See Figures 1, 2, 3, 4, 5 and 6
The OSAC system is used on light duty trucks to aid in control of nitrous oxide (NOx) emissions. The system control the vacuum signal to the distributor vacuum advance unit.
A tiny orifice is incorporated in the OSAC valve which delays the change in ported vacuum to the distributor by 17 to 27 seconds, depending on the engine and truck model, when going from idle to part throttle.
When going from part throttle to idle, the change in ported vacuum is instantaneous.