The understandable resurrection of the in-line six-cylinder engine

The smoothest motor you can build is all about cutting costs, writes David Booth.

It turns out BMW was right all along.

I suspect that the only manufacturer to stick with the classic in-line six-cylinder through thick and thin will soon be celebrated as genius.

Indeed, if imitation truly be the sincerest form of flattery, I suspect it already is.

Mercedes-Benz will be gradually dumping its V6 in favour of its M256 in-line six and Jaguar, by all reports, is about to replace its supercharged V6s and V8s with a turbocharger, having replaced all its pistons in a row.

Furthermore, not only are Mercedes and Jaguar returning to the time-honoured in-line for- mat, but their motivation, their design and even some of their engineering specifics are remarkably similar to what BMW has known all along.

The transition away from the in-line six that started in the 1960s was always about ruthless bean counting. Essentially, automakers were building a boatload of V8s and, modern car design almost always being a triumph of expediency over engineering, when smaller engines were needed to fulfil fuel-economy requirements, what could have been simpler than just lopping a couple of cylinders off a V8?

Never mind that no 90-degree V6 can ever be as smooth as an in-line six, as long as manufacturers were cranking out millions of V8s, the auto industry’s bean counters determined that a V6 was the road to cost-cutting heaven, its lack of internal combustion harmony be damned.

That is, until tightening fuel-economy and emissions standards saw the ascendance of the long-derided four-cylinder engine. Turbocharged or supercharged — and sometimes turbocharged and supercharged — little four-bangers, once the purview of lowly Vegas and disgraced Pintos, are now powering entry-level Audis, newly sporty Volvos and even Mercedes’ imposing S-Class without complaint from their well-heeled clientele. Not going unnoticed by the number crunchers is that an in-line four shares few manufacturing processes with a V6.

But — and I am pretty sure this doesn’t take much imagination — it’s not a huge hop, skip and metaphysical jump from four pistons all in a row to six. Indeed, once you start employing the economies of scale that are every accountant’s dream, the conversion back to the in-line six is a CPA fantasy come true.

Think about it. In-line sixes need one cylinder head, vees need two. If you’re building a modern DOHC engine, an in-line motor reduces the number of camshafts by half. Ditto cam chains, their tensioners and even simple things like sprockets, guides and gaskets. In other words, built in corresponding numbers, an in-line six will always be cheaper to manufacture than a vee.

The true brilliance of BMW’s recent remake of its engine lineup — and which Jaguar and others are quickly copying — is that all its recent power plants are based on identical 500-cc cylinders.

Thus, the company’s threecylinder 1.5-L engine is three modular pistons in a row, the company’s ubiquitous 2.0-L just one more tacked on the end, and the latest version of its iconic 3.0-L in-line six is constructed by simply adding … well, you get the idea.

In other words, the reason the in-line six may be (re) ascendant is the very same cost-cutting economies of scale that have always given corporate bookkeepers their jollies. For once — and, again, the in-line six has long been lauded as the sweetest of internal combustion engines — the engineers and the paper pushers are in line.