Exhaust apparatus for internal combustion engine

Exhaust apparatus for use with internal combustion engines, comprising an outer pipe provided with opposite end portions having diameters nearly the same as that of the engine exhaust pipe, an intermediate portion having a diameter larger than that of said end portions, the respective opposite ends of the outer pipe being joined to said intermediate portion by a pair of oppositely directed tapered sections, an inner pipe contained within said intermediate portion and having a diameter substantially the same as that of said exhaust pipe, said inner pipe being disposed concentrically with respect to the outer pipe and in axial alignment with said exhaust pipe, the respective end margins of the inner pipe being spaced from the inner walls of said tapered sections to provide two parallel concentric spaces for gases moving between said opposite end portions of the outer pipe, means defining a plurality of parallel helical passages extending longitudinally between said end portions in the space between said inner and outer pipes, and at least two groups of helically disposed fins within said inner pipe and positioned respectively near the respective ends thereof.