Exhaust System - Tech Guide

Tech Guide. We take a closer look at exhausts. The problem with the exhaust is that the only part owners ever really see is the last two or three inches – the rest of it is unseen and generally uninteresting. Sadly manufacturers know this so the car’s exhaust is usually always built down to a price and not up to a specification – welcome to the world of marketing.

Bob BMW Written by Thursday, 09 April 2015 22:12
Exhaust System - Tech Guide Exhaust System - Tech Guide 2015 / 2016 Drive-My
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This is a huge disservice to what an exhaust does. Arguably it’s one of the most important parts of the engine and certainly upgrading it can make one of the greatest differences to a car’s power, be it normally aspirated or force induced.

The function of an exhaust is simple enough: take fumes created by the burning of air and fuel, clean them (a bit) and expel them from the vehicle via a length of tube. However, as we’ll see, the reality is a lot more involved…


 

Catalytic converter

A catalytic converter (cat) is the expensive bit of the exhaust. Since 1988 all cars have to have them and since the EU blessed us with extra emissions controls (known as Euro 5) all cars now have to have two – a pre-cat and a main cat. A cat is not like a filter, it doesn’t filter out matter like an air filter does but inside it contains hundreds of cells. These cells are coated with a variety of metals and coatings, the majority of which are expensive, hence the modern scourge of people stealing cats and selling them on.

Cats work by chemically reacting with the nasty gases created by the burning of fossil fuel and converting them into slightly less nasty gases, so the quality of the coatings (together with the number of cells) has a direct effect on the restriction of airflow and the level of emissions from your car. These emissions are measured by a series of sensors attached to the exhaust, commonly known as O2 sensors (as one of the gases they measure is oxygen) or Lambda sensors (referring to the air/fuel mix).

These sensors ultimately feed data back to the car’s ECU to determine fuel mix and to ensure the car’s emissions are within acceptable limits. If they don’t a light on the dash goes on and it’s off to the garage you go. These sensors are the ones that all got fouled when Tesco accidentally sent out contaminated fuel a few years ago. It ended up costing the company quite a few million.


 

The exhaust System

There are five main components to the exhaust: manifold; tube; catalytic converter; silencer; and tip. For diesel engines there is also a particulate filter.

The manifold attaches to the block of the engine or to the exhaust housing of the turbocharger and its job is to channel the airflow out into the rest of the exhaust system for onward extraction from the chassis. The manifold connects to the downpipe, which (as the name suggests) runs down from the engine to the underside of the car. Here it connects to the rest of the system which comprises lengths of tube welded together and spread along the system. There will be: a pre-catalytic converter (known as the pre-cat); the main catalytic converter (cat); one, two or three silencers; and then the exhaust tips (also known as exhaust trims). Diesels can also have a particulate filter (depending when they were built) which is there to filter out some of the soot created when burning diesel fuel. Silencers are the big heavy boxes which keep noise down. They either absorb the sound waves created by the engine with soundabsorbing material (known as absorption silencers) or by bouncing the sound waves around a series of baffles (these are deflection silencers). You can’t tell which type you have by looking at the outside, you need to cut them open, but generally OEM systems use the deflection system and aftermarket ones use the absorption method. Deflection systems are better at controlling noise while absorption ones are better at retaining power. Connecting all these pieces together will be lengths of tube which will be anything from 1” to 4” bore, depending on the power of the car.


 

Materials

Most manufacturers’ exhausts are a combination of stainless steel and mild steel, usually with the visible bits in stainless and the bits you care less about in mild steel.

The problem is mild steel rots and is heavy, so over time the material will fail. Aftermarket systems all tend to be stainless steel, usually type 304 (which is the highest quality) although lower specification types exist so be sure to check if you are upgrading. Given the choice, stainless steel is best but more expensive. There are other, more exotic, materials as well. Titanium has become more popular as it is lighter weight than steel (but is a lot more expensive) and even Inconel, which is used on F1 exhausts. It has a higher heat tolerance but makes titanium look cheap in comparison!


 

Upgrades

Ugrading your exhaust can be one of the best modifications you can do to your car, adding power, torque, improving the sound and saving weight.

An engine needs to expel exhaust gases quickly so it can intake more air and make more power. To expel these exhaust gases faster you need to improve the efficiency of the process by removing any restrictions. This can be done by removing choke points (cats, silencers, bends in the tube) or increasing overall volume (wider pipe, shorter system).

It’s not as simple as just running a straight bit of pipe from the engine to the back of the car, though. For a start it would be so loud that you’d do some serious ear damage, even at low rpm. An F1 car (pre-turbo) generates over 150dB – that’s louder than a jumbo jet on take off. You’d also fail an MoT, set your dash off like a Christmas tree and likely have a negative effect on the fuelling of the car (depending how old it is).

You also need some kind of back pressure to allow the engine to generate power, however this is often overstated. Some back pressure is needed but it’s not the be all and end all for exhausts, simply that a little is needed at some point in the system. So what you need is some sound reduction, some emissions control and some back pressure while looking to speed up airflow and add power. Sound reduction on a standard car is created by deflection silencers. These silencers force the air to bounce about different chambers thus slowing down and breaking up the sound waves to reduce noise. Replacing these with absorption silencers where the air travels directly through but the sound waves are absorbed by a layer of sound deadening material means you can increase airflow for a relatively small increase in sound. This is one of the most cost effective areas where gains can be had.

Cats are another area. Replacing a 400-cell cat with a 200-cell version effectively means you increase the airflow by 150% (it’s not a 1:1 ratio, sadly). However, you must make sure the quality of the cell coating is increased accordingly otherwise the system will fail an MoT and set off issues with the car’s ECU. It’s the coating quality that costs: a 60-gram cat is three times the price of a 30-gram cat but is likely to be needed if you want to reduce the number of cells present and still avoid issues.

The bore of the pipe is important for airflow: a 3” bore will move three times the air a 1” bore pipe will, so maximising the bore will give some good gains to airflow. However the bore has the most effect on back pressure (depending on the length of the pipe) so whilst power maybe increased, torque may be reduced. And torque is very important for driveability.

The whole process of exhaust design is a balancing act; power, torque, noise, emissions are all factors to consider and all are affected by the different elements of exhaust design. Put a cat too close to a turbo and you could melt it due to excess back pressure. Too small a silencer or wrong shape and you could create awful boom in the cabin. And, of course, every car is different in terms of engine and underbody design. Then, and only then, do you get to the one thing you can see, the tips, which leaves the final question: whether you want twin or single exit, steel, carbon fibre or titanium ones!


 

Aftermarket options

Should you get an off-the-shelf or custom exhaust? Both have their positives. Off-the-shelf exhausts usually mean repeatable construction and consistency of fitment. However a custom exhaust means a personal touch and an exhaust that is perfectly fitted to your car. It will have the sound level and look you want rather than what everybody else has. It’s your call. 

Bob BMW

Bob Jr Harper - our specialist in post-war BMW cars

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