In this study, detailed investigations of scavenged prechamber engine combustion are performed experimentally in a Rapid Com-pression Expansion Machine (RCEM), which allows optical access into the main chamber. OH* chemiluminescence measurements combined with pressure measurements are used to study the effect of varying ignition timing on combustion and cycle-to-cycle varia-tions. The variation of ignition timing (pressure at ignition) showed an optimum ignition point for a given injection duration. Earlier ignition resulted in weaker but more reactive jets, coupled to increased cyclic variations. Later ignition did not significantly affect heat release rate, but increased cyclic variation.