The effectiveness generation of HHO gas depends on the characteristic of the electrode plates. This research aims to investigate the effect of the HHO gas generator produced through the water electrolysis technique. The effect of several controlled factors, such as the number, distance, and area of electrode plates used in the water electrolysis process to produce HHO gas, were studied and analyzed using a factorial full design approach (RBFAD). Stainless Steel of 1 mm thickness has been chosen as an electrode plate material. A single-cylinder four-stroke SI engine was used as the test engine. The engine was tested at 16 different setups where the engine speed and exhaust emission of carbon monoxide (CO) and hydrocarbon (HC) gasses were recorded. The statistical significance of these controlled factors contributing to the engine performance and exhaust emission has been established with the analysis of variance. It was found that the number of plates that interacted with a large area of electrode plate is a significant factor that caused an increase in engine speed on an average of 26%, an average reduction in CO by 43%, and a decrease in HC gas by 42%. In conclusion, the higher number of electrode plates and a higher electrode plate area produced more HHO gas that yields completed internal combustion. Thus, it can enhance the engine speed and reduces the CO and HC gas content in exhaust emissions.