This study concerns experimental and theoretical analysis of early stages of the flame kernel development and subsequent stages of combustion and peak pressure in a spark-ignition engine. The simultaneous measurement of engine operating conditions, pressure traces and sequences of combustion images have been made in a single cylinder four stroke engine, in particular, at part load and under lean burn conditions. The early stages of the combustion have been analyzed using a new image analysis methods. These techniques can measure the total kernel growth, the thermal expansion part, the local translational velocity of the centroid, stretching of the flame kernel surface and its roughness as well as the flame contact area with the electrodes. In addition, the fraction of the flame surface area supporting propagation has been determined from the directional variation of flame propagation between successive image frames. The pressure traces measurements were compared with those of theoretical thermodynamic model for the flame kernel development, the latter stages of the combustion process and its subsequent pressure rise. Strong correlation have been demonstrated between the average value of the flame kernel surface area supporting propagation at the early stages of development (typically 0.5-1 ms after spark onset), the overall combustion duration, and the peak pressure. These results demonstrate clear cut evidence that the variability in the stretch of the flame kernel surface area (which influenced by the fluid mechanics and chemical kinetics) is a critical parameter affecting subsequent peak pressure cyclic variability.