PURPOSE:To improve output in intermediate and low speed revolution ranges, by mounting a ring-shaped throttle, which form and location are accurate, to an upper stream portion of an exhaust pipe. CONSTITUTION:The upper stream portion materials 11 of an exhaust passage 3 with an upper stream end 2 are provided with a pair of plate portion materials 12, 12, which are divided into two portions along the axial direction, and exhaust passage portions 3b, 3b, which cross sections are semi-circular and which diameters are small, semi-circular portions 1a, 1a, which project to the inner surfaces of these fixed locations, and flanges 12a, 12a, which project in the axial direction at the outsides of both side edges of the exhaust passage portions 3b, 3b, are integrally formed in the plate portion materials 12, 12. Flanges 12a, 12b are welded to a pair of plate portion materials 12, 12, and the exhaust passage 3 is made up by means of the exhaust passages 3b, 3b and a throttle portion 1 by means of the semi-circular portions 1a, 1a. Thus, the shape and locations of an exhaust portion and the throttle portion are molded accurately and easily by press working the plate portion materials 12.
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