As vehicle fuel economy requirements continue to increase it is becoming more challenging and expensive to simultaneously improve fuel consumption and meet emissions regulations. The Passive Ammonia SCR System (PASS) is a novel aftertreatment concept which has the potential to address NOx emissions with application to both lean SI and stoichiometric SI engines. PASS relies on an underfloor (U/F) SCR for storage of ammonia which is generated by the close-coupled (CC) TWCs. For lean SI engines, it is required to operate with occasional rich pulses in order to generate the ammonia, while for stoichiometric application ammonia is passively generated through the toggling of air/fuel ratio. PASS serves as an efficient and cost-effective enhancement to standard aftertreatment systems.For this study, the PASS concept was demonstrated first using lab reactor results which highlight the oxygen tolerance and temperature requirements of the SCR. It was found that ammonia-SCR performance would be improved significantly with only a few hundred ppm level oxygen, which is a critical requirement for stoichiometric SI engine application. The concept was then evaluated in the context of the aftertreatment system of a spark-ignition direct-injection (SIDI) V8 engine running the hot FTP cycle. The results show that it is possible to achieve incremental fuel economy improvements while using PASS to meet necessary emissions limits. In the specific case of the work demonstrated here, Passive SCR was combined with SIDI-enabled Deceleration Fuel Cut-Off (DFCO) and Lean-Idle (LI) operating strategies to achieve a greater than 11% fuel consumption improvement over purely stoichiometric SIDI operation. This was accomplished while meeting Tier 2 Bin 3 emissions limits with a lower PGM material requirement compared to a conventional SI aftertreatment system.