Everything you’ve ever wanted to know about exhausts but were afraid to ask… Far from just making your car sound good, a decent exhaust system will offer genuine performance benefits and make a huge difference to the driving experience for your BMW. Here’s why… Words: Jamie King.
Fitting a performance exhaust is one of the first upgrades car enthusiasts make in the search for more power. In fact, regardless of the car/engine in question, its displacement capacity, whether it is petrol- or diesel-powered, or if it is naturally aspirated or turbocharged, the old adage of ‘air filter and exhaust’ is widely regarded as the universal Stage 1 setup. Far from just enhancing a car’s soundtrack, a well-engineered performance exhaust system will help release both power and torque from an engine, and is an essential part of car tuning. But why? How exactly does a simple metal pipe improve an engine’s performance?
Well, the exhaust system itself may be a collection of simple, albeit cleverly shaped and mandrel-bent, stainless steel pipes but the science behind it is quite complicated. Therefore the key to a performance exhaust system is all in the way it is designed. Variables such as bore size, the overall length of the system, the volume of headers, the number and style of silencers used, and style of catalytic converters all need to be designed to work in harmony to produce the best possible results. In addition you can start to tinker further with fancy heatresistant coatings and even clever electronically operated valves which allow the gasses to bypass silencers altogether. Altering any one of these variables can and will have an effect on the engine’s performance – meaning they can be tweaked to the give desired characteristics. We’re going to show you how…
System bore size
The bore size refers to the internal diameter of the pipes used. For the majority of the exhaust system the bore size isn’t overly critical – just so long as it is in the right window for the horsepower generated. As a general rule of thumb a 200hp naturally aspirated engine will require a bore size of between two- and two-and-a-half inch diameter. Bigger power engines will require a larger bore size, while less powerful engines would benefit from a smaller bore size.
Forced induction engines tend to warrant larger diameter systems because, unlike the linear way a naturally-aspirated engine expels its exhaust gases, with a turbocharged engine the gases all arrive in one big hit (when the wastegate opens). Therefore turbocharged engines generally require a larger bore exhaust system to cope, typically between three- and three-and-a-half inches.
Manifold bore size
However, when it comes to the manifold end of the system, bore size becomes very critical to performance. Too small and the system will be restrictive and hold back top-end power. Too big and there will be little or no back pressure, causing the gas speed to slow considerably, again restricting the performance potential for the engine.
It’s not all bad though. While a smaller bore manifold will be more restrictive when it comes to outright power, this restriction does also mean that the gas speed is higher, typically resulting in a higher torque output. Conversely a larger bore manifold may mean back pressure, gas speed, and torque are reduced, but a greater overall power is possible.
These traits can be tailored to best suit the desired application. For example, most 16-valve engines – which inherently rev better but produce less torque than eight-valvers – tend to work better with a slightly smaller bore manifold to help keep the torque up. By the same token, more torquey engines can get away with running a slightly larger bore, which will flow more at higher engine speeds. Similarly, lightweight cars that don’t require huge torque outputs are able to utilise larger bore manifolds to help with top-end performance and outright power potential.
As a general rule, road cars tend to work better with a slightly smaller bore manifold as it offers a wider torque spread, but for the ultimate in outright power for race applications a larger bore manifold will be needed. Turbocharged engines are different again. There is no real need to worry too much about back pressure, as the turbocharger itself will act as a huge restriction in the system and create plenty of back pressure. Instead, with turbocharged applications the main concern is ensuring there is a constant stream of exhaust gases at the turbine wheel in order to keep the turbo spinning and reduce lag.
There will actually be an optimum overall length for the entire exhaust system, and there is a complicated way of calculating this. However, it is very rarely done on anything outside of bespoke race cars, simply because the size and shape of the car will nearly always dictate where the exhaust system is going to run. Even most race cars have to adhere to rules stating where the exhaust system runs/exits, so unfortunately there’s not much scope for improvement here. Thankfully, while there is an optimum length, deviating from it makes very little difference in terms of performance for fast road cars and indeed most race cars too.
Header length and volume
The length of the manifold headers, however, is exceptionally crucial to performance. More specifically, it’s actually the volume of the manifold headers that affects performance, but it’s often referred to as the length for easy recognition. That’s where the term ‘equal length headers’ comes from.
Ensuring the manifold primaries all have an equal volume will create what is known as a ‘pulsing effect’. As the exhaust gases exit one cylinder the movement past the other primaries causes a vacuum effect, which in turn helps draw the exhaust gases out from those cylinders. This cycle continues with a push-and-pull effect, with the piston pushing the exhaust gases out of the cylinder, and the vacuum within the manifold helping to pull them out of the other cylinders at the same time.
However, it’s not always feasible to achieve equal volume in all primaries simply because of design and packaging issues, or manufacturing costs. This is one area where a performance exhaust system can differ significantly to the standard equipment; when the focus is shifted from ease (and cost) of production to a performance bias, great gains can be seen by ensuring the manifold primaries are of equal volume.
Most standard production exhaust manifolds are of a pretty rubbish design if we’re honest. They are often made from the cheapest, easiest-to-work-with materials, and are designed to be mass-produced at a cost. Most are made from heavy cast iron, have unequal length primaries, and due to the crude nature of the casting process, aren’t always even the same bore size throughout.
Replacing the standard manifold with a performance item is almost guaranteed to offer increased power potential. For four-cylinder naturally aspirated engines there are two main types of performance manifold available: a 4-2-1, and a 4-1 design.
As the names suggest a 4-2-1 manifold has four primaries which link into two secondaries, and then into one collector just before the rest of the system, whereas a 4-1 manifold simply has four primaries that all link directly into one collector.
A 4-2-1 manifold generally gives more mid-range torque than 4-1 versions, because where the primaries and secondaries link they create a vacuum that helps draw exhaust gases out of the cylinders. This works hand-inhand with equal length headers, and as one wave of exhaust gases passes the primary runner of the paired cylinder it creates a vacuum which helps draw the exhaust gases out from that paired cylinder.
The tricky part is deciding which cylinders to pair when using a 4-2-1 manifold. It can vary depending on application but usually cylinders one and four, and cylinders two and three will be paired.
4-1 manifolds tend to be used in applications where outright power is of more concern than driveability and a wider torque spread. For instance race cars that only ever use the top-end of the rev range will benefit from a 4-1 manifold, as they are less restrictive at higher revs, as the 4-1 design means the manifold can physically expel more exhaust gases more quickly. This also means that typically 4-1 manifolds will have a greater top-end power potential compared to a 4-2-1 design, but at the expense of mid-range torque. Therefore the general consensus is that 4-2-1 manifolds are better suited to fast road cars and 4-1 manifolds are better suited for high-revving race applications.
Another important job the exhaust system has to do is reduce the noise output from an engine. Yes, a system with no silencers at all will be the least restrictive and therefore the best in terms of performance, but the compromise is unbearable volumes of exhaust drone. Not only will this ruin any car you plan to use on the road, it will also mean that you’ll never make it on to a track day anywhere.
Therefore, although silencers are a restriction in the exhaust system, they are a necessity, and as such need to be considered carefully in the design of a system.
There are two main types of silencer; ‘baffled’ and ‘absorption’. Most standard production cars will come out of the factory with baffled-style silencers. They are very good at silencing the exhaust noise, but are also very restrictive – the gases have to wind their way around a series of baffles; this slows everything down and therefore reduces noise.
But slowing the speed of the exhaust gases down causes an increase in back pressure, and ultimately restricts performance of the engine. Therefore most performance exhaust systems feature absorption-style silencers. These units utilise a perforated tube within the silencer, surrounded by packing. As the exhaust gases pass through the silencer, they are allowed to expand – through the perforations – and into the packing, therefore ‘absorbing’ some of the noise. Absorption silencers aren’t as effective as baffled silencers at reducing noise, but they are much less restrictive, and still silence the system enough to make the car driveable.
In order to meet the stricter emissions regulations, cars produced after 1994 will require the use of a catalytic converter within the exhaust system. A cat, as it’s known, contains a selection of precious metals that help reduce the amount of CO and other nasty gases that a petrol combustion engine produces. But what does that mean in terms of performance? Well, it’s adding another huge restriction to the exhaust system so ultimately it’s bad news for performance.
Simply chucking it in the bin and replacing it with a straight piece of pipe – or a de-cat – is great for performance but the car will never pass an MoT again in its life, and even some race regulations now insist a cat is fitted. So a de-cat isn’t really the answer.
However, sports cats are. A typical standard catalytic converter contains between 400-600 cells per square inch, whereas most sports cat are either 100 or 200 cells per square inch. So while not as free flowing as a de-cat pipe, a sports cat is much less restrictive than a standard catalytic converter, allowing the exhaust gases to pass through them much easier and faster, but still reducing the emissions enough to keep the car road legal. For cars produced before 1994 the regulations on emissions are much more relaxed, and can easily be met without the need for a catalytic convertor.
Coatings and wrappings
By its very nature, an exhaust system gets hot – very hot! And heat in an engine bay is always something best kept to a minimum. Whether that’s to avoid powersapping heat soak, potentially damaging radiated heat, or just to make it a little safer to get your hands in the engine bay, keeping underbonnet temperatures down is always a good move. But stopping heat coming out from the exhaust is only half of the story. By keeping the temperature within the exhaust system the exhaust gases will be hotter – therefore they will have more energy and move faster, and ultimately the quicker the exhaust gases exit the system the more power potential an engine has.
Both heat wrap and ceramic coating are great ways of keeping heat within the exhaust system, acting as a thermal barrier between the exhaust and the surrounding atmosphere. Ceramic coating is more costly than heatwrap, but will last longer, looks neater and can deal with very high temperatures. However, if the budget won’t stretch that far, simple heatwrap will offer similar performance benefits for less money.
As we have previously touched upon, choosing the right exhaust system for your needs is always going to involve some form of compromise between outright performance potential and everyday usability. But what if you could reduce some of that compromise by allowing an exhaust to be either free-flowing for ultimate performance, or quiet and considerate for everyday use at the touch of a button?
Well you can, and many new sports cars, including the M-cars, allow you to do just that by utilising electronically controlled valves within the exhaust system. This basically means you can alter the path the exhausts gases take to suit your needs at that time. In crude terms this means you switch between a straight-though system for optimum performance and enhanced noise, to a more conventional silenced system for everyday use or when maximum performance isn’t required.
In the case of the M3/4 the exhaust system employs two exhaust valves (one in each side of the car’s dual exhaust system), which are mounted just before both pipes join a common rear silencer. When the valves are open the exhaust gases virtually bypass the rear silencer altogether and exit straight out of the corresponding tailpipes (left-hand gases exits via left tailpipe, right-hand gases exit via right tailpipe). This offers very little silencing effect (although there is a centre silencer in this system) and provides enhanced engine noise, low back pressure, and improved performance.
When the valve is closed, however, the gases are forced through the rear silencer and out of the opposing tailpipe (left-hand gases exit via right tailpipe, right-hand gases exit via left tailpipe). This not only forces the gases through the silencer but it also effectively increases the length of the exhaust system, resulting in a much greater silencing effect.
In reality things are considerably more complicated than this, and being controlled by the car’s ECU means the valves are much more than just a simple ‘on/off’ switch; they are constantly opening and closing (sometimes partially) to provide the optimum exhaust system for a given set of conditions. For example, even with the car set in ‘comfort’ mode the exhaust valves may remain shut under lower revs, but they will still open to assist performance when you put your foot down. Likewise, even with the car set in ‘sport plus’ mode, the valves may be open when accelerating hard, but they will close (at least partially) when cruising at a constant speed to avoid excessive drone. Many performance exhaust specialists are now factoring exhaust valves into their aftermarket upgrades, combining all the performance advantages of an aftermarket system with the practicality and everyday usability of a standard system!