DNA Motoring HDS-HC88-SL 4-2-1 Exhaust Header for Honda D-Series Made of high quality Steel with 4-2-1 direct bolt-on design TIG welded CNC machine flange O2 sensor bung insertion/ metal gasket included/ inlet: 1.75"/ outlet: 2" Increase high flow air, instantly gain 10 to 15 HP./ TQ Professional installer is highly recommended no instruction included