Milltek Secondary Catalyst Bypass for BMW 5 Series M5 4.4l Twin Turbo Hybrid G90 & G99 (NAS/ROW Spec Vehicles) (2025 onwards) – Maximum Flow, Motorsport Use
This is Milltek's large-bore downpipe and de-cat for the BMW 5 Series M5 4.4l Twin Turbo Hybrid G90 & G99 (NAS/ROW Spec Vehicles). It replaces the factory downpipe with a larger-bore pipe and removes the catalytic converter entirely, clearing the single most restrictive point in the exhaust for the biggest flow gain available at this position in the system. It's usually fitted alongside a Stage 2 remap, since the ECU needs to be told to expect the different readings the lambda sensors give once the cat's gone.
It's built for owners already committed to a serious power upgrade rather than a casual bolt-on, so pair it with the matching Milltek cat-back system for the full effect.
Because it removes the cat, it's intended for off-road and competition use. A de-catted car will fail an MOT emissions test and isn't legal to use on UK roads.
Specifications
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System Type: Secondary Catalyst Bypass
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Pipe Diameter: 80mm (3.15")
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EC Approved: No
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Race System: No
Key Features
- Made from T304L aircraft-grade stainless steel, which is anti-magnetic and resists discolouration
- Mandrel-bent pipework keeps a constant diameter through every bend for unrestricted flow
- 80mm (3.15") pipework
- Removes the factory catalyst for maximum flow (off-road and competition use only)
- Designed, developed and manufactured in the UK by Milltek Sport
Fitment
- BMW 5 Series M5 4.4l Twin Turbo Hybrid G90 & G99 (NAS/ROW Spec Vehicles) (2025 onwards)
Not suitable for: any model, engine or body style not listed above. Check your car before ordering.
Installation
Some versions: Fits to Milltek Axle Back and Downpipes Only. Some versions: Fits to OE Axle Back and Downpipes Only - OE requires cutting. This work involves removing parts close to the turbo and engine, so we recommend fitting by a specialist with a ramp and suitable software support.
Upgrade Further
Also available from Milltek for this car: Milltek Axle Back Exhaust BMW 5 Series M5 4.4l Twin Turbo Hybrid G90 & G99 NAS & ROW Spec Vehicles 25+.
Is it road legal?
No. Removing the catalyst means the car will fail an MOT emissions test, so it is for off-road and competition use only.
Do I need a remap?
A suitable remap is strongly recommended, as the engine management needs to account for the change in flow and sensor readings. Speak to your tuner before fitting.
Can I use it with my standard exhaust?
Yes. Milltek lists it for use with the factory (OE) exhaust system.
Will removing the cat cause a warning light?
Without suitable software it is likely to, as the ECU expects the readings a working catalyst gives. This is why Milltek pairs it with a remap.
What is it made from?
T304L aircraft-grade stainless steel, which is anti-magnetic and resists discolouration better than lower grades.
Is there a warranty?
Yes. Milltek products are covered against defects in materials and workmanship. Contact us with your order number if you have a problem.
