The thermal efficiency of spark-ignition engines can be enhanced by increasing the rate of exhaust gas recirculation (EGR) such that the low temperature combustion regime could be achieved. However, there is a upper limit on the amount of EGR rate, beyond which flame speed becomes slow and unstable, and local quenching starts to hurt the combustion stability, efficiency, and emission. To resolve this issue, the concept of dedicated EGR has been proposed previously to be an effective way to enhance flame propagation under lean burn condition with even higher levels of EGR with reformate H2 and CO. In this study, the effects of thermochemical fuel reforming on the reformate composition under rich conditions (1.2 < ϕ <2.0) have been studied using detailed chemistry for iso-octane and methane, which are the representative components for gasoline and natural gas, respectively. The rich combustion products are then used to represent the composition of the dedicated EGR, whose influence on laminar flame speed and ignition delay time is further analyzed and reported. Moreover, the performance and the dynamic process of the dedicated EGR in a real SI engine system has been simulated using a one-dimensional model in the commercial software GT-Suite. Parametric studies have been performed to provide guidance on the optimal operation conditions for both fuels with dedicated EGR.