Due to growing concerns about air pollution and climate change, stricter emission regulations for road vehicles are affecting the diesel vehicle market, especially in passenger car and light-duty vehicle segments. As a consequence of this, spark ignition (SI) engines are outnumbering the compression-ignition (CI) engines in those segments. But low compression ratio and operating at air–fuel ratio closer to stoichiometric, makes SI engines less efficient than CI engines. Lean combustion can boost SI engine efficiency, however, extreme lean condition increases instability in the combustion. The concept of prechamber ignition is a potential solution to achieve lean combustion by overcoming its drawbacks. The prechamber design is a critical factor that controls the combustion efficiency. Therefore, in this research work, a new prechamber design with two different volumes of 2 and 5% of that of clearance volume was proposed to improve efficiency and reduce the exhaust emissions of the gasoline direct injection (GDI) engine. For this purpose, a numerical investigation was performed using the STAR-CD platform on the proposed model. The engine simulation was carried out for full load condition at 1000 rpm with a stoichiometric air-fuel ratio. Initially, the numerical results of the baseline engine model (four-valve Siamese engine without pre-chamber) were validated using engine experiment results. Then the numerical study was performed to study the effect of pre-chamber design on the mixture distribution, combustion, and emission parameters and the results were compared with that of the base engine model. The results revealed that the model with 2% pre-chamber volume had shown better fuel evaporation and thereby resulted in better homogeneous mixture formation at the start of combustion when compared to the 5% pre-chamber model. However, both models showed inferior evaporation and mixture formation compared to the baseline model. The ignition delay time was decreased for pre-chamber models, with 5% model being the least. In terms of in-cylinder pressure, the 2% volume had higher pressure than the other two models. Furthermore, the carbon dioxide (CO2) was almost similar for all the models. The formation of carbon monoxide (CO) compound was lesser for the 5% pre-chamber volume model than in other models. However, the 5% model showed the highest nitrogen oxide (NO) and soot formation. Overall, the model with 2% pre-chamber volume had shown better performance than the other two models.