MOTOR MOUTH
IN PRAISE OF BMW’S INLINE 6
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 V-6 in favour of its M256 in-line six and Jaguar, by all reports, is about to replace its supercharged V-6s and V-8s 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 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 V-8s 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 V-8? Never mind that no 90-degree V-6 can ever be as smooth as an in-line six, as long as manufacturers were cranking out millions of V-8s, the auto industry’s bean counters determined that a V-6 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 V-6.
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.
But, if simply rearranging the pistons one in front of the other instead of side-by-each was the only lesson to be gleaned from BMW, it might not have been game changing. 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 three-cylinder 1.5-L engine is three modular pistons in a row, the company’s ubiquitous 2.0L 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 V-8s 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. Like BMW, Jaguar’s budding Ingenium lineup is based on multiplying pistons. Currently, that’s limited to the 2.0-L fours — both diesel and gasoline fuelled — that wear the Ingenium nameplate.
But, according to Automotive News, Jaguar Land Rover’s announcement last month that it would buy no more engines from Ford (the source of its current V-6s and V-8s) means an in-line six, based on the same template, is likely in the works. While some analysts are reporting that JLR may start buying twin-turbo V-8s from BMW, others are noting that, with as much as 500 horsepower available from a boosted 3.0-L in-line six, there may no longer be a need for Jaguar to outsource any of its engines.
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. The difference is that only the lowly in-line four is the predominant, not the once all-conquering V-8. 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.
Pundits have always waxed lyrical about the incredible civility of the in-line six engine: “Ripping silk” as metaphor for internal combustion at its most harmonious might well have been coined for Jaguar’s incredible XK6. Engineers have always explained this as the result of the in-line six’s perfect primary and secondary balance. But, unless you have a degree in engineering or an autodidact’s understanding of vector analysis, 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 — basically 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 V-6s offer no such harmony.