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Stone Exhaust

Stone Exhaust BMW 1 Series M135i N55 F20/F21 Valved Switchable Cat-Back (Catback) Performance Sports Exhaust | OEM-Fit Stainless Steel | RWD & xDrive LCI | Back Box/Flap Exhaust | Valvetronic Optional | UK | AP504A

Stone Exhaust BMW 1 Series M135i N55 F20/F21 Valved Switchable Cat-Back (Catback) Performance Sports Exhaust | OEM-Fit Stainless Steel | RWD & xDrive LCI | Back Box/Flap Exhaust | Valvetronic Optional | UK | AP504A

£2,584.32
£3,230.40 £2,584.32
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Exhaust Valve System
Tailpipe Finish

Heads-up on cancellations & returns: if you cancel an order or return an item, you won't get back 100% of what you paid. We apply a 10% restocking fee, the outbound shipping is non-refundable, and we have to pass on Shopify / PayPal payment processing fees; these aren't returned to us when we refund you. International orders also lose on exchange-rate spread in both directions. If the original packaging is damaged or missing, a reboxing charge applies. For international orders specifically, if you refuse to pay our shipping invoice or supply a prepaid label after placing the order, the same deductions apply. We're upfront about this — please be sure of your purchase before ordering.

Switchable Cat Back for BMW 1 Series M135i N55 F20 F21 — Part No. AP504A

Give 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.

Part Number: AP504A

Why this performance system stands out

Stone 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.

Valve control your way

The valve interfaces with the vehicle electronics for factory-style operation. Drivers wanting manual control can add the optional Stone Valvetronic 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.

Main benefits

  • Switchable flap system for two distinct sound profiles with OE-style control
  • Flows from the catalytic converter back for a balanced, resonant tone
  • Optional Valvetronic remote kit with OBD plug for on-demand control
  • One piece layout for smooth flow and straightforward installation
  • Interior resonance reduced compared with the standard rear section
  • Accurate fit on F20 and F21 platforms, rear wheel drive and xDrive, including LCI
  • In-house R&D, dyno checked and sound tuned for the N55 straight six
  • Premium finish with precision welding for long term durability

Engineering and development

OEM 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.

3D scanning and reverse engineering for BMW 1 Series M135i N55 exhaust

7 axis CNC bending

Formed 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.

CNC bending on M135i F20 F21 performance exhaust

Advanced welding

High 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.

Precision welding on BMW M135i N55 system

Direct spot welding details on BMW 1 Series exhaust

Vehicle compatibility

  • BMW 1 Series F20 M135i and M135i xDrive 2011+
  • BMW 1 Series F20 LCI M135i and M135i xDrive 2015+
  • BMW 1 Series F21 M135i 2011+ and M135i xDrive 2012+
  • BMW 1 Series F21 LCI M135i and M135i xDrive 2015+

Fitting and setup

The tailpipe section is designed to be trimmed to suit different diffuser options, giving you flexibility on the rear look. Professional installation is recommended.

Need this installed We offer expert fitting at MJ Modifications in Warwickshire. Request a quote at info@mjmodifications.co.uk.

Video

BMW N55 F20 M135i with Stone catless downpipe plus Valvetronic cat back sound

Warranty

Manufacturer warranty: 2 years.

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Manufacturer

Stone Exhaust

Part number

AP504A

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