Take your BMW X3M or X4M to the next level with the MAD 3.5" FatBoy Catless Downpipes. Designed for enthusiasts who demand maximum performance, our latest FatBoy design features a massive 3.5-inch diameter for significantly improved exhaust flow over traditional downpipes.

ETA: Sept 2026

Engineered specifically for the S58 platform, the MAD FatBoy Downpipes reduce exhaust backpressure, allowing the turbochargers to spool faster, improve throttle response, and unlock substantial horsepower and torque gains. Combined with an aggressive exhaust note and increased turbo sound, this is one of the most impactful upgrades you can make to your X3M or X4M.

Performance Benefits

The factory catalytic converters create a major restriction in the exhaust system. The MAD 3.5" FatBoy Catless Downpipes eliminate this bottleneck, providing:

• Up to 30+ WHP on stock turbochargers

• Even greater gains with upgraded turbochargers

• Faster turbo spool

• Sharper throttle response

• Reduced exhaust backpressure

• Increased horsepower and torque

• Lower exhaust temperatures

• Improved overall driving experience

Why the FatBoy Design?

Unlike conventional downpipes, the MAD 3.5" FatBoy design maximizes exhaust volume immediately after the turbocharger, allowing exhaust gases to expand more efficiently for improved flow and performance.

The result is:

• Maximum exhaust flow

• Improved turbo efficiency

• Increased horsepower potential

• More aggressive exhaust tone

• Louder turbo whistle

Premium Construction

Every MAD FatBoy Downpipe is built to perform under the most demanding conditions.

Features include:

• Massive 3.5-inch FatBoy design

• Heavy-duty 304 stainless steel construction

• Precision mandrel bends

• TIG welded and hand-finished

• Smooth internal transitions for unrestricted flow

• Integrated flex sections to reduce stress on the exhaust system

• Precision CNC-machined flanges

• Direct bolt-on installation

Designed for both left-hand drive (LHD) and right-hand drive (RHD) vehicles.

Built for Performance

Whether your vehicle is completely stock or running upgraded turbochargers, the MAD 3.5" FatBoy Downpipes are designed to maximize performance while maintaining OEM-quality fitment.

Compatible with both factory and most aftermarket exhaust systems, they deliver an aggressive exhaust note without unnecessary turbulence or restriction.

Features

• NEW 3.5" FatBoy design

• Catless construction

• Maximum exhaust flow

• Faster turbo spool

• Sharper throttle response

• Up to 30+ WHP on stock turbos

• Even greater gains on upgraded turbos

• More aggressive exhaust tone

• Increased turbo whistle

• Heavy-duty 304 stainless steel construction

• Precision TIG welded

• Integrated flex sections

• Spot-on fitment

• Compatible with LHD and RHD vehicles

• Works with factory and aftermarket exhaust systems

• Lifetime Warranty against manufacturing defects and fitment

• Ships in 1–2 business days

Installation Notes

Downpipe gaskets are not included.

Required for installation:

• BMW Gasket Part #18308053348 (Quantity: 2)

Fitment

• 2019+ BMW X3M (F97)

• 2019+ BMW X4M (F98)

SKU

MAD-041

Emissions Disclaimer

This product is not legal for sale or use on pollution-controlled vehicles operated on public roads, highways, or streets. It is intended for off-road, race, or closed-course competition use only, where such modifications are permitted.

By purchasing this product, you acknowledge and agree that:

• You are responsible for complying with all applicable federal, state, and local emissions laws and regulations.

• MAD and its affiliates disclaim any liability for improper or unlawful use of this product.

• Use of this product on public roads may violate the Federal Clean Air Act (42 U.S.C. § 7522) and similar state laws.

Please verify your local emissions regulations before purchasing or installing this product.