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Inline-6 Helped Forge BMW Brand

Since 1933, BMW inline 6-cyl. engines have set a premium-engine standard.

The Ward’s 10 Best Engines competition celebrates 13 years of recognizing outstanding powertrain development. In this last of a 10-part series, Ward’s highlights the legacy of the inline 6-cyl. engine – a layout BMW has embraced with distinction for cars since the 1930s.

BMW AG unassumingly celebrated its 90th anniversary this past summer, and a look at its history shows an auto company irrefutably linked to engines like no auto maker before or since.

Woven throughout BMW’s history is its signature inline 6-cyl. engine – a layout to which the company owes its very origin.

Starting as an aircraft engine from BMW’s beginning in 1917 through today, the I-6 – a Ward’s 10 Best Engines winner for 12 of the competition’s 13 years – is so much a part of the brand’s tradition that the letters BMW almost cannot be said without mentioning the magnificent I-6 in the same breath.

It was this engine architecture that spawned BMW in 1917, when Max Friz joined Munich’s Rapp-Motorenwerk as a chief engineer. The idea he brought from his former job at Daimler-Motoren-Gesellschaft: a high-compression aircraft engine that would enable military aircraft to fly at high altitudes, a coveted tactical objective in that nascent era of air combat.

His answer – doubtless inspired by the work of a colleague at Daimler-Motoren by the name of Karl Maybach (yes, that Maybach) – was a large-displacement, high-compression piston engine with six cylinders arranged inline.

The layout was not happenstance, as Friz knew the optimized balance of the I-6 – its almost total lack of vibration – would be a great advantage in the fragile airframes of WWI fighters.

Those same attributes would serve BMW cars, first in 1933 and continuing with the company’s contemporary vehicles.

The intrinsic balance and smoothness of the straight 6 was not the only favorable attribute for aircraft. Its slimness allowed a narrower airframe, improving the fighter’s aerodynamics, and Friz’s engine design also called for an aluminum block and pistons, beginning valuable materials experience that would be leveraged for BMW auto engines.

BMW history says the drawings were not even complete when representatives from the German war department paid a visit. After a debriefing from Friz, they ordered 600 of the engines on the spot.

The overnight success prompted a reorganization of Rapp-Motorenwerk, and on July 21, 1917, the company’s new name was entered into the business register: Bayerisch Motoren Werke.

Its logo went from a black horse resembling a knight chesspiece to the now-famous stylized white spinning propeller on a blue-sky background. Blue and white are the historic colors of Bavaria.

These engines powered German fighter aircraft until the end of the war, and immediately afterward a larger I-6 sent a plane to an altitude record of more than 32,000 ft. (9,754 m). After WWI, German aircraft production was forbidden, and BMW aimed its engine expertise at motorcycles; the first BMW motorcycle was produced in 1923 under Friz’s direction.

Automobiles naturally followed; the first true BMW automobile arrived in 1931. But it was not until the famous BMW 303 of 1933 that the company called its I-6 into service for the first time in an automobile.

That engine, code-named M78 – for the next 70-odd years, all BMW I-6 engines would use “M” designations – was a modest 1.2L unit developing 30 hp and 50 lb.-ft. (68 Nm) of torque.

Based on an earlier BMW 4-cyl. design, the engine had several unique features, including irregular spacing between the bores. The area between the second and third cylinders and between the fourth and fifth cylinders was larger in order to accommodate the crankshaft main bearings, which had to be situated in the middle because of the assembly techniques of the day.

That seminal BMW I-6 was expanded to 2.0L for the 1937 BMW 328 roadster, one of the company’s greatest models. By 1941, BMW engineers were even testing this I-6 with fuel injection.

By 1968, BMW had introduced another generation of the I-6 that began to resemble an engine more recognizable today. The MO6 family featured an overhead camshaft, and the hallowed smoothness was enhanced by a 12-counterweight forged crankshaft, supported by seven main bearings.

The 3.5L M88, introduced in 1978 for the legendary M1, was BMW’s first use of 4-valve cylinder heads for an I-6.

Fourteen years later in 1992, the I-6 had two camshafts and the first use of BMW’s VANOS infinitely variable valve timing for intake and exhaust cams.

In 2001 came the most recent valve-timing innovation, the Valvetronic system that also varies intake-valve lift, eliminating the traditional throttle and markedly cutting pumping losses.

Recent materials advances and the latest high-tech fueling mark the evolution to today’s N-family engines.

The 3.0L N52, launched in the U.S. in 2005, brought a new innovation: a composite-metal construction technique that uses magnesium – 25% lighter than aluminum – for the upper and lower crankcase, while aluminum comprises the structural core and cylinder bores.

There’s some steel inserted into the magnesium lower crankcase bedplate to assure structural integrity, particularly for the journals in which the forged steel crankshaft rides.

BMW’s inspiration for this choice might be traced to a mid-1920s melding of two I-6 engines to form a stupendous V-12 for all manner of aircraft, as WWI constraints on German aviation were reduced.

The 580-hp V-12 was constructed with aluminum, and portions of the crankcase were magnesium. It made headlines as the powerplant for the bizarre “rail Zeppelin” in the early 1930s, a streamlined railcar using a rear-mounted propeller for thrust to achieve 143 mph (230 km/h) – a prodigious and record-setting speed for the era.

The newest variant of BMW’s timeless I-6 also enjoys a direct connection to BMW’s aircraft-engine origins.

BMW’s N54 3.0L doesn’t employ the novel aluminum-magnesium cylinder block of the N52, but it uses an innovative twin-turbocharger design, marking BMW’s first return to turbocharging since 1981.

BMW engineers say they will no longer be making engines larger to achieve power targets, thus the revival of turbocharging to hike specific output.

But the N54 also marks another important first for the I-6 format: the first application of gasoline direct injection, using piezohydraulic injectors. BMW says the N54 is the industry’s first all-aluminum I-6 to combine turbocharging with direct injection.

But to find BMW’s first experience with gasoline direct injection, return to the aircraft engines that seem a perpetual source of BMW’s automotive-engine inspiration: The company’s 132F 9-cyl. radial engine used GDI – in 1937.

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