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Resurgence of the inline six-cylinder engine: Why Toyota Supra, BMW M3, Genesis GV80 and many others are choosing the straight-six

The Supra series is well-known for its straight-sixes, so Toyota made sure the new BMW-co-developed model had the right engine.

Given the world’s (well, most of it, anyway) march to embrace electrification, it seems a bit strange that new internal-combustion engine technologies continue to evolve.

The fact is, of course, that even with Britain’s recent announcement that effectively kills the internal-combustion engine for new cars from 2030 (2035 for hybrids), there’s life in the old ICE girl yet.

And the latest trend to emerge is one that will please fans of the ICE concept in its purest form.

Yes, carmakers are rediscovering the joy of six.

Specifically, the inline six-cylinder engine; a layout that fell out of favour in the 1980s and 1990s as engineers struggled to meet designers’ and safety engineers’ packaging requirements as everybody was hustling for a five-star crash-test result.

At that point, the physically more compact V6 became the default setting ,with really only one carmaker clinging to the I6 layout.

Take a bow, BMW, because the Bavarian manufacturer was the one that never lost faith in the inline-six concept.

Of course, that was at least partly to do with the fact that the I6 had been perhaps BMW’s finest hour over the years, and its inline sixes were generally reckoned to be the best in the business.

The M3 is powered by a twin-turbo 3.0-litre six cylinder. The M3 is powered by a twin-turbo 3.0-litre six cylinder.

So, better to suffer the packaging problems than to turn its back on its key point of difference.

But other carmakers? Not so much, and the I6 was shuffled into the history books.

Mercedes-Benz went from inline sixes to V6s in the late 1990s, Jaguar went V6 a few years before that and Toyota ditched the inline six in the first decade of this century after many decades of successfully powering its LandCruiser off-roader with just such a layout, and building legendary engines such as the revered 2JZ.

And then, quite recently, it all started to turn around.

Mercedes announced the M256 straight-six seen here in its mild-hybrid AMG applications and the 3.0-litre turbo-diesel in its W213 E350d.

Land Rover and Jaguar are sharing a bunch of 3.0-litre inline sixes dubbed Ingenium for a range of mild-hybrid applications, and Toyota’s new Supra uses a straight-six, although, controversially, it’s built by BMW.

And there will be more.

The just-released Genesis GV80 large SUV in diesel form uses a straight-six, Mazda is about to unveil a new-age I6, and there’s talk that the Mazda unit will be shared with Toyota and Lexus in forthcoming models.

The GV80 is offered with a 3.0-litre straight six. The GV80 is offered with a 3.0-litre straight six.

Not only that, but there’s also chat that the Mazda six will be available in diesel and innovative SkyActivX compression-ignition petrol forms.

And, with the luxury of being able to say, “I told you so”, BMW switched its M3 back from a V8 to an inline six way back in 2014.

One thing the I6 hasn’t escaped, of course, is the march towards forced induction, but in reality, it’s this fact that has largely given the idea new legs.

The problem in previous years was that the inline six was difficult to package in increasingly aerodynamic shaped cars.

Those same cars where then difficult to engineer so that they crashed `properly’ without the engine protruding into the passenger cell.

The increasing prevalence of front-wheel-drive and east-west engines was also stacking the deck against the longer I6 which now had to fit between the front suspension towers.

In the end, it all got too hard and the world switched to the shorter (but wider) V6.

But now, with renewed interest in rear-wheel-drive layouts and a raft of other technologies, it seems the inline six is more than just viable again; it has become an aspirational format.

So what’s changed?

The difficulty in packaging a long, inline six to crash properly has been somewhat overcome with the general downsizing of engines thanks to turbocharging.

When once a carmaker would specify a 4.0-litre engine, it can now use a 2.5-litre turbo unit for the same net performance result (and lower emissions), which means the engine itself is physically smaller.

But straight sixes are also not the same shape as they once were.

Where the water pump and cooling fan were once driven off the end (typically the front) of the crankshaft, these days, those elements are electrically driven and can, therefore, be placed anywhere in the engine bay.

Electric turbochargers are showing up in cars with increasingly powerful (48-volt) electrical systems and no longer need to be mounted on the exhaust manifold.

Both those factors give designers a lot more flexibility in where components are placed around the engine.

An inline six-cylinder engine is also a simpler piece of equipment compared with a V6.

The inline engine has just one cylinder head, one set of camshafts, one cylinder-head gasket, one exhaust manifold and, with a modern cross-flow design, it’s easier for designers to keep the hot and cold sides of a turbo-motor separate for greater efficiency.

That can mean a smaller intercooler, simplified plumbing and a cheaper engine to build thanks to the smaller list of components.

An inline six also, unlike a V6, boasts what’s called perfect primary balance.

Put simply, there’s always a piston going up its cylinder-bore for every piston going down, and the power strokes are evenly spaced and actually overlap each other.

That means smooth operation that can’t – generally - be matched by a V6.

The straight six is also a cinch for modular design.

Carmakers are increasingly fond of using one set of cylinder spacings and bore sizes to arrive at different capacities simply by adding or subtracting cylinders.

An inline six is very adaptable in this regard, and it’s easy to use the same architecture to build a five-, four- or three-cylinder engine, not something that is as easy with a V6.

Then there’s the actual driving experience: If you’re ever zinged a good inline six to redline through the gears, you’ll know that it can be a truly refined and high-end experience.

And in an age where perception is reality (and has it not always been thus?), that makes a powerful argument for the classic inline six.

Australian car buyers are pretty switched on to the straight-six, too.

Aside from being the basis of the bread-and-butter family cars many of us grew up in, locally made inline sixes have been variously the quickest-accelerating Aussie cars of the day (Valiant Charger E49), quicker than their V8 stablemates to 100km/h (Ford Falcon EF XR6) and a Bathurst 1000 winner (Brock’s 1972 Torana XU1).