Stone Exhaust BMW N55 F20 F21 M135i OEM Integrated Valved Cat-back Exhaust System
If you are searching for the best BMW M135i exhaust, BMW M135i cat-back exhaust, BMW F20 M135i exhaust, BMW F21 M135i exhaust, N55 M135i valved exhaust or an OEM-fit performance exhaust for the M135i in the UK, this Stone Exhaust system is the upgrade you want. Precision engineered for the BMW 1 Series M135i with the N55 engine, this OEM integrated valved cat-back delivers a refined daily-drive tone with the valve closed and an aggressive soundtrack when open, all with exact OEM fitment and proven performance development.
Part Number: AP504A
Why choose the Stone Exhaust M135i cat-back
Since 1998, Stone Exhaust has been developing world-class performance exhaust systems using 3D scanning, reverse engineering and in-house dyno validation. This BMW M135i N55 cat-back exhaust is designed around the factory system for 100 percent OEM fitment and seamless integration with the car’s electronics. The result is a BMW M135i valved exhaust that enhances sound and throttle response, eliminates cabin drone compared to the standard exhaust and maintains everyday usability.
We have installed this system on customer cars at Nforcd and feedback has been consistently positive for fitment quality, finish and sound. If you want a premium BMW N55 M135i performance exhaust that looks and sounds right, this is it.
OEM integrated valve control with optional Valvetronic
The OEM integrated valve allows the ECU to control the exhaust valve automatically, just like the factory system. For drivers who want manual control, you can add the Stone Valvetronic System (single valve) with a plug-and-play OBDII controller and remote. This gives you on-demand control of quiet or loud modes, ideal for daily driving, long motorway journeys or weekend blasts.
Key features and benefits
- BMW M135i N55 cat-back exhaust with OEM integration for factory-like operation
- Valved exhaust for the BMW M135i - quiet when you need it, aggressive when you want it
- Optional Stone Valvetronic System with plug-and-play OBDII controller
- One piece design for clean flow and easy installation
- Precision mechanical welding for strength and durability
- In-house research and development, 3D scanning and reverse engineering
- Dyno developed in-house by Stone Exhaust
- Designed to eliminate the drone found on the standard BMW M135i exhaust system
- Exact OEM fitment for BMW F20 M135i and BMW F21 M135i, including xDrive and LCI models
R&D and 3D scanning
Stone Exhaust engineers reverse engineer OEM parts by 3D scanning to understand factory geometry, pipe diameter, routing and bend angles across the N55 exhaust path. Using 3D simulation software, they optimise exhaust gas flow, thermal behaviour and pressure zones, then validate on the dyno to deliver reliable performance and sound.
7 axis CNC bending for accuracy
Constructed with a 7 axis CNC bending machine for aerospace-level precision. Stone achieves an industry-leading 98 percent roundness and controls outer bore wear to only 25 percent at 180 degrees. With integrally moulded pipework and an accuracy error under 0.025 mm, airflow obstruction is minimised and weld fracture risks are reduced.
Advanced welding techniques
Critical joints see extreme temperatures, so Stone uses crisscross and overlapping welding to increase weld width and penetration. Direct spot welding by mechanical welding arms further improves precision and strength for long-term reliability and a premium finish.
Compatibility - BMW N55 M135i
- 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+
Installation and fitting
This exhaust is adjustable to enable use with different diffusers. The tailpipe section is designed to be cut to suit your desired diffuser setup.
Professional fitting available at Nforcd in Warwickshire. Request a fitting quote at Sales@nforcd.com.
Video
BMW N55 F20 M135i with Stone Exhaust Catless Downpipe + Valvetronic Cat-back System Sound
Warranty
Stone Exhaust 2 year warranty.
At a glance - search terms customers use
BMW M135i exhaust, BMW M135i cat back, BMW F20 M135i exhaust, BMW F21 M135i exhaust, N55 M135i exhaust, BMW M135i valved exhaust, BMW M135i performance exhaust UK, Stone Exhaust AP504A, OEM integrated valve exhaust, Valvetronic M135i exhaust, BMW 1 Series M135i catback.
Manufactured by
Stone Exhaust
Part number
AP504A