The Milltek Sport Secondary Catalyst Bypass is designed to maximize exhaust flow, power delivery and sound on F8X generation M3 and M4 models. By removing the factory-installed secondary catalysts (while leaving the primary catalysts in place), the system provides a significant improvement in airflow and sound permeability – resulting in a wider, deeper and more aggressive sound profile that is ideal for track and performance-oriented use.
This upgrade is especially popular with F8X enthusiasts who want more sound and dynamics without the need to change the entire exhaust system.
Better flow and more intense sound
Secondary Catalyst Bypass Solution:
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Improves exhaust gas flow rate and reduces back pressure
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Deepens the sound and increases the transparency of engine tones
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Provides an aggressive, motorsport-style exhaust note
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Retains optional compatibility with the original Cat Back system
The S55 engine's familiar metallic high-rev sound comes through cleaner and stronger than before, especially during sporty driving.
Bolt-on installation to original mounting points
Milltek's design ensures:
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direct compatibility with the factory exhaust system
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bolting without welding or structural changes
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precise fit and high-quality finish
Bypass tubes integrate seamlessly with either the standard or Milltek Cat Back system.
304L grade stainless steel
Milltek's 304L Aircraft Grade stainless steel offers:
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excellent heat resistance
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long service life even under high thermal loads
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high-quality TIG welding
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flow-optimized internal structure
Pipes made from this material can withstand even the most demanding operating conditions.
Compatibility: BMW F8X M3/M4 (non-OPF/GPF)
This secondary catalytic converter bypass kit fits the following 2014-2018 models: