The BMW G80 M3 and G82/G83 M4 have the performance to make every drive memorable. Their S58 twin-turbocharged inline-six delivers serious power, but for many owners, the factory exhaust leaves room for a more distinctive sound.

That is where our MAD 4-Inch FATBOY Single Midpipe comes in.

Built specifically for the G8X M3 and M4, the FATBOY combines a full-length 4-inch single center section with a minimum pipe diameter of 3 inches throughout. It is designed to maintain exhaust flow while giving the S58 a deeper, more aggressive voice with reduced rasp.

Whether you are upgrading a stock exhaust setup or adding the finishing touch to a more extensive build, the FATBOY gives your M car a new sound and presence.

A Full-Length 4-Inch Center Section

The FATBOY name comes from the midpipe’s defining feature: its full-length single 4-inch center section.

The midpipe carries exhaust gases between the downpipes and the rear section of the exhaust. Its design plays an important role in both the path those gases take and the sound that reaches the back of the car.

Our single-pipe layout is designed around maintaining exhaust flow through the center of the system. With a minimum diameter of 3 inches throughout, the FATBOY provides a generously sized path from front to back.

For S58 owners planning a complete exhaust setup, it is a substantial centerpiece that brings together flow-focused design and a distinctive sound.

A Deeper, More Aggressive S58 Sound

The S58 has plenty of performance, and the right exhaust configuration can bring more of that character into the driving experience.

The MAD FATBOY Midpipe is designed to give the G80 M3 and G82/G83 M4 a deeper, more aggressive exhaust note, with significantly less rasp. The goal is a sound that feels strong and purposeful, whether you are accelerating through the gears or enjoying the car at a more relaxed pace.

The rest of your exhaust setup will also influence the result. Your choice of downpipes and axle-back can change the volume and character, making the FATBOY a foundation for a sound tailored to your build.

Reduced Rasp with Stock or Aftermarket Downpipes

One of the challenges of building an S58 exhaust system is finding the sound you want without introducing excessive rasp.

Our FATBOY Midpipe is designed to produce a more refined exhaust tone with reduced rasp, including when paired with aftermarket downpipes. It can also be used with the factory downpipes, giving owners different ways to approach their exhaust upgrade.

If you are starting with a largely stock car, the midpipe can provide a noticeable change in exhaust character. If you already have aftermarket downpipes, it offers a way to develop the sound of your existing setup further.

The final volume and tone depend on the complete system, so consider how each component contributes to the sound you are looking for.

Keep Your Factory Axle-Back or Upgrade It

An exhaust build does not have to happen all at once. Some owners prefer to start with the midpipe and keep the factory rear section, while others already have an aftermarket axle-back they want to retain.

The MAD FATBOY Midpipe includes 3-inch and 2.75-inch reducer couplers to connect to compatible 3-inch aftermarket axle-back systems, including ours, or the factory axle-back.

That flexibility gives you options when planning your setup. You can install the midpipe as part of your current configuration and decide on further changes later.

MAD Extended Chassis Brace Included

The FATBOY’s large single center section is paired with our MAD Extended Chassis Brace, which is included with the midpipe.

Including the brace means it is accounted for as part of the package, alongside the couplers needed for the listed axle-back connections. It is another detail that makes the FATBOY a complete upgrade designed around the G8X platform.

Factory Cut Location and Clamp Design

Our FATBOY Midpipe uses the OEM cut location and a clamp design for installation and reversibility.

This approach allows the midpipe to connect at the designated factory location while giving owners the ability to return to the original configuration if desired. Installation involves cutting at that location, so planning and accurate fitment are important.

Together with the supplied reducer couplers and chassis brace, the design makes it easier to integrate the FATBOY into your chosen exhaust setup.

Key Features

• Full-length single 4-inch center section.
• Minimum 3-inch pipe diameter throughout.
• Designed to maintain exhaust flow through the midpipe.
• Deeper, more aggressive exhaust note with reduced rasp.
• Compatible with stock or aftermarket downpipes.
• Includes the MAD Extended Chassis Brace.
• Includes 3-inch and 2.75-inch reducer couplers for compatible aftermarket or factory axle-back connections.
• Uses the OEM cut location and clamp design for installation and reversibility.
• Designed specifically for the BMW G8X M3 and M4.

BMW M3 and M4 Fitment

The MAD 4-Inch FATBOY Single Midpipe is designed for the following S58-powered models:

• 2021+ BMW G80 M3
• 2021+ BMW G82 M4 Coupe
• 2021+ BMW G83 M4 Convertible

Whether you drive the four-door M3, the M4 Coupe or the M4 Convertible, the FATBOY is designed to give your G8X a deeper exhaust character.

Installation Note: Downpipe Gaskets

Downpipe gaskets are not included with the midpipe.

Installation requires two gaskets, part number 18308053348. Make sure both are available before beginning the installation.

Give Your G8X the FATBOY Treatment

A great exhaust upgrade should make you want to take the long way home. The MAD 4-Inch FATBOY Single Midpipe is built to do exactly that, giving your S58 a deeper, more aggressive sound while maintaining a flow-focused design.

With its full-length 4-inch center section, included extended chassis brace and reducer couplers, the FATBOY gives G80 M3 and G82/G83 M4 owners a versatile way to upgrade their exhaust setup.

Shop the MAD BMW S58 4-Inch FATBOY Single Midpipe for the G80 M3 and G82/G83 M4