Experimental optical and thermal tests were carried out in a constant-volume combustion chamber and a single cylinder gasoline direct injection (GDI) engine to obtain a comprehensive understanding of the effects of spark plug electrode gap on flame kernel development, engine performance, mid emissions. High-speed Schlieren Visualization was utilized to study the flame kernel growth at different equivalence ratios. Planar Laser Induced Fluorescence (PLIF) was employed to investigate the combustIOn zone and the flame front development on the horizontal swirl plane after spark ignition. High-speed imaging technique was carried out to study turbulent flame propagation. Combustion analysis, using in-cylinder pressure data and Mass Fraction Burned (MFB) was employed, along with exhaust emissions measurement to obtain a better understanding of the spark plug gap effects on engine performance and emissions. It is found that the flame kernel growth area increases as the spark plug gap increases. PLIF imaging for the combustion process inside the GDI engine demonstrate a larger flame kernel associated With the larger gap. The maximum in-cylinder pressure, turbulent flame speed, heat release rate, and the mass fraction burned increases with the spark plug gap. The engine output increases slightly and the combustion process becomes more stable due to the reduction in cyclic variations as the spark plug gap increases. With the maximum spark plug gap, the engine produces minimum hydrocarbon emissions and particulate number concentration. NOx emissions are increased as the spark plug gap becomes wider due to the higher temperature accompanied with the increase in flame speed and in-cylinder pressure. (C) 2017 Published by Elsevier Ltd.