{"product_id":"stone-exhaust-bmw-n55-f20-f21-m135i-cat-back-valvetronic-exhaust","title":"Stone Exhaust BMW 1 Series M135i N55 F20\/F21 Valved Switchable Cat-Back (Catback) Performance Sports Exhaust | OEM-Fit Stainless Steel | RWD \u0026 xDrive LCI | Back Box\/Flap Exhaust | Valvetronic Optional | UK | AP504A","description":"\u003cdiv class=\"product__description rte\"\u003e\n  \n  \u003ch2\u003eSwitchable Cat Back for BMW 1 Series M135i N55 F20 F21 — Part No. \u003cstrong\u003eAP504A\u003c\/strong\u003e\n\u003c\/h2\u003e\n  \n  \u003cp\u003eGive your turbocharged straight six the soundtrack it deserves with this premium switchable cat back for the BMW 1 Series M135i. Developed around the factory hardware for OE-style integration, it delivers a refined tone for daily use with the valve shut and a crisp, motorsport-inspired note when opened. Exact fit for rear wheel drive and xDrive, including facelift models. If you are shopping for a quality UK performance system that fits right first time and sounds superb, this is a top-tier choice.\u003c\/p\u003e\n  \n  \u003cp\u003e\u003cstrong\u003ePart Number:\u003c\/strong\u003e AP504A\u003c\/p\u003e\n  \n  \u003ch3\u003eWhy this performance system stands out\u003c\/h3\u003e\n  \u003cp\u003eStone has been building high quality exhausts since 1998, combining 3D scanning, reverse engineering and in-house dyno validation to create systems that bolt up like factory and elevate character without the headaches. We have fitted multiple kits at MJ Modifications in Warwickshire and customer feedback has been consistently excellent for fit, finish and sound quality.\u003c\/p\u003e\n  \n  \u003ch3\u003eValve control your way\u003c\/h3\u003e\n  \u003cp\u003eThe valve interfaces with the vehicle electronics for factory-style operation. Drivers wanting manual control can add the optional \u003cstrong\u003eStone Valvetronic\u003c\/strong\u003e single-valve setup with plug and play OBD controller and remote fobs, letting you choose quiet or loud at the press of a button. Ideal for early starts, long motorway runs and weekend blasts.\u003c\/p\u003e\n  \n  \u003ch3\u003eMain benefits\u003c\/h3\u003e\n  \u003cul\u003e\n    \u003cli\u003eSwitchable flap system for two distinct sound profiles with OE-style control\u003c\/li\u003e\n    \u003cli\u003eFlows from the catalytic converter back for a balanced, resonant tone\u003c\/li\u003e\n    \u003cli\u003eOptional Valvetronic remote kit with OBD plug for on-demand control\u003c\/li\u003e\n    \u003cli\u003eOne piece layout for smooth flow and straightforward installation\u003c\/li\u003e\n    \u003cli\u003eInterior resonance reduced compared with the standard rear section\u003c\/li\u003e\n    \u003cli\u003eAccurate fit on F20 and F21 platforms, rear wheel drive and xDrive, including LCI\u003c\/li\u003e\n    \u003cli\u003eIn-house R\u0026amp;D, dyno checked and sound tuned for the N55 straight six\u003c\/li\u003e\n    \u003cli\u003ePremium finish with precision welding for long term durability\u003c\/li\u003e\n  \u003c\/ul\u003e\n  \n  \u003ch3\u003eEngineering and development\u003c\/h3\u003e\n  \u003cp\u003eOEM parts are 3D scanned to capture exact geometry, diameters and routing. Using simulation tools, engineers optimise gas flow, pressure distribution and temperature behaviour, then verify on the dyno and the road for dependable results and an authentic tone that complements the M135i.\u003c\/p\u003e\n  \u003cp\u003e\u003cimg src=\"https:\/\/cdn.shopify.com\/s\/files\/1\/0284\/6300\/5795\/files\/1_9336c8b0-c9ea-4822-891c-83490f630707.jpg?v=1572705528\" alt=\"3D scanning and reverse engineering for BMW 1 Series M135i N55 exhaust\"\u003e\u003c\/p\u003e\n  \n  \u003ch3\u003e7 axis CNC bending\u003c\/h3\u003e\n  \u003cp\u003eFormed on 7 axis CNC bending equipment for aerospace-level precision. The process achieves up to 98 percent roundness with outer bore wear controlled to roughly 25 percent at tight 180 degree bends. With an accuracy error under 0.025 mm and integrally formed pipework, airflow disruption is minimised and weld stress is reduced.\u003c\/p\u003e\n  \u003cp\u003e\u003cimg src=\"https:\/\/cdn.shopify.com\/s\/files\/1\/0284\/6300\/5795\/files\/bending.jpg?v=1572739395\" alt=\"CNC bending on M135i F20 F21 performance exhaust\"\u003e\u003c\/p\u003e\n  \n  \u003ch3\u003eAdvanced welding\u003c\/h3\u003e\n  \u003cp\u003eHigh temperature joints use crisscross and overlapping techniques to increase width and penetration, while direct spot welding by mechanical arms improves consistency and strength. The result is a robust, clean finish that resists cracking and looks as good as it sounds.\u003c\/p\u003e\n  \u003cp\u003e\u003cimg src=\"https:\/\/cdn.shopify.com\/s\/files\/1\/0284\/6300\/5795\/files\/Stone_Exhaust_Art_of_Welding_-_1_1024x1024.jpg?v=1572828150\" alt=\"Precision welding on BMW M135i N55 system\"\u003e\u003c\/p\u003e\n  \u003cp\u003e\u003cimg src=\"https:\/\/cdn.shopify.com\/s\/files\/1\/0284\/6300\/5795\/files\/Stone_Exhaust_Art_of_Welding_-_2_1024x1024.jpg?v=1572828178\" alt=\"Direct spot welding details on BMW 1 Series exhaust\"\u003e\u003c\/p\u003e\n  \n  \u003ch3\u003eVehicle compatibility\u003c\/h3\u003e\n  \u003cul\u003e\n    \u003cli\u003eBMW 1 Series F20 M135i and M135i xDrive 2011+\u003c\/li\u003e\n    \u003cli\u003eBMW 1 Series F20 LCI M135i and M135i xDrive 2015+\u003c\/li\u003e\n    \u003cli\u003eBMW 1 Series F21 M135i 2011+ and M135i xDrive 2012+\u003c\/li\u003e\n    \u003cli\u003eBMW 1 Series F21 LCI M135i and M135i xDrive 2015+\u003c\/li\u003e\n  \u003c\/ul\u003e\n  \n  \u003ch3\u003eFitting and setup\u003c\/h3\u003e\n  \u003cp\u003eThe tailpipe section is designed to be trimmed to suit different diffuser options, giving you flexibility on the rear look. Professional installation is recommended.\u003c\/p\u003e\n  \u003cp\u003e\u003cstrong\u003eNeed this installed\u003c\/strong\u003e We offer expert fitting at \u003cstrong\u003eMJ Modifications\u003c\/strong\u003e in Warwickshire. Request a quote at \u003ca href=\"mailto:info@mjmodifications.co.uk\"\u003einfo@mjmodifications.co.uk\u003c\/a\u003e.\u003c\/p\u003e\n  \n  \u003ch3\u003eVideo\u003c\/h3\u003e\n  \u003cp\u003e\u003cstrong\u003eBMW N55 F20 M135i with Stone catless downpipe plus Valvetronic cat back sound\u003c\/strong\u003e\u003c\/p\u003e\n  \u003cp\u003e\u003ciframe width=\"1182\" height=\"665\" src=\"https:\/\/www.youtube.com\/embed\/_L1ekU3Vj9I\" title=\"BMW M135i N55 switchable cat back exhaust sound with Valvetronic\" frameborder=\"0\" allow=\"accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture\" allowfullscreen\u003e\u003c\/iframe\u003e\u003c\/p\u003e\n  \n  \u003ch3\u003eWarranty\u003c\/h3\u003e\n  \u003cp\u003eManufacturer warranty: 2 years.\u003c\/p\u003e\n  \n  \u003ch3\u003ePopular search phrases\u003c\/h3\u003e\n  \u003cp\u003eM135i sports exhaust UK, M135i switchable exhaust, BMW 1 Series performance exhaust F20 F21, N55 cat back UK, M135i sound upgrade, BMW 1 Series back box upgrade, M135i flap exhaust, M135i exhaust with valve control, F20 M135i performance system, F21 M135i stainless catback, M135i xDrive exhaust upgrade, LCI M135i exhaust UK, BMW 1er M135i Abgasanlage, AP504A Stone Exhaust, plug and play OBD valve controller, M135i rear section system, BMW M Performance alternative, UK-fitted M135i exhaust, turbo six exhaust upgrade.\u003c\/p\u003e\n  \n  \u003ch3\u003eManufacturer\u003c\/h3\u003e\n  \u003cp\u003eStone Exhaust\u003c\/p\u003e\n  \n  \u003ch3\u003ePart number\u003c\/h3\u003e\n  \u003cp\u003e\u003cstrong\u003eAP504A\u003c\/strong\u003e\u003c\/p\u003e\n  \n\u003c\/div\u003e","brand":"Stone Exhaust","offers":[{"title":"OEM Integrated Valve \/ Flat Cut Gloss Carbon Fiber (82mm)","offer_id":56008706654594,"sku":null,"price":2584.32,"currency_code":"GBP","in_stock":true}],"thumbnail_url":"\/\/cdn.shopify.com\/s\/files\/1\/0871\/5445\/8937\/files\/8200CG_FlatCutGlossCarbonFibre_102mm-2_1946x_394740de-4217-4859-b96b-6649f6839982.png?v=1764091078","url":"https:\/\/mjmodifications.co.uk\/products\/stone-exhaust-bmw-n55-f20-f21-m135i-cat-back-valvetronic-exhaust","provider":"MJ Modifications","version":"1.0","type":"link"}