Internal combustion engines are still the main choice when considering propulsion technology in the transport sector. Spark ignition units offer the advantage of good efficiency with simpler after-treatment systems. Lean operation is a promising strategy that would further improve efficiency, but requires mitigation of cycle-to-cycle variability. Within this context, and given the increasing trend of using simulation based evaluations during engine development, the current work investigated combustion in an optical spark ignition engine through measurements and quasi-dimensional simulation. The possibility of visualizing in-cylinder processes provides unique insight, but also introduces complications with respect to commercial engines. For this reason, quasi-dimensional simulation was applied so as to better understand the factors that induce cycle-to-cycle variability. For the specific case of the investigated engine, cycle-to-cycle measured exhaust air-fuel ratio was found to be directly correlated to variations of engine output. Several routes of incorporating these effects into simulations were evaluated. Introducing a random component in the period of laminar-turbulent flame transition was found to ensure good grounds for simulating peak pressure variability. Indicated mean effective pressure on the other hand was found to depend less on the initial stages of combustion and was strongly correlated to aforementioned variability of exhaust air-fuel ratio.