The Straight Six is the Best Six - And It's Making a Comeback

Henry Leung
by Henry Leung
the straight six is the best six and it s making a comeback

As a parent of two young children, I watch a lot of movies at home. Most of the blockbuster movies I’ve watched this year are remakes. This month alone, I watched Ghostbusters, Star Trek Beyond, and Star Wars: The Force Awakens. All three are part of franchises that died a decade (or more) ago and have been reborn successfully in 2016.

In the same way, inline-six engines have returned to Mercedes-Benz after nearly a 20 year hiatus in North America.

Why are straight six engines making a comeback?

Guess who’s back, back again?

The straight six is the ideal six cylinder engine from a simplicity, smoothness, and efficiency standpoint.

The smoothness and balance come from the engine’s configuration. The front three cylinders and rear three cylinders are mirror images and move in offset pairs. This allows for perfect primary and secondary balance without power-robbing balance shafts, and allows the engine to be much smoother than a V6 or inline four, especially in high-rev applications.

In addition, because of the inline layout, only a single set of camshafts is required for the valvetrain, compared to two sets for multi-bank V6 and V8 engines.

The design’s simplicity has driven the use of inline sixes for larger vehicles such as monstrous Caterpillar earth movers and even mammoth ships.

Why are (almost) all six cylinder engines V6?

The V6 engine is the engine of accountants; it was developed for packaging and modular manufacturing. In every other area, it has more vibration, a more expensive valvetrain, complexity, and worse exhaust noise than the inline six.

In the 1990s, Daimler switched from inline sixes to V6s because of cost; the new V6 configuration allowed the engine to share architecture with its V8, reducing overall development and manufacturing costs.

Other car manufacturers adopted V6s for similar reasons. In the 1980s, there was a push to adopt front-wheel drive, especially for midsize and compact cars to maximize interior room and save on cost. The other trend was the downsizing of engines; four-cylinders were substituting six-cylinders as base engines, while six-cylinder engines were substituting V8s. In addition, it’s almost impossible to fit an inline-six into the engine bay of a midsize car. The length of the motor makes it either too wide or too long, depending on its orientation under the hood.

As a result, by the late ’90s, almost all car makers had switched to V6 engines for their passenger cars. Volvo was one of the rare exceptions with its 2.9-liter inline six in the S80, but it was only able to achieve this with a special gearbox. BMW is another exception, as its tradition (at least in the ’90s) dictated inline-six engines and rear-wheel drive. Furthermore, BMW’s front engine, rear-wheel-drive layout meant it had sufficient engine bay room for a longitudinally mounted inline six.

The packaging benefits of the V6 engine did come with downsides. To share engine architecture with a V8, a V6 engine needed to have a 90-degree angle between cylinder banks. This type of arrangement needed to have extra balancing shafts to neuter vibration. A 60-degree V6 was the better design from a balance standpoint, but it couldn’t share any architecture with the V8 and had to be designed separately. In addition, because of the two separate cylinder banks, there needed to be twice as many camshafts for the valvetrain as an inline engine. This lead to additional cost and complexity.

Return of the King

After more than two decades, the stars have now realigned. Now that modular inline engines are once again cost feasible, the technically superior inline six configuration is once again the engine of choice.

In recent years, fuel economy requirements have led to engine sizes and the reduction of the number of cylinders in engines. Turbo inline-four engines are replacing V6s; turbo sixes are replacing V8s.

OEMs can now make a modular line of inline three, four and six cylinder engines on the same manufacturing line and share development costs. They also benefit from reduced valvetrain cost for an inline engine compared to a V6.

We are seeing this resurgence this year from several European manufacturers. Mercedes-Benz is abandoning its V6 and replacing it with an inline six that can also share architecture with its inline four engines. Jaguar Land Rover is rumored to be developing an inline six engine based on its Ingenium line of four-cylinder engines. For BMW, the inline six engine is here to stay for the same reasons. While there are still some BMWs available with V8s, the majority are based on its new modular turbocharged inline three, four and six cylinder engines.

It would not be surprising to see a similar revival from Japanese and American manufacturers. For example, Toyota and Nissan had turbocharged six cylinder engines in their lineups until the late ’90s. With the lack of market for new V8 engines, the next generation of engines could be developed on an inline platform.

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  • Don1967 Don1967 on Dec 15, 2016

    My Volvo 3.2 I6 has a nice smooth snarl to it. It hustles 4,000 pounds of steel along at a decent pace, while asking for little more than 87-octane fuel and a fresh crankcase of 5w30 conventional oil every 12 months. Try that with your turbo-four. The story of why they chose the transverse-I6 layout is about cost + safety. It's a mechanically-simple engine with lots of empty crush space fore and aft of the block.

  • DJEmir DJEmir on Oct 13, 2017

    Star Wars was not a reboot, remake or redo though it was a continuation whereas the others were remakes. So not quite the same. They didn't have to completely redo Star Wars, they were pioneers and were partly responsible for furthering the Special effects technology and were ahead of their time enough that it still plays well with or without the digital touches added in later.