DNA Motoring HDS-HC88-H Stainless Steel 4-1 Exhaust Drag Header for Honda D-Series Made of high quality Steel with 4-1 direct bolt-on tubular design TIG welded CNC machine flange O2 sensor bung insertion/ metal gasket included/ inlet: 1.5"X1.25" (oval)/ outlet: 2.375"/ main piping diameter: 2.5"/ extension adaptor pipe included Increase high flow air, instantly gain 10 to 15 HP./ TQ Professional installer is highly recommended no instruction included