In Spark Ignition engines, the heat release rate is not only piloted by the mixture reactivity but also by its sensitivity to stretch effects. Only few results can be found in the literature about flame stretch effect in SI engine configurations.For this study, three different fuels (Methane, Propane, Iso-octane) were studied, but at different air-fuel lean mixture conditions, to present almost equivalent laminar flame speeds and thermo-dynamical properties at ignition timing condition. Besides those mixtures present different Lewis numbers which are relevant parameters to describe flame-stretch interactions. Mie-scattering tomography was then performed in an optical Spark Ignition (S.I.) engine. Using a high speed camera, flame propagation images were acquired through the piston. Thermodynamic analyses based on in-cylinder pressure traces were performed to estimate in-cylinder temperature and burnt mass fraction during the engine cycle.From the determination of flame areas, the global flame stretch and an equivalent propagation speed have been then defined and estimated. Results prove that mixtures have the same ranking in terms of flame stretch sensitivities as in the laminar regime. Probability density functions of flame curvature are centered on 0.05 mm−1 for the three mixtures whereas differences are observed on the global wrinkling. Finally the impact of the Lewis number on the Burned Mass Fraction curved was studied thus showing a linear relationship between crank angle corresponding to 5 and 10 % of burned mass fraction and the Lewis number.