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BMW’s S55 is fast and responsive, but it doesn’t sound like the iconic M Power straight-sixes we remember. Here’s why – and how a single exhaust upgrade helps it find its voice.
On paper, the S55 is everything we expect from a modern M engine. When it debuted in the F80 M3 in 2014, it set a new bar in terms of throttle response for turbocharged engines, and revved to 7500rpm. It’s reliable, efficient and as we all know… highly tuneable. The S55 was deemed so good it earned BMW the 2015 International Engine of the Year award.
But in the singular pursuit of throttle response, perhaps still weighed down by the legacy of scalpel-sharp, naturally aspirated M engines, BMW overlooked one crucial ingredient: the soundtrack.
Compared to its predecessors or even the turbocharged N55, the S55’s flat, nasally note leaves a lot to be desired. Even M die-hards will admit it sounds harsh under load. And while some of that can be blamed on turbocharging and emissions regulations, there's a deeper reason why the S55 doesn’t quite hit the mark.
The S55 runs a pair of mono-scroll turbos – one feeding cylinders 1–3, the other 4–6. That in itself isn’t unusual for a twin-turbocharged setup. But the way BMW configured the exhaust system effectively creates two separate inline triples, rather than a unified six-cylinder.
The main culprit is the downpipes. They’re unequal in length, producing opposing exhaust pulses that remain out of sync all the way through the mid section – only merging at the rear silencer.
On top of that, the centre-section has been designed with emissions and noise regulations in mind, utilises a combined silencer containing a small internal crossover to reduce drone. But it’s also restrictive and fails to correct the imbalance. If anything, it amplifies the uneven, slightly synthetic tone many owners find frustrating.
The answer lies in equalising those exhaust pulses, and that’s exactly what an equal length mid pipe does. By ensuring the gases from both banks travel an identical distance before merging, the mid pipe brings balance back to the force. Exhaust pulses become more synchronised, smoothing out the sound and restoring that classic straight-six growl.
Instead of two competing triples shouting over each other, you get a single, coherent voice – richer, deeper, and more satisfying across the entire rev range.
There is more good news too. If you’re just looking to improve the sound of a standard car but don’t want to draw too much attention – or if you’ve already got catless downpipes installed – you can add Resonator Pipes to the package to keep the decibels under control.
If you’ve ever stepped out of your M3 or M4 thinking it should sound better for the way it performs, an equal length mid pipe might be the most transformative exhaust mod you can make – and it’s done through proper engineering, not artificial sound design. For us, it’s a no-brainer.
Find out more about the Active Autowerke mid pipe in this YouTube series:
If you’re unsure which mid pipe is right for your setup, just chat with one of our real, human team members. We’ll help you find the perfect part for your build – and can even get you booked in at Xatrix for professional fitting.