Quick recommendations
- Our preferred brand: Cobra Sport – excellent quality, hand TIG-welded and manufactured in the UK.
- Best road-car exhaust: Cobra GPF-back system, which retains the factory catalyst and GPF.
- Want more sound: Choose a box-delete GPF-back while retaining the factory emissions hardware.
- GPF delete: Gives a little more exhaust noise, but not a huge change on its own.
- Cat-back: Cobra's MK4 ST cat-back removes the factory GPF as well as replacing the centre and rear exhaust sections.
- Stage 2: Ideally use a complete turbo-back system. A downpipe plus GPF-delete section is also sufficient when combined with the required intake, intercooler and calibration.
- Downpipe gain: Around 10–15hp is realistic on a correctly tuned Stage 2 car in our experience.
- Sports cat vs de-cat: Very little performance difference at this power level.
- Road use: Once the factory GPF is removed, the exhaust configuration should be treated as motorsport/off-road use only.
- Hatchback vs estate: Exhaust systems are different – always order the correct body-style version.
MK4 Focus ST exhaust systems and downpipes buying guide
The MK4 Focus ST exhaust range can initially be confusing because the car uses both a conventional catalytic converter and a GPF / petrol particulate filter.
This means terms such as GPF-back, cat-back, GPF delete, downpipe and turbo-back refer to genuinely different sections of the exhaust.
Choosing the correct system depends on whether your priority is sound, road use or maximum performance from a tuned car.
For Stage 1, Stage 1+ and Stage 2 hardware recommendations, read our MK4 Focus ST tuning guide.
How is the MK4 Focus ST exhaust laid out?
In simple terms, the factory exhaust is arranged as:
Turbo → catalytic converter / downpipe → GPF → centre exhaust → rear exhaust
Understanding this makes the different Cobra systems much easier to choose.
What is a GPF-back exhaust?
A GPF-back system starts after the factory petrol particulate filter.
It keeps:
- Factory catalytic converter
- Factory GPF
And replaces:
- Centre exhaust section
- Rear exhaust section
The Cobra Sport MK4 Focus ST GPF-Back Exhaust uses 3-inch pipework while retaining both factory emissions devices.
For a normal road-going Focus ST where the customer wants a better exhaust note without removing the GPF, this is our preferred setup.
Does a GPF-back exhaust add much power?
Not really.
We primarily regard a GPF-back system as a sound and appearance modification.
The factory catalyst and GPF remain in place, so the major restrictions further forward in the system have not been removed.
Fit one because you want:
- A deeper exhaust note
- More volume
- Larger or different tailpipes
- Higher-quality stainless-steel construction
Do not fit one expecting a large increase in peak power.
How loud is the Cobra GPF-back?
The normal Cobra GPF-back gives the car a noticeably deeper and louder exhaust note while remaining relatively sensible for road use.
Because both the catalytic converter and GPF remain in place, they continue to suppress a significant amount of exhaust noise.
This makes the standard GPF-back a good option for a daily-driven car.
Resonated versus non-resonated
Where resonated and non-resonated versions are available, the difference is mainly sound.
Resonated
Includes an additional centre silencer and is the quieter, more refined option.
Non-resonated
Removes the centre silencer and gives a louder, more aggressive exhaust note.
There is no significant reason to choose one over the other for power – choose according to the sound level you want.
Cobra box-delete GPF-back
The Cobra Sport Box Delete GPF-Back Exhaust is the louder GPF-retaining option.
It removes more silencing from the rear exhaust while leaving the original catalyst and GPF in place.
This is a good choice if you want considerably more exhaust sound without removing the factory particulate filter.
Cobra Venom exhaust
Cobra's Venom range removes the rear silencer completely and is designed for customers looking for the most aggressive sound.
It is considerably louder than the conventional systems.
Cobra's own testing quotes approximately an 8.75dB increase for its GPF-back Venom configuration.
This is Cobra's manufacturer test figure rather than a Wayside measurement.
What is a Cobra cat-back on the MK4 Focus ST?
This is an important distinction because Cobra's terminology is slightly different from what some customers may expect.
The Cobra Sport MK4 Focus ST Cat-Back Exhaust starts after the catalytic converter and removes the factory GPF.
It replaces:
- Factory GPF
- Centre exhaust
- Rear exhaust
This is therefore very different from the GPF-back system.
GPF-back versus cat-back
GPF-back
- Retains factory catalyst
- Retains factory GPF
- Replaces centre and rear exhaust
- Mainly a sound and appearance modification
- Our preferred option for a normal road car
Cat-back
- Retains factory catalyst
- Removes factory GPF
- Replaces GPF, centre and rear sections
- Requires appropriate engine calibration
- Should be treated as an off-road / motorsport configuration on a GPF-equipped car
MK4 Focus ST GPF delete
The Cobra Sport GPF / PPF Delete replaces only the factory particulate-filter section.
Removing the GPF gives the car a little more exhaust noise, although the difference on its own is not enormous.
It also removes a restriction from the exhaust system, making it useful as part of a Stage 2 setup.
Because the factory emissions device has been removed, this should be treated as a motorsport/off-road modification and appropriate engine calibration is required.
Is a GPF delete worth fitting just for sound?
It does make the car louder, but the increase is relatively modest compared with replacing the complete rear exhaust or fitting a box-delete system.
If sound is your only objective on a road car, we would normally choose a GPF-back exhaust instead and retain the factory emissions hardware.
The GPF delete makes more sense when it forms part of a performance-oriented Stage 2 exhaust setup.
MK4 Focus ST downpipe upgrade
The Cobra Sport MK4 Focus ST downpipe replaces the restrictive front exhaust section immediately after the turbocharger.
It uses a large 3.5-inch / 88.9mm section from the turbo before reducing to 3-inch / 76.2mm.
This large initial section gives the exhaust gases more room as they leave the turbocharger before transitioning into the rest of the 3-inch system.
This is one of the most important exhaust upgrades when moving to Stage 2.
Sports cat versus de-cat downpipe
The Cobra downpipe is available with either:
- High-flow sports catalyst
- De-cat section
In our experience, there is very little performance difference between the two at normal Stage 2 MK4 Focus ST power levels.
We would not choose a de-cat simply because you expect a substantial power advantage over a good sports catalyst.
Sports cat
The sports-cat version retains a high-flow catalytic converter while substantially reducing restriction compared with the original front section.
However, Cobra states that its sports catalyst replaces a factory emissions device and is not EC homologated.
When combined with GPF removal as part of the Stage 2 setup, the exhaust configuration should therefore be treated as motorsport/off-road use only.
De-cat
The de-cat removes the catalytic converter entirely.
This version is supplied for motorsport and off-road use only.
At the power levels we normally see from a Stage 2 MK4 Focus ST, we do not see a major performance advantage over the sports-cat version.
Can I use the Cobra downpipe with the standard GPF?
We would not recommend building the exhaust this way.
Cobra itself advises against fitting its downpipe to the standard GPF section because of differences in the sensor arrangement and the effect this has on GPF functionality.
For a Stage 2 car, we would combine the uprated downpipe with a GPF-delete section.
Best Stage 2 exhaust setup
Ideally, we would fit a complete Cobra Sport Turbo-Back Exhaust.
This replaces the exhaust from the turbocharger backwards and gives you a complete matched 3-inch system.
However, you do not necessarily need to replace the entire rear exhaust to achieve the required Stage 2 flow.
A good Stage 2 exhaust setup can consist of:
- Uprated downpipe
- GPF-delete section
- Factory or performance rear exhaust
Combined with:
- Performance intake
- Uprated intercooler
- Correct Stage 2 ECU calibration
We also strongly recommend considering the clutch because the increased torque produced by tuned MK4 Focus STs can expose a weak or worn factory clutch.
How much power does the Stage 2 exhaust hardware add?
In our experience, approximately 10–15hp of the Stage 2 improvement can reasonably be attributed to removing the restriction in the downpipe / catalyst / GPF area.
The exact result varies depending on:
- ECU calibration
- Fuel quality
- Intake system
- Intercooler efficiency
- Ambient temperature
- Condition of the engine and turbocharger
The exhaust hardware works as part of the complete Stage 2 package rather than as an isolated modification.
Does Stage 1 need an exhaust?
No.
Our Stage 1 MK4 Focus ST calibration requires no supporting hardware.
You can fit a GPF-back exhaust at Stage 1 if you want additional sound, but it is not necessary for the power increase.
Does Stage 1+ need an exhaust?
No.
Our Stage 1+ setup focuses primarily on:
- Performance intake
- Uprated intercooler
The standard catalyst and GPF can remain in place.
A GPF-back exhaust is optional if you want additional sound.
Does Stage 2 need a downpipe?
Yes.
Removing the restriction in the front exhaust section is one of the key differences between our Stage 1+ and Stage 2 hardware packages.
For best results, combine the uprated downpipe with the GPF-delete section or use a complete turbo-back system.
Cat-back and GPF-back power gains
We would primarily fit either rear exhaust system for sound and appearance rather than expecting a major horsepower increase.
The meaningful performance restriction is further forward around the original catalytic converter and GPF.
For this reason:
- Want sound: Upgrade the rear exhaust.
- Want Stage 2 power: Concentrate on the downpipe and GPF area.
Which Cobra exhaust sounds best?
There is no single best sound because it depends on how loud you want the car.
Normal GPF-back
Deeper and louder than standard while retaining the factory emissions hardware. Our preferred road-car option.
Box-delete GPF-back
Considerably louder while still retaining the original catalyst and GPF.
Cat-back / GPF-delete configurations
Removing the GPF adds some extra volume and allows more exhaust character through, although GPF removal alone does not transform the sound.
Venom
The most aggressive option, with the rear silencer removed for maximum exhaust volume.
Why we recommend Cobra Sport
Cobra Sport is our preferred exhaust brand for the MK4 Focus ST.
In our experience, the quality and fitment are excellent.
The systems are manufactured in the UK from T304 stainless steel and are hand TIG-welded.
Cobra also offers one of the widest ranges for this platform, allowing customers to start with a GPF-back system and later move towards a cat-back or complete turbo-back configuration.
Hatchback versus estate exhaust fitment
This is very important.
The MK4 Focus ST hatchback and estate use different exhaust systems.
Do not assume that a hatchback exhaust will fit the estate or vice versa.
Always check the body style stated in the product title and description before ordering.
MK4 Focus ST Estate exhausts
Dedicated estate versions are available, including the Cobra Focus ST Estate GPF-Back Exhaust.
The same general advice applies:
- GPF-back for a road car primarily wanting sound
- Box-delete GPF-back for more volume
- Downpipe and GPF-delete hardware for Stage 2
- Complete turbo-back for the most comprehensive performance setup
But the actual physical exhaust system must be the estate-specific version.
Can I upgrade the exhaust in stages?
Yes.
This is one advantage of the Cobra system.
A sensible route could be:
- Start with a GPF-back exhaust for sound.
- Later add the GPF-delete section if building a Stage 2 car.
- Add the uprated downpipe.
- Complete the system as the rest of the vehicle specification develops.
If you already know the car will be going straight to Stage 2, buying the complete turbo-back from the start can be the cleaner option.
Frequently asked questions
What exhaust do you recommend for a road-going MK4 Focus ST?
The Cobra Sport GPF-back. It improves exhaust sound while retaining the factory catalyst and GPF.
Does a GPF-back remove the GPF?
No. A GPF-back starts after the factory particulate filter and leaves it in place.
Does the Cobra cat-back remove the GPF?
Yes. On the MK4 Focus ST, Cobra's cat-back starts after the catalyst and replaces the GPF, centre and rear exhaust sections.
Does removing the GPF make the car much louder?
It gives the exhaust a little more volume, but removing the GPF alone does not create a huge increase in noise.
What is the loudest Cobra option?
The Venom / rear-silencer-delete configurations are the most aggressive options.
Does a GPF-back add much horsepower?
No. We primarily consider it a sound and appearance modification.
Does a cat-back add much horsepower?
The main performance improvement comes from removing restriction further forward in the exhaust rather than the rear system itself.
Does Stage 1 need an exhaust?
No.
Does Stage 1+ need an exhaust?
No. The key Stage 1+ hardware is the intake and intercooler.
What exhaust does Stage 2 need?
Ideally a complete turbo-back. An uprated downpipe plus GPF-delete section is also sufficient when used with the other required Stage 2 hardware.
How much power does the Stage 2 exhaust hardware add?
Approximately 10–15hp is realistic in our experience as part of a correctly calibrated Stage 2 package.
Sports cat or de-cat?
There is very little performance difference at normal Stage 2 power levels.
Can I fit the Cobra downpipe to the standard GPF?
We would not recommend it. Cobra also advises against this configuration because of the sensor arrangement and its impact on GPF functionality.
Does exhaust modification require tuning?
A normal GPF-back does not require Stage 2 calibration because the factory catalyst and GPF remain in place. Removing the GPF or changing the downpipe requires appropriate engine calibration.
Are hatchback and estate exhausts the same?
No. They use different exhaust systems, so it is essential to select the correct body-style version.
MK4 Focus ST tuning guide
For our Stage 1, Stage 1+ and Stage 2 upgrade paths, expected dyno figures and supporting hardware recommendations, read our MK4 Focus ST tuning guide.
More MK4 Focus ST performance parts
Browse our complete range of MK4 Focus ST performance parts for induction, cooling, clutch, braking, suspension, servicing and styling upgrades.
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