Klarius explains exhaust system back pressure: video

Monday 31st October 2016 | 0 Comments

 
Klarius Products manufactures over 2 million exhaust systems every year in the UK
Klarius Products manufactures over 2 million exhaust systems every year in the UK

UK exhaust manufacturer Klarius has posted a series of web training videos to YouTube, explaining the nuances of exhaust design, and such necessities to engines as back pressure. Since back pressure is controlled by the design of the entire exhaust system – including diameter, shape and length of the exhaust and any baffles present in the muffler – Klarius states that detailed design and testing is a necessity. The company manufactures more than two million exhaust systems every year in the UK.

Head of R&D, Doug Bentley explains everything you need to know about back pressure: “Aftermarket sports exhausts are usually the first place we turn to for an example of how to get things right and wrong, they vary from titanium works of art through to generic universal big bore mufflers. They are usually tuned for more volume, which is usually thanks to fewer sound deadening materials and baffles in the silencer boxes. If the pipe is the wrong diameter though the system, a ‘freer breathing’ low-end exhaust system can actually reduce power.

“We make the race series exhaust systems for a range of competition cars based on road cars, the Mazda MX5 series and the BMW compact cup / 330 Challenge are obvious examples. The balance we have to strike there is to make sure the cars pass track noise regulations, and emissions tests, as well as maximising engine power, particularly at higher revs as needed during racing events.

“Customers are often surprised to find that the exhaust pipe they see at the back of the car is only slightly wider than the road car, and if you remove the cosmetic tips and heat shields, then it can look smaller. There is a reason for this; an engine is designed for a specific flow of gas through the piston chambers, valves and exhaust system. The engine tune relies on some degree of back pressure to push against in order to balance the gas pressure in the engine and maximise power and torque.

A type approved replacement Exhaust, CAT or DPF item for a road car or light commercial vehicle will maintain the correct, balanced, back pressure of the original at normal engine speeds

A type approved replacement Exhaust, CAT or DPF item for a road car or light commercial vehicle will maintain the correct, balanced, back pressure of the original at normal engine speeds

“Take a drinks straw as an example, if you blow through a small straw with a small diameter the pressure in your cheeks rises and becomes uncomfortable, choose a wide straw and the pressure reduces, blow out of your mouth and there is no pressure at all, just atmospheric pressure and you soon run out of puff. If you remove the exhaust from an engine entirely it will run very badly, misfire and produce less power, conversely if you put too many restrictions into an exhaust system the engine will labour unnecessarily, getting pressure right – is important.

“This is an extreme example, but it illustrates the principle. The same applies for turbo charged and normally aspirated engines, the only difference being there is a pressure drop across the turbo which has to be compensated for. The pressure maintained in the exhaust manifold in a turbocharged engine is a higher than that in the rest of the exhaust system.

“If an exhaust system fitted to a road car breaks or corrodes and the familiar blowing noise appears then it will also reduce its power and start using a lot more fuel. Many modern engines rely on ECU calibration and use sensors attached to the exhaust system to check exhaust gas temperature and content of exhaust gas, i.e. is the engine running lean or rich, which will have an impact on emissions too. In these cases the ECU might put the car into limp home mode, or in extreme cases shut the car down entirely.

“The issue with a break or a hole caused by mechanical damage or corrosion is that it will change the back pressure in the exhaust system, reducing it. We have seen some cheap aftermarket exhausts that already have the incorrect back pressure and have disastrous effects on engine performance, efficiency, emissions and engine wear. Klarius puts every single exhaust design through a type approval process, this means the official independent government body (the VCA) that sets and polices product performance standards, tests our products and issues a certificate of conformity, which is a guarantee that each product either matches or exceeds the performance of the original.”

Klarius type approval policy

Exhaust design is vital, get it wrong and an engine will have reduced power, use more fuel and generally require servicing more often

Exhaust design is vital, get it wrong and an engine will have reduced power, use more fuel and generally require servicing more often

Bentley adds that the company’s full type approval policy incurs more R&D costs, though the production of a large number of exhausts means the cost is amortised over a large run. This also applies to catalytic converters (CATs) and Diesel Particulate Filters (DPFs). Both of these systems can create back pressure, as Bentley explains: “the CATs we employ on the race systems for example have a lower cell density within the monolith, (often referred to as 200 cell units) they are designed to minimise back pressure at high engine speeds, but are more expensive because they are made in lower volumes than standard units.

“At normal engine speed ranges however a standard CAT is more effective. The CAT is a ‘flow through’ device, so gas passes through it and reacts with precious metal on the surface of the ceramic or metallic monolith, which makes exhaust gas emissions less harmful. Providing it is the right size for the engine capacity, it won’t affect back pressure at anything but very high engine speeds.

“DPFs are actually filters, so exhaust gas has to pass through the ceramic core material, this means they are generally larger than CATs and in some cases have CATs built-in to them or as part of the same monolith design. The important thing is that the ceramic monolith has to be the right size and cell density, or there is an increase in resistance to gas flow, which increases back pressure.

“DPFs in cars that are only driven short distances tend to clog up because the cleaning routine (Regeneration), which is a controlled burn of collected soot, into ash, doesn’t happen. A blocked DPF increases back pressure hugely and will lead to an ECU taking action or eventually the car stalling. Some drivers have removed the DPF to avoid this issue, but apart from being illegal and very bad for the environment, it will also alter the back pressure in the exhaust and lead to increased fuel usage, most likely a drop in power and eventually engine problems.”

For all of these reasons, balancing back pressure is an important aspect of the exhaust system for both race and road vehicle engines. This, in Klarius’ view makes type approval a necessary step in the design process, in ensuring engine efficiency, power and emissions levels are maintained, as well as reducing engine wear.

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Category: Product News