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The CMST valved exhaust system for the Porsche Boxster (and its Cayman sibling) is a SUS304 stainless steel cat-back unit with electronically controlled bypass valves, replacing the factory muffler assembly while retaining the original catalytic converters and downpipes. Where the factory Boxster exhaust prioritises legal noise limits and cabin comfort, the CMST system gives the driver active control over exhaust character — a quieter closed-valve mode for everyday driving, and an open-valve mode that delivers the flat-six's mid-range sound far more directly. The system is noted to produce genuine performance improvement alongside the sound upgrade, attributed to reduced backpressure through the valve-bypass circuit.

• Vehicle: Porsche Boxster (mid-engine roadster) and Cayman (mid-engine coupe) — confirm generation
• System type: Cat-back valved exhaust
• Material: SUS304 (AISI 304) stainless steel, brushed finish
• Valve operation: Electronic bypass valve, remote-controlled
• Weight: Lighter than OEM equivalent
• Mounting: Cat-back (connects at factory cat exit; all emissions equipment retained)

The Boxster Sound Equation

The Porsche Boxster's mid-mounted flat-six is one of the most distinctive engine architectures in production sports cars. Located behind the driver and ahead of the rear axle, the engine exhales through a compact exhaust system that routes under the luggage compartment floor and exits centrally below the rear bumper. This layout gives the Boxster a unique exhaust character — different from front-engined 911s, closer to the race-car midship placement used in the Cayman GT4 RS's Weissach package.

Despite this mechanical foundation, the factory Boxster exhaust is tuned conservatively. European and Australian ADR noise limits require exhaust systems to pass drive-by and static noise tests; the factory muffler achieves compliance with significant sound attenuation. The result is an engine that sounds restrained under normal driving conditions, only opening up meaningfully at high revs with the throttle fully depressed — a situation rarely available on public roads.

A valved cat-back system changes this balance by introducing a low-restriction bypass path. The valve-open mode routes exhaust gas through the bypass, producing a sound level and character that is closer to what the engine's displacement and combustion architecture naturally generate. For flat-six Boxster and Cayman owners who bought the car partly for its engine character, this is the modification that delivers what the factory withheld.

SUS304 Construction and Weight Comparison

CMST specifies SUS304 stainless steel for the Boxster/Cayman exhaust for the same reasons it applies across the CMST exhaust range: the alloy combines corrosion resistance with sufficient heat tolerance for sustained exhaust temperatures, and it is machinable into the complex bent-tube geometries that the Boxster's compact underbody requires. The brushed exterior finish conceals minor abrasion better than a mirror polish and requires less maintenance to keep presentable.

The factory Boxster exhaust muffler is a heavily constructed unit, as it needs to suppress the flat-six's sound signature across a wide RPM range. CMST's valved system achieves comparable quiet-mode attenuation through the valve architecture rather than through raw muffler volume, allowing a lighter physical construction. The weight saving is located at the car's rear, which on a mid-engined car like the Boxster affects the polar moment of inertia more than the same saving would on a front- or rear-engined layout — though the practical effect on a road car at legal speeds is modest.

Cat-Back Configuration: What It Retains and What It Changes

A cat-back system modifies only the exhaust components aft of the catalytic converter. On the Boxster, this means the catalytic converters (typically two, one per bank of the flat-six) and the primary downpipes remain factory equipment. Only the secondary pipe routing, muffler assembly, and exit tips are changed by the CMST system.

This matters for three reasons. First, the catalytic converters are the primary emissions control components; retaining them keeps the car compliant with Australian ADR emissions standards. Second, the factory oxygen sensors upstream and downstream of the cats remain in their factory positions, so the ECU's fuelling trims operate normally without detecting the modification. Third, the cat-back configuration means the CMST system qualifies as an aftermarket exhaust modification rather than an emissions deletion, which is a relevant distinction for roadworthy inspections in Australian states.

The Boxster's flat-six generates significant heat in the compact mid-engine bay; the exhaust routing under the car reaches elevated temperatures during sustained spirited driving. SUS304's heat tolerance is appropriate for normal exhaust operating temperatures. The bypass valve actuator — a small electromechanical component — is positioned to avoid the highest-heat zones in the exhaust routing.

Sound Character: Boxster vs Cayman Differences

While the Boxster and Cayman share the same flat-six engine family (in generation-matched variants), their body structures produce slightly different sound signatures even with the same exhaust system. The Boxster is an open car — with the roof down, the driver and passengers hear a more direct version of the exhaust note because there is no fixed roof to block or muffle sound. The Cayman, as a closed coupe, filters more sound into the cabin through the fixed roof and rear glass, but also focuses more sound pressure toward the tail through the closed body structure.

In open-valve mode, the CMST system produces a flat-six sound that is notably different from the six-cylinder note produced by inline or V configurations. The flat-six's firing order and the symmetric two-bank exhaust layout create a characteristic burble and bark that is most prominent at mid-RPM under moderate to hard acceleration. At idle, the valve-open note is a low, slightly loping idle rather than the smooth idle the factory tune achieves. The sound at high RPM in valve-open mode is sharp and direct — closer to the character of the Cayman GT4, which uses a less restricted factory exhaust.

Fitment Across Boxster Generations

CMST's valved exhaust for the Boxster is generation-specific. The 986-series Boxster (1997–2004), 987-series (2004–2012), 981-series (2012–2016), and 982-series (2016-onward) each have different cat-back geometries, mounting flange positions, and rear bumper tip aperture sizes. The system must be specified for the correct generation.

The 982-series Boxster (marketed as the 718 Boxster from 2016) uses a turbocharged 2.0-litre or 2.5-litre flat-four rather than the naturally aspirated flat-six of earlier generations — a change that significantly altered the car's sound character and led many owners to seek exhaust upgrades specifically to restore more engine character. The CMST valved system is available for the 718-series and addresses the four-cylinder's more industrial note effectively in valve-open mode. Confirm your exact generation and engine variant when ordering from AME Motorsport.

Browse the complete CMST cat-back exhaust range to see current stock, or view the full CMST brand catalogue. For Porsche-specific aero and body products alongside the exhaust upgrade, see the CMST body kit for Porsche and CMST bumper for Porsche collections.

Performance Considerations

The source material notes genuine performance improvement on the Boxster/Cayman after fitting the CMST exhaust, attributed to improved exhaust flow. This is plausible for naturally aspirated engines, where exhaust backpressure reduction can allow marginally better scavenging of exhaust gases from the combustion chamber, slightly improving volumetric efficiency. On turbocharged 718 variants, the cat-back position is less impactful on power output — the turbocharger operates between the cat and the engine, and backpressure at the muffler has less direct effect on boost and fuelling.

Without supporting ECU calibration, any power increase from a cat-back exhaust on either naturally aspirated or turbocharged Boxster engines is small. The performance improvement most owners report is the improved throttle response feel — a subjective experience influenced by the changed exhaust sound as much as by any measurable power gain. This is not a criticism of the system; it is an accurate description of what an exhaust upgrade delivers and what it does not.

Frequently Asked Questions

Does the CMST valved exhaust fit both the Boxster and the Cayman?

The flat-six Boxster and Cayman of the same generation share exhaust architecture, and CMST exhaust systems are typically compatible with both. The 718 Boxster and 718 Cayman (flat-four) require a separate generation-specific system. Confirm your exact model and year when ordering.

Is the exhaust louder than the factory system in closed-valve mode?

In closed-valve mode the CMST system is slightly richer in tone than factory — most owners describe it as sounding like what the factory exhaust should have sounded like. It is not dramatically louder at idle or normal driving RPM with the valve closed. Open-valve mode is substantially different in character and volume.

Will the valved exhaust cause the car to fail a roadworthy inspection in Australia?

A cat-back exhaust system that retains factory catalytic converters and does not exceed state noise limits in its standard closed-valve operating mode is generally roadworthy compliant. Driving continuously in open-valve mode in noise-regulated areas could create a compliance issue depending on the measured dB output. Use closed-valve mode in environments where noise compliance matters.

How do I control the bypass valve?

The standard CMST installation includes a remote fob that opens and closes the bypass valve electronically. Some users have the valve actuator wired to a toggle switch on the dashboard or integrated with a Bluetooth OBD controller for smartphone control. The remote fob is the simplest and most reliable method for most users.

Does fitting the exhaust affect the Boxster's factory sport exhaust button (where fitted)?

Some Boxster variants have a factory sport exhaust option that uses a similar valve mechanism in the OEM system. Replacing the full cat-back unit with the CMST system removes the factory valve and replaces it with the CMST actuator and remote. The factory sport exhaust button on the dashboard will no longer control exhaust sound after the retrofit; the CMST remote becomes the sole valve control.

What is the installation time for a Porsche workshop?

An experienced Porsche workshop or performance exhaust specialist can typically complete a Boxster cat-back installation in 1.5–3 hours, depending on the system configuration and access to a hoist. The Boxster's central exhaust routing requires working under the car with the mid-section accessible from below; a proper hoist makes this straightforward.