Great Day for a Great Car - The M116 and M117 V-8 engines of 1969-1992 | MBCA

Graham Robson

The M116 V-8 engine, and its M117 variant, powered the flagships of Mercedes-Benz for more than two decades.

September 1969 – mark that date clearly. Not only was it the month in which Mercedes-Benz first showed its C111 Wankel-engined prototype, but it also signaled the arrival of a brand-new production-line overhead-camshaft V-8 engine – the M116. Both these novelties were unveiled at the Frankfurt Motor Show that year. Naturally, it was the C111 that generated all the headlines, so it was really a couple of years before the vital ground-breaking importance of the sturdy new engine became obvious.

Perhaps you would expect me, as an enthusiastic historian of all things Mercedes-Benz, to gush a little about this impressive piece of mechanical kit, which I fully intend to do – but I really need only relate the most basic list of facts.

The M116 in context

First announced in 1969, the always-improving M116 remained in production for twenty-two years. The M116 was joined by its big brother the M117 for a similar period, after which both were replaced by an analogous dual-overhead-camshaft unit. That engine covered a further 10 years to the end of the 20th century.

By the way, the M116 wasn’t a never-changing lump. There would be myriad derivatives of the M116/M117 pair over the years – we will refer to the 3.5, 3.8, 4.2, 4.5 5.0 and 5.5-liter varieties that were manufactured on the state-of-the-art production lines at the Stuttgart-Untertürkheim factory.

Looking back, we can see that Fritz Nallinger and his engineering team had been incredibly busy, resourceful, and successful throughout the 1960s. The department conceived, introduced and refined several important new model ranges and transmissions in that period. Although the firm’s first-ever production V-8 engine had appeared in 1963, that unit was originally meant only for the company flagship, the grand 600. A range of four-cylinder and six-cylinder gasoline and diesel-powered engines propelled the balance of the fleet.

When, why, where, and how did the company take the next, very big, step? In September of 1969, the Technical Editor of Autocar magazine, Geoffrey Howard, summed it up well when he surveyed the design of the firm’s new power unit:

"It is a well-known fact that the six-cylinder engines of the Daimler-Benz car program are becoming a bit dated. … It has been known for a number of years already that Daimler-Benz were working on a smaller vee-8 engine on lines rather similar to the famous 6.3-litre originally introduced for the 600."

The company, in fact, had clearly taken a very long, measured, and deep breath before committing itself to a major overhaul of the machine shops at Untertürkheim. It was not merely that the firm would be investing many millions, but engineers were also digging deeply into the philosophy underlying the type of engines they believed would underpin the future of Mercedes-Benz.

A quick history lesson here – not a single Mercedes-Benz road car had used V-8 power until 1963. Even then, the 600 was the first and only model to use one. By the mid-1960s, when secret design work began on the new M116 project, modern V-8 engines for road cars had almost all been designed in the United States. On the other hand, when the new German engine was finally deemed ready for launch, a diligent look through the listing of European series-production road cars of 1969/1970 shows that only four other European V-8 engines existed – one a redeveloped American design, another a copy of Cadillac and Chrysler V-8s, a Masarati V-8 and the last was frankly just odd. Those rivals, in order, were produced by Rover, Rolls-Royce, Masarati and Tatra.

Designing the M116

Back in Stuttgart, the ever-analytical design team had sat down, considered everything, and evolved an engine that owed nothing except its basic 90-degree vee-angle to any other known layout. This was probably the most carefully chosen group of experts the company had ever assembled: The intention was that the new powerplant could eventually be produced in a variety of engine capacities, using a range of materials and fuel supply systems, and that the new engine could be matched to a variety of body shells, marketing sectors, and uses.

Right from the start, the team made the base engine a 3.5-liter unit, with plans to push capacity up to 5.0 liters and beyond. At first there were no plans to use the new V-8 in any competition cars, but even that, as we now know, would change over the years.

As a 90-degree V-8, the M116 could be neatly packaged to fit into almost any existing or future Mercedes-Benz engine bay. The standard models were spacious enough to accept a sturdy old inline six-cylinder such as the 2.8-liter and 3.0-liter units the M116 was planned to succeed. Although as a 90-degree V-8, the M116 was going to be rather wide, a main design requirement was that it proved not too deep, so as not to compromise ground clearance. Weight was sure to be something of a problem, but by the accepted industrial abilities of the 1960s an aluminum block was still economically out of the question.

Preliminary design work on the M116 commenced soon after its predecessor, the 6.3-litre V-8 M100, appeared in 1963. With the M100 on the market, there seemed to be no immediate need to rush development. After all, Mercedes-Benz had many other projects underway in the 1960s, including several Fintail models, the Pagoda SL, the first of the S-Class and the New Generation range.

The basics of the M116 were at once obviously conventional, but beautifully detailed. The cylinder block was cast iron, the single-overhead-camshaft cylinder heads were aluminum, and it was clear from a study of the cross-sectional drawings that there was space built in for the cylinder bore and perhaps the stroke to be increased in due course.

The new engine’s crankshaft was supported by five main bearings. The wide and shallow oil pan was a casting rather than a pressing, while the inlet passages, the fuel injection installation, and the massive air cleaners were all squeezed in atop the unit. Camshaft drive was by chain from the nose of the crank, and the heads were designed so that the line of valves were somewhat inclined towards the center of the engine to lessen the overall width of the assembly.

Only a brave and experienced mechanic would wish to work on the front of this engine, for the camshaft drive chains shared a limited space with no fewer than five narrow vee-belts which drove auxiliary units, including the alternator, the power steering pump, and the engine-bay cooling fan.

A remarkable success

It was immediately clear that the new M116 was a remarkable engine, one that was both more powerful and also more fuel-efficient, as well as capable of dealing with all the power-stifling exhaust emission regulations the American authorities would shortly impose. Coupled with the latest in Mercedes-Benz automatic transmissions, the M116 provided the sort of performance which was so effortless that drivers could forget what was under the hood and enjoy the fact that their Mercedes-Benz was smooth, peaceful, and effortless.

Enter the M117

After its rather understated launch in 1969, M116 engines began to be fitted to more and more of the company product line, but from 1971 the new V-8 was joined by the closely related M117, which was manufactured on most of the same machine tooling equipment. However, the important difference between the two types was that the cast iron cylinder block of the M117 engine was enlarged so that the stroke could be increased to no less than 85mm, which meant that a new crankshaft was also required.

The most significant change came in 1977, when the limited-production 450SLC 5.0 model made its appearance, flaunting a very special 5,025cc version of the M117 power unit, with an aluminum block, which shaved 95 pounds off its weight. As it transpired, this would be a unique model and engine size combination. In fact, it was not until the 1981 model year that the light-alloy block and revised range of 3.8- and 5.0-liter engines finally took over from the original cast iron units. The 4.2-liter version of the revised engine appeared in 1985, soon followed by the final variation, the 5.5-liter M117, which would last until the firm’s complete V-8 range was replaced by a new dual-overhead-camshaft V-8 model for the 1990s and beyond.

A Tale of Two Engines: M100 & M116

Although there were visual similarities between the two power units, no parts were shared between the M100 and the M116. Bore and stroke were dramatically different, requiring all-new engine parts.

ENGINE DISPLACEMENT BORE X STROKE PEAK POWER

M100 6,332cc 103 x 95mm 250 bhp at 4,000 rpm

M116 3,499cc 92 x 65.8mm 200/1951 hp at 5,800/5,500 rpm

M116/117 Installation Log

M116 – 3,499cc: 280SE 3.5 • 280SEL 3.5 (W108, W111)

300SEL 3.5 (W109)

350SL • 450SL • 350SLC (R107/C107)

350SE • 350SEL (W116)

M116 – 3,818cc: 380SL (R107)

380SE • 380SEL (W126)

M116 – 3,839cc: 380SL • 380SLC (R107/C107)

380SE • 380SEL • 380SEC (W126)