What are catback exhausts and are they legal in the UK?

Fitting an aftermarket performance exhaust is one of the all-time most popular basic tuning modifications – an upgrade almost as old as the car itself. But what are they exactly, how do they impact performance, and are they even legal in the first place? We reveal everything you need to know about catback exhausts.

What is a catback exhaust?

Whether it’s a standard exhaust system or an aftermarket performance upgrade, a catback exhaust is the portion of the system that runs from the catalytic converter to the tailpipe at the rear of the vehicle. Although, technically speaking, “DPF back” would be a more accurate description, “catback” is also a common term used to describe the part of the exhaust system on a diesel car that is fitted after a diesel particulate filter.

In most cases, a catback system can be broken down into a number of components such as the centre pipe, back box and tailpipes, along with the addition of various silencers and resonators depending on the design for that specific vehicle.

Aside from the catback portion, all exhaust systems contain other components that make up the full exhaust tract – namely the other parts that connect the engine to the rest of the system. This includes components such as manifolds, downpipes and – on vehicles that use them – turbochargers.

While these components can be upgraded for performance, doing so is often significantly more involved (not to mention more expensive) to install than a straightforward catback upgrade bolted on after your emission control device. In essence, the catback components are the simplest part of the system, the most cost effective to upgrade, and the easiest to design and manufacture.

How do emissions control devices fit into this?

Perhaps the most important exhaust component you’ll find on your vehicle is either the catalytic converter or the diesel particulate filter. These factory-fitted emission control devices are not only extremely expensive to upgrade – if, of course, you can find a suitable upgrade at all – but here in the UK it’s also a legal requirement to have these in place for vehicles driven on the public highway. During your annual MOT test, these devices are checked both by electronic emissions testing and the MOT tester confirming that they’re physically in place. In other words, they need to be there, and they need to do their job effectively in order for your vehicle to pass.

Catalytic converters work by promoting a reaction to convert toxic gases into less toxic emissions. Because they catalyse a reaction between unburned hydrocarbons (HC) and carbon monoxide (CO) to produce carbon dioxide (CO2) and water (H2O) they’re also referred to as “catalysts”.

Diesel particulate filters (DPF), on the other hand, control exhaust emission levels by catching the soot produced by diesel engines. These then go through a regeneration cycle to burn off up to 95% of the soot before the emissions leave the tailpipe. So, while these factory devices aren’t optimal for engine tuning, they are essential for the environment, and that’s why they’re legally required.

Every petrol vehicle registered after 1993 must have a catalytic convertor, and every diesel car registered since 2013 must have a DPF. Failure to comply with these regulations will not only stop you getting your MOT certificate but, if you’re caught on the road without one, you could face a £1,000 fine. Because of this, when it comes to upgrading your exhaust, it’s far more convenient to leave the emissions control device in place and replace the rear part of the system with a performance catback exhaust.

How does a catback upgrade impact performance?

Like the name suggests, performance exhaust upgrades are mostly about getting the optimum gas flow for each specific application for better engine performance. Essentially your exhaust system affects how effectively your engine can “breathe out” and any hardware upgrade will be designed to help it perform this process as efficiently as possible.

Basically speaking, an engine “breathes in” through the inlet tract. The oxygen it draws into the cylinders is matched with a specific amount of fuel, compressed and then burned. The resulting explosion pushes down the pistons, turning the crankshaft to produce the horsepower that gets you down the road. The by-product of all this is a large volume of waste exhaust gas which must be released from the cylinders – and out through the exhaust – so the engine can repeat the cycle. All this happens within a fraction of a second.

It stands to reason then, that the more efficiently your exhaust system can get the gas out, the quicker the engine can draw in more air, essentially freeing up more power. In this way, designing a performance catback system for a road car is all about cutting down on gas flow restrictions, but crucially without making the overall system loud enough to cause an uncomfortable driving experience – or any legal difficulties.

What’s wrong with standard catback exhausts?

Like most OEM parts, standard exhaust systems are built to a budget and they’re rarely optimised for the best performance. It’s simply not a priority for most vehicle manufacturers over other considerations like noise, comfort and manufacturing costs. Large silencers that are over-engineered to keep volume to a minimum, along with small diameter pipework and excessively tight bends are all problematic when it comes to getting exhaust gases out quickly and efficiently simply because they restrict how quickly the gas can flow through (and ultimately out of) the system.

But this is exactly where aftermarket manufacturers and engine tuners can engineer their products to make power gains, simply by optimising the design and cutting down on these kinds of restrictions. Various tricks of the trade, including using larger diameter pipe for more volumetric flow, less restrictive silencers and back boxes, and smoothing out the bends for the straightest route (within reason) are all employed to help optimise gas flow and effectively increase horsepower.

How much extra power will I get?

There are limits as to what any exhaust manufacturer can do for out-and-out performance, simply because it’s not the only consideration. In tuning terms at least, a huge diameter pipe exiting in a straight line, with no back boxes and silencers to interrupt the gas flow, would offer the least restrictive route for waste gases.

For a road car though, a cost-effective exhaust designed and constructed in this way isn’t possible. First because it wouldn’t fit without a serious amount of custom fabrication work on the rest of the vehicle, but mostly because it would be so loud that it would land you in trouble with the law.

Instead, designers of performance systems will closely follow the route of the original exhaust and employ specially developed high-flow silencers and resonators to tune the sound to an acceptable level. In fact, all the best aftermarket manufacturers go through an extensive R&D process centring around sound tuning to give a pleasant performance grumble without excessive droning at speed or you having to face legal ramifications of running race car noise levels on the road.

So, while the routing is critical and high-flow silencers will always offer some restriction in gas flow, when combined they’re not as suffocating as the restrictions you’ll find in a standard exhaust system. The idea is to give you the best of both worlds here. The optimisation process is simply a trade-off between performance and practicality.

The power gain will always depend on the specific vehicle’s design and the engine fitted. Turbocharged vehicles typically net larger gains from performance exhausts than naturally aspirated cars. It also works differently for petrol and diesel vehicles. And, as the cats and DPFs often offer the biggest restriction of all, there’s always that kind of upgrade to consider, too. Generally speaking, though, a good quality catback exhaust can increase power and torque by around 2-4% on its own, and add a lot more if the other engine parameters (such as timing and fuelling) are optimised with an ECU remap.

Are catback exhausts legal in the UK?

Yes, for the most part catback exhausts are road legal in the UK. As you’re not physically removing the catalytic converter or DPF, essentially all you’re looking to achieve is bolting a freer-flowing system (post emission control device) to free up a few extra horsepower. So, just as long as you inform your insurance company about the modification, a well-designed catback exhaust from a reputable manufacturer will be a road legal upgrade.

The only thing to watch out for here is “off-road” systems or any other exhaust components that are denoted as “race-only” items. In a bid to optimise gas flow to the extreme, some manufacturers produce catback systems which don’t offer a whole lot of sound tuning, if any restrictions at all. Even though these will inevitably be freer-flowing and net you even more power, they may be a little too loud for use on the road. Ironically these can also be too loud for trackday use. Nowadays most circuits impose a strict noise limit for cars wishing to go out on track. Although these types of catback systems are perfectly legal to sell, they’re definitely not legal for road use.

In a similar vein, cat or DPF delete pipes – which replace your emissions control device with nothing but a straight pipe – are readily available and legal to buy, but they’re not intended for road use. Again, the key is to opt for a reputable exhaust manufacturer for your road car, and always do your research before buying.

Are there any other advantages aside from performance?

Improved styling is the most obvious reason why most car fans will fit a performance exhaust. A nice shiny tailpipe is much more attractive than a rusty exit pipe, after all, right? But one of the best reasons to consider a performance catback exhaust is the longevity it offers.

Standard exhaust systems, on most cars at least, are constructed from mild steel. This material may be relatively cheap and reasonably resilient to knocks and scrapes (which is ideal for the car manufacturers), but over time it will rust through and need replacement.

Exhaust systems are subject to heavy concentrations of corrosive road salt and moisture under the car, and this will only accelerate the oxidation process. So, while a standard exhaust can last for a good few years, eventually they will fail, at which point they will be out of warranty. Performance exhausts, however, are usually constructed from stainless steel, with some extremely high-end systems crafted in lightweight titanium. What these have in common – aside from looking amazing – is that they won’t rust. In most cases an aftermarket exhaust will last well beyond the lifetime of the vehicle. That’s why, for many, these are seen as something of an investment.

How do I install a performance catback exhaust?

Discounting universal exhausts and back boxes, which you will inevitably have to adapt to fit properly (not a job for the home mechanic, or the faint-hearted), there’s basically two types of aftermarket exhaust: custom-built and direct-fit.

The thing to bear in mind is that every exhaust design is different depending on the specific make, model and year of the vehicle, along with the trim level and engine option. It’s not a one-size-fits-all solution; you need the correct part to fit your actual car.

Custom-built exhausts are perhaps the simplest to install, simply because you never have to do it yourself. Here you book your vehicle into a specialist who will remove the old item and build a stainless-steel replacement there and then. It’s a great option for any vehicle – especially those rarer cars where finding direct-fit items may pose a problem – and you can be 100% certain it’s going to fit. But it goes without saying that these come with quite the price tag.

Direct-fit is another way of saying bolt-on. Just like a standard exhaust replacement you may pick up at your local motor factors, these are designed as an off-the-shelf direct replacement part that should be relatively easy to fit. These are by far the most common and, all things considered, most cost effective.

In the vast majority of cases these are initially developed on a car of the same make, model and engine, and mass produced on a jig, in larger and larger numbers depending on the popularity of the vehicle. You can take these for fitment at a professional garage if you choose to, but direct-fit items are designed to be no more difficult to fit than the standard item. In fact, if you can get the old one off, there’s no doubt you’ll be able to install the new one. Most will even use the same type of flanges, bolts, gaskets or clamps. So, while you will require some mechanical knowledge and a few hand tools to do the job, installing a direct-fit performance upgrade is unlikely to be beyond the skills of any home mechanic. In fact, that’s probably what makes these one of the greatest DIY tuning mods.

Words by Midge Burr

With 20 years’ experience writing and editing modified car magazines, Midge Burr is an expert in all things cars. He draws on his experiences as a qualified mechanic, long-time car builder and eager road rally participant, and is convinced he’s pretty much driven every type of vehicle under the sun, from touring cars and racing lorries to monster trucks and rally cars.

Choose a specialist insurance broker that offers like-for-like cover on mods

Finding an insurance company that covers modifications can be difficult, especially one that offers like-for-like cover on modifications. At Adrian Flux, we offer modified car insurance that’s tailored to your vehicle, providing you with the peace of mind that your mods – including your catback exhaust – are covered. Call us on 0800 369 8590 or book a callback at a time that suits you.