Audi S4 Catalytic Converter B7 (2004–2008)

The catalytic converter in the Audi S4 B7 (2003-2008, 4.2 V8 Quattro 344 HP) is a crucial component of the exhaust system designed to reduce harmful emissions. It converts toxic gases from the engine exhaust into less harmful substances before they exit the tailpipe, helping the vehicle meet emissions standards.

Yes, stainless steel sport catalytic converters and decatalysers are available as replacements to reduce exhaust restrictions and improve performance. For example, Milltek offers high-flow catalytic converters designed specifically for the S4 B7 4.2 V8 Quattro. These sport versions are TIG-welded and feature precise flanges, offering durability and potential power gains while often complying with emission regulations.

The original equipment manufacturer (OEM) catalytic converters for the Audi S4 B7 feature consistent quality and sizes suitable for different body styles (saloon, Avant, Cabriolet):

• Rear axle catalytic converter diameter: approx. 300 mm (ventilated disc brakes)
• Front axle catalytic converter diameter: approx. 312 mm
• Compatible with 4.2 V8 Quaattro engine, maintaining emissions compliance (Euro 4)

These converters are designed to fit precisely within the exhaust system, providing effective emissions control while ensuring engine performance is not compromised.

Yes, aftermarket exhaust lines and catback systems are available to complement upgraded catalytic converters to enhance sound and performance:

• Milltek and Diapason Motorsport offer exhaust lines with 57.15 mm tubing diameter and larger quad or twin 76.2 mm outlets
• These systems often omit intermediate silencers for a more aggressive exhaust note
• 034Motorsport supplies high-flow adaptations with stainless steel and TIG welds, improving exhaust flow and engine response

Upgrading the post-catalytic exhaust components can enhance both sound and power delivery when combined with the proper catalytic converter upgrade.

Legality varies depending on location and vehicle regulations. In many European countries, removing or replacing the catalytic converter with decatalysers for road use is illegal. These modifications may be acceptable on closed circuits or off-road use only.

• Always check local emissions laws before modifying catalytic converters
• High-flow catalytic converters that meet or exceed OEM emissions standards can be a legal alternative
• Using non-approved catalytic converters or removing them can cause engine warning lights and potential legal issues

It is recommended to use approved replacements compliant with Euro standards (such as Euro 4 for the S4 B7) to ensure emissions compliance and avoid penalties.

Replacement options vary, with sport or OEM-style fitments commonly including:

• Tubing diameters around 57.15 mm for high-flow Milltek sport catalytic converters
• Some replacement pipes with larger 63.5 mm diameters for reduced restrictions
• Precise flanges designed for direct fit on 2003–2008 Audi S4 B7 models, including saloon, Avant, and Cabriolet

Correct selection based on vehicle model and transmission (manual or automatic) ensures proper installation and function.

Genuine quality catalytic converters are manufactured with high-grade INOX 409 stainless steel and certified materials to ensure durability, emissions compliance, and performance. Choose converters with:

• High-performance monoliths with rare metal coatings for efficient conversion
• Certification by recognized automotive authorities (e.g., TÜV SÜD)
• Direct compatibility with your Audi S4 B7 engine and exhaust layout

Ordering from trusted suppliers who provide warranties and factory support helps guarantee product authenticity and quality.

To maintain your catalytic converter and exhaust system in peak condition:

• Use high-quality, compatible replacement parts matching OEM specifications
• Perform regular inspections of sensors and exhaust components for damage or wear
• Ensure engine is properly tuned to avoid misfires or unburnt fuel that can damage the converter
• Address any warning lights promptly to prevent further damage

Proper care and choosing certified high-flow or OEM replacement catalytic converters can help maintain fuel efficiency, performance, and compliance with emissions regulations for your Audi S4 B7.