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The CMST valved cat-back exhaust for the BMW 640i is a bolt-on stainless-steel system built around an electronically controlled valve and twin carbon-fibre tips. It is fabricated from brushed 304 stainless steel, runs a dual-side, twin-exit layout, and uses a remote-operated valve so you can switch between a quiet daily tone and a deeper, fuller note on demand. It is designed for the BMW 6 Series 640i (F06 Gran Coupe, F12 convertible and F13 coupe) fitted with the N55 turbocharged inline-six.

• Vehicle: BMW 640i (F06 / F12 / F13), N55 3.0L turbo inline-six
• System type: Cat-back, dual-side twin-exit, electronically valved
• Material: 304 stainless steel, brushed (drawn) finish
• Tips: Twin carbon-fibre tips per side (four total)
• Valve control: Wireless remote, open/closed on demand
• Fitment: Direct bolt-on to factory mounting and hangers

What the CMST 640i exhaust actually is

This is a cat-back system, which means it replaces everything from the cat-back connection rearward: the mid-pipes, the rear silencers and the tips. It leaves the factory catalytic converters and the car's emissions hardware in place, so the standard exhaust note signature of the N55 is retained while the volume, valving and tip design change. On a 640i that distinction matters: keeping the cats means the system is far less likely to throw fault codes or interfere with the car's closed-loop fuelling than a full turbo-back de-cat would.

The 640i left the factory with BMW's N55 single-turbo 3.0-litre six, an engine that responds well to reduced back-pressure on the cold side of the exhaust. CMST builds this system specifically around that car's underbody routing rather than adapting a generic kit, so the pipe bends, the silencer placement and the hanger positions follow the original 6 Series layout.

304 stainless steel construction

The pipework and silencers are made from 304-grade stainless steel with a brushed (drawn) external finish. 304 is the alloy most reputable exhaust fabricators use for street systems: it contains roughly 18% chromium and 8% nickel, which gives it strong corrosion resistance against road salt, moisture and the heat-cycling an exhaust endures every drive. For an Australian car that sees coastal air or winter road treatment, that corrosion margin is the difference between a system that stays clean for years and one that surface-rusts early.

Compared with the factory 640i exhaust, a mandrel-bent stainless cat-back is typically lighter. The OEM system uses heavier-gauge steel and large production silencers sized for maximum noise suppression; a performance stainless system uses thinner-wall tube and smaller, more free-flowing mufflers, which removes unsprung-adjacent weight at the very rear of the car. We avoid quoting an exact kilogram saving because it varies by exact OEM specification, but the direction is consistent: less mass, hung behind the rear axle line.

How the remote valve works

The defining feature is the electronic valve. Each rear silencer carries a butterfly valve that is driven by a small servo motor, and that motor is controlled by a wireless remote you keep in the cabin. With the valve closed, exhaust gas is routed through the muffler's longer, more restrictive path, so the car stays quiet and resonance-free at cruise. With the valve open, gas takes a shorter, straighter route, the system breathes more freely, and the note becomes louder and deeper.

This is what makes a valved system practical to live with. You can leave the valve shut for an early-morning start in a quiet street or a long motorway run where drone is tiring, then open it on a back road or at a track day when you want the full character of the N55. The valve switches independently of the car's drive modes, so it is not tied to Sport or Comfort — you control the sound directly.

Sound character

With the valve open the system delivers a low, full-bodied note rather than a high, raspy one — the turbocharged straight-six has a naturally smooth, deep timbre, and a well-designed valved cat-back amplifies that without turning it harsh. Closed, the car returns to something close to a refined factory tone. Sound is subjective and depends on whether you also change the mid-pipe, so we describe the system's intended behaviour rather than promising a specific decibel figure.

Carbon-fibre tips

The system finishes in twin carbon-fibre tips per side — four tips across the rear. The carbon weave is matched to the kind of carbon aero many 6 Series owners run (front lip, boot spoiler, diffuser), so the rear of the car reads as a coherent build rather than a mismatched bolt-on. Carbon tips also stay visually clean: unlike polished stainless tips, they do not blue or discolour from heat over time. The tip housings clamp to the system's outlets, so they sit at the correct depth in the rear bumper cut-outs.

Fitment and installation

The system is a direct bolt-on. It connects to the factory cat-back joint, reuses the original rubber hanger locations, and the rear silencers locate in the standard positions so the carbon tips line up with the bumper apertures. No cutting of the floor or permanent modification is required for a standard 640i, which means the change is fully reversible if you return the car to stock.

Because the valve adds a servo and wiring, fitment is best done on a hoist with the rear bumper-area access clear, and the valve harness needs a tidy, heat-aware routing path. We recommend professional fitment in Australia so the valve actuator is mounted clear of heat, the hangers are seated evenly, and the tips are set to an even depth on both sides. A workshop will also confirm there are no exhaust leaks at the cat-back joint before the car goes back on the road.

Power and flow notes

A cat-back system's main job is to reduce restriction downstream of the catalysts, which helps the turbocharged N55 evacuate spent gas more efficiently at higher rpm. On a factory-tuned 640i the headline power change from a cat-back alone is modest — the larger gains on these engines come from tuning and intake/charge-pipe work — but the exhaust improves how freely the engine breathes at the top end and sharpens throttle response. We deliberately do not publish invented dyno numbers; any genuine figures depend on the rest of the car's specification and a proper dyno run.

Frequently asked questions

Will the CMST exhaust fit my 640i Gran Coupe and the coupe?

The system is built for the 640i across the F06 Gran Coupe, F12 convertible and F13 coupe, all of which share the N55 drivetrain and 6 Series exhaust routing. Confirm your exact chassis code and rear bumper style when ordering, since the tip cut-outs differ between body styles.

Is it loud all the time?

No. The remote valve lets you keep it quiet for daily driving and open it when you want volume. Closed, it behaves close to factory; open, it gives the deep, full note the system is designed around.

Will it set off a check-engine light?

Because it is a cat-back and leaves the factory catalytic converters in place, it does not alter the emissions hardware the car monitors, so it should not trigger fault codes on a standard car. Always have any exhaust work checked for compliance with your state's regulations before road use.

Are the carbon tips going to discolour?

Carbon-fibre tips do not blue or rainbow the way polished stainless tips can, because the visible surface is lacquered carbon rather than bare metal. They wipe clean and keep their finish through normal heat cycling.

Can I fit it myself?

It is a bolt-on, but the electronic valve and its wiring make professional installation the safer choice. A hoist, correct hanger seating and tidy, heat-aware harness routing all matter for a clean result.

Build your 640i

A valved cat-back is the natural pairing for a carbon-aero 6 Series. If you are specifying the rest of the car, browse the CMST cat-back exhaust range, match it with the carbon pieces in our CMST body kits for BMW, and see the full CMST catalogue for diffusers, spoilers and lips that share the same carbon weave. Contact AME Motorsport for fitment guidance and current pricing on the 640i system.