Examining exhaust design

The casual observer could be forgiven for thinking that an exhaust pipe is just a tube with a silencer and technological change is rare. Nothing could be further from the truth, one aftermarket industry supplier tells Jon Lawson

Specialising in exhausts, Milltek has crafted a reputation over the past four decades for fabricating components from stainless steel and titanium, selling over 16,500 systems in the last 12 months alone. Kev Hall is the technical development manager, joining the company after years as a customer working in product development and calibration.

Pondering industry changes, he notes, “One area we have seen significant advances in, and have heavily invested in, is laser technology. Since the introduction of our £2.5 million advanced manufacturing facility in 2022, we’ve been utilising our twin 5-axis robotic laser along with a tube and sheet laser to optimise the speed and efficiency of our production process, although we’re still heavily invested in our workforce and pride ourselves on the capabilities of our welding team.

He continues, “This last year we’ve been testing the capability of laser welding equipment and have embraced this technology, adding multiple laser welding bays to our production facilities. This equipment is perfect for more intricate parts as there is less heat generated, therefore less heat affected zone (HAZ) and material distortion. Whilst it won’t replace traditional welding methods, it’s an expansion of our technical expertise and is an easier skill to teach new apprentices.”

And this isn’t the only novel technology the company is directing its efforts towards, as Hall explains. “We were early adopters of 3D printing technology,” he says. “We’ve used plastic prototyping for aesthetic reasons for tips and panels for years, increasingly we are now using more robust materials in the manufacturing process, for example adapters in the valve control system.”

The exhaust market has been undergoing change, according to Hall driven by exhaust legislation dating from 2019. “There’s always been a requirement for a better quality sounding exhaust,” he observes. “But there’s been a shift in the balance between customers wanting power and sound, to just wanting sound improvements.”

Noise tuning is a very subtle business. Hall explains, “What we do here depends on where the vehicle will be sold. Europe, China, America – the legislation is different in each market. For example, cars sold in the EEC area must emit no more than 2dB above stock specification, our modular design is more to enable us to be efficient in our production and stocking of exhaust systems. We aim to have as much crossover of parts between our different systems to ensure customers can easily change the exhaust configuration (if required) and for production efficiency. We rely on different silencer designs, resonated and non-resonated, and where OEMs fit valves into the exhaust we can redesign these to give a bigger sound differential.”

Another ongoing trend which directly affects the exhaust designer’s life is the increased use of forced induction. This offers some level of muffling within the turbocharger which has a knock-on effect for the silencer design. With its motorsport heritage, Milltek has an advantage here and despite access to the modern CAE tools, the company often bypasses this stage, preferring to jump to prototyping and physical testing early on.

“On a new vehicle, we like the real-world scenario rather than a testbed,” Hall confirms. “We will do rudimentary flow work but for the 90% of new-builds we will purchase a vehicle because it’s the fastest way to get the design correct. This aspect is important when dealing with common platforms such as a number of Volkswagen or Audi models where the silencer design and emissions side of it might be common to all, but the exhaust layout is not.”

So, the majority of testing, whether it be for durability or performance, is done either on the road or the track, with dyno work if necessary. Heat management is key here, measured both with standard OEM and additional sensors.

It’s a subject that becomes acutely important when dealing with hybrid vehicles. “We have been interested in hybrids for years,” says Hall. “Right from the base models up to Porsche Cayenne Turbo E-Hybrids, which we are currently doing development work on, in particular where sound is concerned. As it jumps from electric to engine power, the valve control in the exhaust is crucial here. We’ve also worked with BMWs and JLR products, and when implementing a full system we spend a lot of time looking at heat management where there are large battery packs in the floor. Thermal wrapping and coating is something we do frequently with the higher power models. The job is actually getting easier as there’s more space now on some models due to additional sensors and also other aspects such as the installation of an Otto Particulate Filter (OPF) or Gasoline Particulate Filter (GPF)which may not have been there a decade ago.”