Whether you’re looking for more power, a better soundtrack, or simply want to give your Ford ST or RS a more aggressive look, upgrading your exhaust system is one of the most popular modifications you can make.
However, with terms such as cat-back, sports cat, resonated, non-resonated and de-cat being used everywhere, it can be confusing to know which option is right for your car.
This guide explains everything you need to know before buying an aftermarket exhaust.
Why Upgrade Your Exhaust?
An aftermarket exhaust offers several benefits over the restrictive factory system:
- Improved exhaust flow
- Increased engine performance (when combined with other modifications)
- Better throttle response
- More aggressive exhaust note
- Reduced weight (on some systems)
- Improved appearance with larger or upgraded tailpipes
For many Ford enthusiasts, the sound alone is enough reason to upgrade.
What Is a Cat-Back Exhaust?
A cat-back exhaust replaces everything after the catalytic converter.
Typically this includes:
- Centre pipe
- Resonator (if fitted)
- Rear silencer(s)
- Tailpipes
The original catalytic converter remains in place.
Advantages
- Usually MOT friendly
- Straightforward installation
- Noticeably improved sound
- Small performance gains
- No engine management issues on most vehicles
For most road cars, a cat-back system is the best balance between performance, legality and cost.
What Is a Sports Cat?
A sports catalytic converter replaces the original catalytic converter with a higher-flow version.
Factory catalytic converters often contain dense ceramic cores designed to prioritise emissions.
Sports cats typically use:
- Larger cell sizes
- Less restrictive construction
- Improved gas flow
Common ratings include:
- 200 Cell
- 100 Cell
The lower the cell count, the less restrictive the converter.
Benefits
- Better exhaust flow
- Increased horsepower
- Faster turbo spool (turbocharged cars)
- More aggressive exhaust note
- Usually emissions compliant if correctly designed and fully warmed up
A quality 200-cell sports cat is often the preferred choice for performance road cars.
What Is a Resonated Exhaust?
A resonated exhaust includes an additional silencer known as a resonator.
Its purpose is not to make the car quiet.
Instead, it removes certain frequencies that cause:
- Cabin drone
- Harsh rasp
- Unpleasant motorway noise
Ideal For
- Daily drivers
- Long-distance commuting
- Family cars
- Drivers wanting a sporty sound without excessive volume
Many people are surprised that resonated systems still sound significantly louder than standard.
What Is a Non-Resonated Exhaust?
A non-resonated exhaust removes the centre resonator completely.
This allows exhaust gases to flow with fewer restrictions and significantly increases volume.
Expect:
- Louder cold starts
- More pops and burbles
- Increased crackles on overrun
- More turbo noise (where applicable)
- More cabin drone at motorway speeds
Ideal For
- Weekend cars
- Track cars
- Drivers who want maximum sound
If your car is used every day, consider whether extra noise is something you’ll enjoy after several months—not just on day one.
Sports Cat vs De-Cat
Many enthusiasts wonder whether they should fit a sports cat or remove the catalytic converter entirely.
Sports Cat
Advantages:
- Better performance
- Usually road legal when emissions standards are met
- Reduced smell
- Less likely to trigger warning lights
- Better for resale
Disadvantages:
- More expensive
- Slightly more restrictive than a de-cat
De-Cat Pipe
A de-cat completely removes the catalytic converter.
This creates the least restrictive exhaust path possible.
Advantages:
- Maximum exhaust flow
- Loudest possible exhaust note
- Often cheapest option
- Popular on dedicated track cars
However, there are significant downsides.
The Pitfalls of Running a De-Cat in the UK
Although a de-cat can offer small performance gains, it comes with important compromises.
MOT Failure
If your vehicle was originally fitted with a catalytic converter (which almost all modern petrol cars were), removing it will generally result in an MOT failure because the required emissions equipment is missing and the vehicle is unlikely to meet emissions limits.
Road Legality
Driving a car on UK roads without the required emissions equipment may make it non-compliant with construction and use regulations.
Engine Warning Lights
Many modern vehicles monitor catalytic converter efficiency using oxygen sensors.
Removing the catalyst can cause:
- Engine Management Light (EML)
- Fault codes
- Limp mode on some vehicles
Some vehicles require ECU calibration to minimise these issues, although this does not change the legal or MOT position.
Strong Fuel Smell
Without a catalytic converter you’ll often notice:
- Raw fuel smell
- Exhaust fumes around the vehicle
- More noticeable odours when stationary
Many owners find this becomes unpleasant during everyday driving.
Insurance
Failing to declare a de-cat (or any performance modification) could invalidate your insurance. Always tell your insurer about exhaust modifications.
Will an Exhaust Add Power?
The answer depends on the rest of your setup.
Typical gains:
Cat-back only
- 2–5 bhp
Sports cat with cat-back
- 5–15 bhp
Full exhaust combined with ECU remap
- Much larger gains, especially on turbocharged engines
Naturally aspirated engines generally benefit more when the exhaust upgrade is combined with intake improvements and ECU tuning.
Which Exhaust Is Right for You?
Daily Driver
Choose:
- Resonated cat-back
- 200-cell sports cat (if upgrading)
You’ll enjoy a sporty sound without excessive motorway drone.
Fast Road Build
Choose:
- Non-resonated cat-back
- Quality sports cat
- ECU remap
This offers an excellent balance of sound, performance and drivability.
Track Car
Choose:
- Non-resonated system
- De-cat (for track use only where permitted)
- Full performance exhaust
This setup prioritises performance over comfort and road legality.
Choosing a Quality Exhaust
Not all exhaust systems are created equal.
Look for:
- T304 or T409 stainless steel construction
- Mandrel-bent pipework
- TIG welded joints
- Quality mounting brackets
- Lifetime warranty where available
- Proven fitment on your specific Ford model
Trusted manufacturers such as Cobra Sport, Milltek Sport, Scorpion Redpower, Airtec Motorsport and other reputable performance brands produce systems designed specifically for Ford ST and RS models.
Final Thoughts
An exhaust upgrade is one of the most rewarding modifications you can make. The right setup can transform how your car sounds, improve performance, and make every drive more enjoyable.
For most road-going Ford ST and RS owners, a resonated or non-resonated cat-back paired with a quality 200-cell sports catalytic converter offers the best combination of performance, reliability and everyday usability.
Before making any changes, consider how you use your car. The loudest system isn’t always the most enjoyable, and while de-cat pipes remain popular for dedicated motorsport applications, they come with significant legal and practical drawbacks for road use in the UK.
Choosing quality components and ensuring they’re professionally installed will help you enjoy years of trouble-free performance.
