CMST Mustang Valved Cat-Back Exhaust (SUS304)
The CMST valved cat-back exhaust for the 2015โ2017 (S550) Ford Mustang is a SUS304 stainless system with remote-controlled valves that switch between a quiet bypass and an open, free-flowing path. It is built for both the 2.3L EcoBoost four-cylinder and the 5.0 V8 GT, replacing the factory rear section from the catalytic converters back while leaving the standard cats in place. The finish is a brushed (hairline) stainless with polished tips, and the valves let an owner run a softer note around town or a fuller, deeper tone once the road opens up.
- Vehicle: Ford Mustang, S550 generation (2015โ2017 facelift-era cars)
- Fitment: 2.3L EcoBoost and 5.0 V8 GT (confirm exact model year and engine before ordering)
- Material: International-grade SUS304 stainless steel, TIG-welded
- Configuration: Dual single-exit as shown; dual-dual-exit also offered by CMST
- Valve type: Vacuum-actuated baffle valves with remote control
- Finish: Brushed/hairline body with polished stainless tips
AME Motorsport supplies this system as part of our CMST cat-back exhaust for Ford range, shipped to Australian Mustang owners who want a factory-clean look with a switchable note. The sections below cover what the system actually is, how it is made, how the valves behave, and what to think about before fitting one here.
What the CMST valved cat-back is
A cat-back exhaust replaces everything from the catalytic converter rearward: the over-axle (mid) pipe, the rear mufflers and the tips. Because it starts behind the cats, the factory emissions hardware stays exactly where Ford put it, which is the main reason a cat-back is generally a tidier path than a full turbo-back or downpipe change. The CMST kit for the S550 follows that pattern, supplying the rear mufflers with built-in valves, the connecting pipework and a set of polished exit tips.
The system shown here is the dual single-exit layout, meaning one tip exits each side of the rear bumper. CMST also produces a dual-dual-exit version with two tips per side for owners who want a four-tip look. Both share the same core construction and the same remote valve system; the difference is purely how many tips you see at the back. The mufflers connect to the mid-pipe with band clamps rather than fixed flanges, which gives a workshop room to rotate and align the tips evenly in the bumper cut-outs.
https://cdn.shopify.com/s/files/1/0841/1922/4633/files/img_00_7388d186-1c21-4b01-9d18-fcd7bc6abe87.jpg?v=1782816612Construction: SUS304 stainless and TIG welding
The whole system is made from SUS304 stainless steel, the international designation for the common 18/8 austenitic grade. Compared with the aluminised mild steel used on many original-equipment exhausts, SUS304 resists rust and high-temperature scaling far better, so it holds its appearance and its wall integrity for longer in Australian conditions, where coastal salt air and long highway heat both take a toll. The body carries a brushed (hairline) finish, while the visible tips are polished to a mirror surface.
Joints are TIG welded. TIG (tungsten inert gas) welding lays a clean, controlled bead and is the method serious exhaust fabricators use on stainless because it keeps the weld tidy and consistent. The pipework is mandrel bent, which keeps the internal diameter constant through each bend instead of crushing or wrinkling it the way a cheaper press bend can; a consistent bore is what lets exhaust gas flow without the restrictions a pinched bend would add.
| Property | SUS304 stainless | Aluminised mild steel |
|---|---|---|
| Corrosion resistance | High; resists rust and salt-air pitting | Moderate; coating wears and rust can start at cuts |
| Heat tolerance | Strong resistance to high-temperature scaling | Lower; coating degrades over time |
| Finish | Takes a brushed or polished surface | Dull grey, not intended to be shown |
| Longevity | Long service life, suits a visible system | Shorter, typical of hidden factory pipework |
How the remote valves work
Each rear muffler houses a vacuum-actuated valve, a baffle that opens or closes inside the muffler to change the path the exhaust gas takes. A vacuum canister sits on the muffler and moves the valve when signalled, and the system is operated remotely so the driver can change the setting without leaving the seat. Switching the valve changes both the exhaust note and how freely the system flows, which is the whole point of a valved design: one exhaust, two distinct characters.
https://cdn.shopify.com/s/files/1/0841/1922/4633/files/img_03_373ce141-c34f-410a-8318-41768ccf667f.jpg?v=1782816616Quiet mode
With the valves closed, exhaust gas is routed through the longer, more muffled path inside the rear boxes. This is the setting for cold starts in a quiet street, early-morning departures, underground car parks and long, steady motorway running where a constant drone gets tiring. The tone drops back and the system behaves close to a restrained factory-style exhaust.
Open mode
Open the valves and the gas takes the shorter, freer route, bypassing much of the muffling. The note becomes louder, fuller and deeper, and the system flows more freely. This is the setting for an open road or a spirited drive, where the source describes the result as full and deep, lighter in tone than stock and noticeably less restricted.
Sound and flow
On sound, the CMST system gives the S550 a fuller, deeper voice than the factory exhaust, with the valves deciding how much of it reaches the street. On the 5.0 V8 GT the effect is obvious because the engine already has a strong natural tone for the exhaust to carry. On the 2.3 EcoBoost the turbocharger sits in the exhaust stream and absorbs a lot of sound energy, so a cat-back changes the character and depth of the note more than it lifts raw volume; EcoBoost owners should expect a richer tone rather than a dramatic increase in loudness.
https://cdn.shopify.com/s/files/1/0841/1922/4633/files/img_02_7856f8c3-d45b-489c-88bd-3780b5174a7e.jpg?v=1782816621On power, it is worth being straight. A cat-back exhaust mainly delivers freer flow, a small weight saving over heavier factory pipework, and a better sound. On both a naturally aspirated V8 and a turbocharged four, the power change from a cat-back alone is modest. Real, measurable gains come from a downpipe and a supporting tune, and a cat-back includes neither, so do not expect a cat-back on its own to transform the dyno sheet. What it reliably changes is how the car sounds and how the rear of the car looks. If you are building a wider package, the cat-back pairs naturally with the rest of our CMST exhaust for Ford components, all from the same CMST catalogue.
Fitment in Australia
Because this is a cat-back, the factory catalytic converters and the emissions hardware ahead of them stay in place; the kit only replaces the rear section. It is designed as a bolt and clamp-on fit using band clamps at the mid-pipe, and we recommend it be fitted by a workshop so the hangers, clamps and tip alignment are set correctly. A correct fit-up matters on a valved system because the tips need to sit evenly in the bumper and the valve canisters and their vacuum lines need to be routed and connected properly.
On noise, a valved exhaust gives owners a genuine advantage here: the quiet valve setting lets the car run a softer note for street use and noise-conscious situations. Australian noise rules and ADR requirements vary by state and are enforced on the road, so we do not make a blanket compliance claim. Many owners value the quiet valve setting for street and noise considerations; confirm your local requirements before fitting and running any aftermarket exhaust. The install photo also shows the system exiting through a carbon rear diffuser, and the polished tips suit a colour-matched or carbon aero treatment; if you are styling the rear to match, look at the CMST diffuser for Ford and the broader Ford Mustang body kit range.
Frequently asked questions
Does it fit my Mustang?
It is made for the S550 Mustang in 2015โ2017 form and is offered for both the 2.3L EcoBoost and the 5.0 V8 GT. The two engines use different exhaust layouts, so the correct version must be matched to your engine. Always confirm your exact build year and engine against the listing before ordering, as fitment is engine and year specific.
Will it add power?
Only modestly. A cat-back improves flow, saves a little weight and changes the sound, but it leaves the cats and factory downpipe in place. Meaningful power gains on either the EcoBoost or the V8 come from a downpipe plus a supporting tune, which a cat-back does not include. Treat this system as a sound, flow and appearance upgrade rather than a dyno modification.
How loud is it, and can I keep it quiet?
With the valves open it is louder, fuller and deeper than stock; with the valves closed it settles back to a restrained note. The remote control lets you choose at any time, so you can run quiet through town or on a long motorway stint and open it up on the right road. The 2.3 EcoBoost stays more subdued than the V8 because the turbo muffles the tone.
Is it road friendly in Australia?
The quiet valve setting is exactly why many owners choose a valved exhaust, because it lets the car run a softer note for street and noise-conscious use. Noise limits and ADR rules differ between states and are checked on the road, so confirm your local requirements before fitting and using the system. We do not claim blanket compliance; the responsibility for meeting local rules sits with the owner.
What is the difference between dual single-exit and dual-dual-exit?
Dual single-exit gives one tip on each side of the rear bumper, the layout shown here. Dual-dual-exit gives two tips per side for a four-tip look. Both use the same SUS304 construction and the same remote valve system; the choice is about the appearance you want at the back of the car and how the tips suit your bumper and diffuser.
Does it keep the cats, and should a workshop fit it?
Yes, it keeps the factory catalytic converters because it is a cat-back that only replaces the rear section. We recommend professional fitment so the band clamps, hangers, vacuum lines and tip alignment are all set correctly, which is what gives the system its even, factory-clean finish in the bumper.
Order the CMST valved cat-back for your Mustang
If you want a SUS304 stainless exhaust with remote valves, a fuller note on demand and a quiet setting for the street, browse the full CMST cat-back exhaust for Ford range at AME Motorsport. You can also explore our wider CMST cat-back exhaust selection across other models, and our team can help you match the right configuration and engine fitment for your S550 before you buy.