Orifice Spark Advance Control: Description
Orifice Spark Advance System (OSAC) is used on all light duty emission engines to aid in control of NOx (Oxides of Nitrogen). It controls vacuum to vacuum advance actuator of distributor. A tiny orifice incorporated in OSAC valve delays change in ported vacuum to distributor by about 17 seconds (or 27 seconds on some models) when going from idle to part throttle. When going from part throttle to idle, change in ported vacuum to distributor will be instantaneous.