Exhaust system for engine

An exhaust system for an engine, comprising:

• a supercharger having an exhaust turbine housing and an air compressor housing;
• an intake pipe connected to the air compressor housing;
• an exhaust relay pipe;
• and
• a cast-iron exhaust manifold having a collector, the collector including a plurality of exhaust inlet flanges and a single exhaust outlet flange;
• wherein:
• a direction of a crankshaft of the engine defines a front-to-rear direction, a lateral direction being oriented orthogonally to the front-to-rear direction,
• the plurality of exhaust inlet flanges being positioned along a lateral side of the collector and the exhaust outlet flange being positioned along a top side of the collector,
• the intake pipe, the air compressor housing, the exhaust turbine housing and the exhaust relay pipe being placed in line, above the collector, in order from front to rear,
• the exhaust outlet flange being positioned closer to a rear end of the collector than a front end of the collector,
• the exhaust turbine housing being fastened to the exhaust outlet flange;
• the exhaust relay pipe being fastened to the collector rearward of the exhaust outlet flange;
• and
• the exhaust relay pipe being fastened to a rear of the exhaust turbine housing;
• and
• a screw fastener that fastens the exhaust turbine housing to the exhaust outlet flange, the exhaust outlet flange having an upper surface that includes a pressure receiving surface that receives a fastening force of the screw fastener,
• wherein an oxide film of triiron tetraoxide is formed on the pressure receiving surface, the oxide film being configured to resist plastic deformation by the fastening force of the screw fastener, thereby preventing the fastening force of the screw fastener from decreasing.