The venerable in-line six is resurrected
Smoothest motor also keeps costs in check
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, having replaced all its pistons in a row.
Furthermore, not only are Mercedes and Jaguar returning to the time- hon- oured in- line format — as early as 1910 there were almost 100 manufacturers producing I6s — 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. When smaller engines were needed to fulfil fuel-economy requirements, what could be simpler than just lopping a couple of cylinders off aV 8? 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 bean-counters determined that aV 6 was the road to cost- cutting heaven, its lack of harmony be damned.
That is, until tightening fuel-economy and emissions standards saw the ascendance of the long- derided four- cylinder. Turbocharged or supercharged—sometimes turbo charged and supercharged — little fourbangers are now powering entry-level Audis, newly sporty Volvos and even Mercedes’ imposing S- Class with- out complaint.
But 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.
In- l i ne 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 sprockets, guides and gaskets. In other words, built in numbers, an in- line six will always be cheaper to manufacture than a vee.
But, if simply rearranging the pistons one in front of the other instead of sideby- each was the only lesson to be gleaned from BMW, it might not have been gamechanging. No, the true genius of BMW’s engineering — as Motor Mouth has lauded before — has been to manufacture every one of its engines from all but identical components.
Yes, 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 constructed by simply adding … well, you get the idea.
Pistons, rings, connecting rods, even valves, valve guides and their seals are all shared across the line. Were it not for the need for the big V8s on M cars and its top-end luxury sedans, all of BMW’s — and Mini’s — engines could be built on the same production line and from the same parts bin.
Indeed, this modularity is almost assuredly the reason Jaguar followed BMW’s 500-cc-per-cylinder strategy.
Pundits have a l ways waxed lyrical about the civility of the in-line six: “Ripping silk” as metaphor for internal combustion at its most harmonious might well have been coined for Jaguar’s incredible XK6. Engineers explain this as the result of the in- line six’s perfect primary and secondary balance. That eerie smoothness is really down to something we can all understand: equal and opposite forces cancelling each other out.
Essentially, the overall format of the in- line six — two three- cylinder engines in a row with crankpins arranged 120 degrees apart — always has one piston doing the exact polar opposite of another. In-line fours and V6s offer no such harmony.