Abstract: A tuned exhaust gas relief system for marine propulsion systems, for example an outboard motor, includes a lower drive shaft housing coupled to a two stroke engine by a pair of intermediate stacked exhaust extension plates. The housing directs the exhaust gas downwardly to a through-the-hub exhaust propeller for exit therethrough. With the unit in reverse or idling, exhaust gases are trapped within the housing. A pair of tuned exhaust relief passageways may be formed by cavities in the mating faces of the two extension plates with a pair of inlet openings in the lower wall of the bottom plate. A baffle member may overlie the inlet openings. The passageways define constant cross-sectional area channels which terminate in exhaust openings in the rear wall of the drive shaft housing. A resonant cavity, which may be integrally formed by mating recesses in the opposed faces of the extension plates, is in close spaced relation to each idle exhaust passageway.