Synthesis gas (syngas) aided combustion is of increasing interest in boosted lean burn SI engines due to its impact on dilution tolerance and knock resistance. Because of this interest in combustion of reformed fuel, a variety of fuel reforming systems have been developed including in-cylinder reformation, catalytic reformer using waste heat, and solid oxide fuel cell. Because of this interest in reformed fuel, more concrete understanding of how to leverage syngas supplementation is essential to optimize engine performance and derive the most benefit from the availability of syngas in the combustion process. While combustion stability with syngas supplementation has been studied, detailed understanding of the impact of syngas quantity on engine knock is still limited. This study investigates the effect of syngas (H2/CO) addition on knocking tendency under high Exhaust Gas Recirculation (EGR) and air diluted conditions. Syngas amount is controlled on an energy basis from 0% to 15%, to compare the difference between EGR and air diluted combustion with boosted intake pressure and spark timing held constant. Knocking tendencies with respect to the burn duration and combustion phasing are analyzed by the intensity and duration, and heat release analysis, emissions, and further statistical analyses are discussed as well. The results show that as the syngas supplementation ratio increases, knocking tendency in more strongly suppressed with EGR dilution than it is with air dilution.