preferred embodiment of the present invention described above in conjunction with fig1 7 promotes a generally uniform flow of exhaust gas through the catalyst module whether the catalyst module comprises a plurality of catalyst devices 23 25 or whether it comprises a single larger catalyst device 123 with reference to fig2 and 6 the exhaust gas e flowing into the exhaust manifold 22 or first exhaust conduit flows from regions of relatively smaller cross sectional area e g the exhaust ports of the engine to a plenum area 44 having a greater cross sectional area as a result of this increase in cross sectional area along the path of the exhaust gas e the velocity of the exhaust gas decreases this allows the exhaust to more uniformly seek areas of lower pressure along the inlet surfaces of the catalyst devices 23 25 in other words without the plenum area 44 the exhaust gas stream would be more subject to the influences of gas stream velocity and momentum that could urge the exhaust to flow through limited portions of the inlet area of the catalyst devices however when a plenum 44 is provided the velocity of the gas stream slows and allows the exhaust to more uniformly seek lower pressure areas along the inlet surfaces of the catalyst devices rather than directing the exhaust gas stream with a restrictive conduit the expanded area 44 of.