decrease in fossil energy resources requires a diversification of the liquid and gaseous fuels potentially consumable in internal combustion engines. The use of these fuels modifies the combustion process and the heat released as well. In a Spark Ignition (SI) engine, the heat released is mainly piloted not only by the mixture reactivity but also by its sensitivity to stretch effects. Only a few results can be found in the literature about stretch effects for SI engine configurations. The purpose of the present paper is to evaluate stretch effects on the flame front propagation in an optical SI engine and to investigate the relative importance of these effects depending on the fuel considered.Different air-fuel mixtures presenting different flame stretch sensitivities were selected. Four different engine regimes (1400, 1600, 1800 and 2000 rpm) were studied for all the mixtures in order to evaluate the impact of different turbulence intensities. In-cylinder pressure analyses were performed to determine the heat release rate and the crank angle corresponding to 5% of mass burned. At the same time, direct visualizations of the flame through the piston were performed using an intensified high-speed video camera. From the recorded images, a global flame stretch and an equivalent propagation speed were defined and their evolution studied. An increase in the stretch rate is observed for higher engine speeds but the combustion process for the mixtures presenting a strong sensitivity to stretch is slowed down when the regime is increased.