Stone Exhaust Valvetronic Cat-Back for BMW N55 F25/F26 — X3 35i xDrive, X4 35i xDrive, X4 M40i xDrive | Valved Performance Exhaust | Plug & Play OBDII Controller | AP531S/AP531BSQ

Switchable Cat Back System for BMW F25 F26 N55 - X3 35i xDrive, X4 35i xDrive, X4 M40i xDrive

Unlock the soundtrack and response your turbocharged straight six deserves. This Stone valve-controlled cat back transforms the F25 and F26 platform with a deep performance tone under load and calm, refined manners on the motorway. Developed using 3D scanning, reverse engineering and in-house dyno validation, it delivers OEM-style fitment, smooth flow paths and the flexibility of quiet or loud at the touch of a button.

Ideal for drivers searching for a switchable sports exhaust for BMW X3 35i or X4 35i, a flap-controlled system for the X4 M40i, or a plug in OBD port controller solution for the N55B30 engine. If you are comparing rear section upgrades, muffler replacements, performance systems for the X3 petrol turbo or X4 petrol turbo, this is the premium option that balances civility with genuine performance character.

Vehicle compatibility

• BMW X3 F25 35i xDrive - N55
• BMW X4 F26 35i xDrive - N55
• BMW X4 F26 M40i xDrive - N55

What makes it stand out

• Switchable valves with remote control - choose quiet, loud or automatic operation
• Comfortable, low-drone cruising with a rich, aggressive tone under throttle
• OBD port plug in module - no cutting, no splicing, simple installation
• Automatic mode can be set to open at your chosen RPM for the best of both worlds
• 3D-scanned and reverse-engineered for true OEM-like fit and alignment
• Dyno-developed to sharpen response and optimise flow on the N55 platform
• Precision mechanical welding with long, smooth bends for consistency
• 7-axis CNC-bent sections promote roundness and reduce turbulence

Valve control - sound on your terms

• ON - valves open for maximum flow and an authentic straight six soundtrack
• OFF - valves closed for neighbour-friendly cold starts and long journeys
• AUTO - user-defined RPM trigger opens the valves when you want the noise
• True plug in OBDII controller - quick to install and easy to live with

Engineering detail that matters

• CAD-led development for perfect F25 and F26 chassis integration
• Flow path simulation guides pipe routing and diameters for smooth gas speed
• CNC bending keeps the tube profile round for cleaner, faster flow
• Advanced weld techniques provide strength and durability at high temperature

Tailpipe options and part numbers

• AP531S - Dual exit with twin tailpipes - requires M40i rear valance
• AP531BSQ - Dual exit with quad tailpipes - requires M40i rear valance

Installation

• Direct bolt-on using OEM-style hangers and mounts for F25 X3 and F26 X4
• OBDII controller installs in minutes
• Professional fitting recommended to ensure perfect tip alignment and clearance

Prefer a hassle-free install by specialists who know these cars inside out? Book fitting with MJ Modifications in Warwickshire. Request a quote: info@mjmodifications.co.uk.

Tried and loved by our customers

We have supplied and fitted these Stone systems on X3 and X4 N55 models and feedback has been consistently brilliant. Owners praise the premium sound quality, the calm cruising with valves shut and the instant drama at the press of a button. If you are searching for a switchable sports exhaust for BMW X3 35i, X4 35i or X4 M40i in the UK, this is the upgrade we recommend.

Warranty

Stone Exhaust 2 year warranty.

Brand

Stone Exhaust

Watch and listen

Stone Valvetronic System Installation

BMW X4 35i catless downpipe sound with Stone system

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