Musings on the Motoring World

Straight-six engines are making a comeback – thanks to downsizing

Nothing meek ever made history. If you want a mention, you got to go grand. And when it is the subject of automotive history, there will never be an era quite as glorious, or as excessive, as the German uber-saloons of the mid-noughties. It was a time when BMW stuck a racy V10 into the nose of its latest M5, AMG developed a gargantuan 6208cc naturally-aspirated V8 that saw duty in the E63, and Audi’s quattro division took the nuclear option by arming the RS6 with a V10 that has a pair of turbos strapped on for good measure.

If you were hard up for big hearts, there was never a better time to be alive than such a time as this, as what followed quickly in the trio’s wake was an era of caution and trepidation with the pre-2008 commodities boom sending fuel prices through the roof and Eurocrats starting to get iffy about the gases cars were pumping into the atmosphere.

As taxes were raised and regulations were tightened, car makers quickly started downsizing and turbocharging their next generation of engines to deliver the power improvements customers expected with every successive generation, whilst keeping the bureaucrats satisfied that we aren’t letting our hedonism burn down the planet with everyone in it.

Thanks to the downsizing of engines “progress”, as measured by the reliable metric of…metric horsepower, continued on its trajectory though at the expense of some of the automotive world’s most prized powerplants. The M Division’s shortlived – in both lineage and longevity – S85 V10 was succeeded by a unit with two fewer cylinders and a pair of turbos. AMG’s new 4-litre M178 V8 traded more than two litres of displacement from its 6.2-litre predecessor for turbochargers. Audi’s subsequent RS6 would eventually sport a more powerful – and more sensible – biturbo V8, though on a related note, its parent company, Volkswagen, retired the venerable narrow-angle VR6 engine in the Golf R32 in favour of a souped-up blown four-banger from the Golf GTI in the first R-focused product, the Golf R. It was certainly austere times in all but where it matters in the European theatre of power outputs.

Though fuel prices never reached the levels of the fevered dream of a Mad Max doomsday prepper, the downsizing trend had enabled carmakers to trim their emissions on the test cycle while turbocharging gave customers the torque-rich powerplants they so desired. For customers, manufacturers, and bureaucrats, it was a win-win deal – except for the “no replacement for displacement” brigade.

While the era of mechanical exuberance fades away under the shadow of environmental prudence, it has however allowed the return of one of the automotive world most celebrated engine types, the straight-six engine. Ever since engineers got their heads around the nuances of engine design, the straight-six engine has always been an engineer’s favourite.

Prized for its inherent smoothness due to the movements of the motor’s first three cylinders being mirrored by the movement of the last three cylinders, canceling the rocking motion from each set of cylinders. The layout was held in such high regard that nearly every big name manufacturer from Toyota right up to Rolls-Royce has had a straight-six engine in its arsenal. Unfortunately, by the time the era of our aforementioned uber-saloons rolled around, the straight-six engine was all but extinguished as engineering preferences give way to the two-pronged constraints of economies of scale and packaging.

As front-wheel drive layouts quickly became more prevalent in any carmaker’s line-up it was clear that it was no place for the long straight-six arrangement – unless you were the mad vikings running Volvo – thus paving the way for the V6 which can be fitted to both front-wheel drive and rear-wheel drive layouts. Furthermore, the lower and shorted dimensions of the V6 engines meant it was safer for pedestrian crash safety and allowed for the fitment of bigger frontal crash structures respectively.

That being said, though similar in cylinder count, the V6 layout, which is basically two inline-three engines joined at the crank, could never deliver the same smoothness as the traditional inline-six layout without the use of balancing shafts to “cheat” past its inherent imbalance.

Like any big engine, however, when the downsizing trend rolled around the V6’s very existence was threatened as newer turbo four-cylinder engines were able to match its power outputs without having to deal with its weight or handicap in fuel consumption test cycles. For many premium makers, like Jaguar and Mercedes-Benz who based some of their V6 engines on a bigger V8 unit with it 90-degree angle between cylinder banks – as opposed to the smoother-running 60-degree angle favoured by most V6 manufacturers – the economies of scale for both engine layouts were put in jeopardy as demand for both V6s and V8s declined in favour of tiny blown four-bangers.

There was however an elegant solution, and that was first demonstrated by the stubborn engineers at BMW who stuck to their love for the buttery smoothness of the straight-six engine. During the mass proliferation of downsized engines in 2011, BMW debuted a new engine series known as the N20, which featured a 2-litre four-cylinder turbo unit that was engineered with the ideal 500cc per cylinder arrangement. More pertinently the N20 engine was meant to be a part of BMW’s bigger modular engine strategy which maintains a critical bore spacing – the space between each cylinder – allowing BMW to make inline-three, inline-four and straight-six iterations.

Seeing as there would be a greater demand for four-cylinder engines than there would ever be for V6s or V8s in the near future, building a modular design out of an inline cylinder arrangement was both technically and financially sound. Adding to that, with the public being quickly normalised to the punchy low-end torque delivery of turbocharged units, the straight-six engine allows for a simpler and more cost-effective turbocharger and exhaust setups, which in turn becomes a huge boon for the adoption of electrically assisted turbochargers.

This combination of inherent engineering advantages and market acceptance of turbocharged inline-four engine has seen Mercedes-Benz and, more recently, Jaguar Land Rover reintroduce a new generation of straight-six engines, each based off a modular architecture that features the ideal 500cc per cylinder displacement and electrically-assisted turbochargers. That isn’t all. Rumours abound that Chrysler too will follow suit with a new straight-six engine that might replace the Pentastar V6, while Aston Martin is planning their own straight-six to replace the V8s they source from AMG.

Considering that the downsizing era of post-2008 had brought power to the masses with engineers being able to wring out big horsepower figures from relatively tiny hot-hatch sized engines and now precipitated the return of the glorious straight-six, it wasn’t just the end of an era of excess, but the beginning of another promising and celebrated one.

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