Engine of Destiny -- The M186 3-liter of 1951
 
Article by Graham Robson
Photography courtesy of Graham Robson and Daimler Archives
 
Think back, if you can, to 1947, when Mercedes-Benz finally cleared the rubble of war, rebuilt vital factory units, and started producing private cars once again. With an evolutionary use of rescued prewar tooling and designs, the company managed to produce 214 170V machines in 1946; 1,045 would follow in 1947.
 
Planning for the Future
 
But what could be accomplished for the future? Perhaps the company could not immediately hope to get back to the 1930’s standard when 4-, 6-, and 8-cylinder engines – gasoline and diesel – were all manufactured at the same Stuttgart plant. But if there were to be any postwar expansion, production would have to increase beyond the side-valve “four” built with tools and equipment that had survived Germany’s bombardment. If investment capital could be found, management and engineers were determined to add a more ambitious engine to the range.
 
What sort of engine? Take into consideration that the technical standards in Europe, as well as the United States, were still quite low. Jaguar’s magnificent twin-cam XK engine was still under wraps, BMW had nothing to offer, and in North America, all of Cadillac’s cars were powered by an L-head (side-valve) V-8 dated back to 1936.
 
It was time for a deep breath. After much thought and a study of what emerging markets might accept, top management in Stuttgart decided to commission an all-new, 3-liter “six” that was to include as much modern engineering as possible. Such enterprise meant spending heavily on new equipment – with an expectation of a long production life. And so it would be, for the last of these magnificent power plants would not be assembled until 1967, 20 years after design studies began.
 
During that time, no enlarged version of the engine was offered for sale – as if the 85mm bore and the 88mm stroke were sacrosanct – even though power ratings increased steadily, particularly after the Bosch fuel injection was added to the specification. In some ways, that is what makes this an amazing, standard-setting design; it was still a remarkably capable engine when production finally ended.
 
Three Years of Work
 
When beginning the design work, automobile engineer Fritz Nallinger and his colleagues had a major problem. Unless an engine is designed without regard to costs, compromises must be made. But for Mercedes-Benz, there was more to take into account. The new M186, as it would be designated, not only had to deliver technically, but had to be produced by a company battered to a pulp just two years earlier and one that then still lacked almost every facility – except experience and sheer guts.
 
For all these reasons, Nallinger’s team concentrated on designing an all-new car – the W186 family of sedans and cabriolets – and the engine that would power them all. It was three long years – early 1951 – before the new model debuted, and not until November 1951 when series production began.
 
And there must have been times when designers became misty eyed when recalling past achievements. Not only had they built the sturdy, supercharged 8-cylinder engines that powered the 540K super cars of the 1930s, but there were also the 475-horsepower 3-liter V-12s used in the GP single-seater racecars of 1939. And let us never forget – what some rate as their greatest engineering achievement to date – the DB600 series of inverted V-12 aero engines used in Germany’s Messerschmitt Bf 109 fighter aircraft in the early 1940s, where the original 33.9-liter/1,350 horsepower output was later increased to a 35.7-liter and up to 2,000 horsepower.
 
In place of all this, a pragmatic approach was needed. What was needed was an ultra-sturdy straight-six design, one that could be built in tens of thousands, one that would be absolutely reliable in service, and whose original design had to hide away a degree of improvement that could be unlocked in future years. What finally appeared was a 2,996cc unit, which in its first generation produced 115 horsepower at 4,600 rpm, but which weighed no less than 585 pounds.
 
The use of a cast-iron cylinder block with a seven main-bearing crankshaft may have been conventional, but the use of single overhead camshaft valve gear and a unique head/valve architecture was not. The company’s prewar high-performance engines used conventional overhead-valve layouts, but now was the time for Nallinger’s enthusiasts to draw on what they knew from their prewar and wartime engines; not only would the M186 have single-overhead camshaft valve gear, but the camshaft would operate the valves via fingers in the same way as the legendary racecar engines of the 1930s.

The first application of the M186 engine was on the W186 chassis for the 300 Sedans.

 
Practical Innovations
 
There was more, though, much more. Some years earlier, a respected and experienced British designer firmly stated, “When designing any all-new engine, work should always start at the inlet valve. …” Daimler-Benz surely agreed with this, for the secret of the new M186 engine’s potential was in the layout of the light-alloy cylinder head, its relationship to the cast-iron cylinder block, and the location of the valves.
 
Simply and basically, the team angled the joint face between the head and the block at 30 degrees horizontal, provided the head with an absolutely flat machined face, and formed the combustion chamber in the wedge-shaped space that developed between the head face and piston tops. Those pistons, of course, were oddly profiled – but it all worked.
 
Because gasoline quality standards were still increasing and contemporary octane ratings were relatively low, the compression ratio was limited to 6.4:1.
 
The angled head-face solution was not, I believe, a Daimler-Benz innovation, because in order for V-8 engines to maintain a relative slimness, certain new American engines had adopted the same layout. Not only that, but in Britain, Rover (a tiny company in comparison with Daimler-Benz), had done much the same thing, with the added complication of using a side-mounted exhaust valve.
 
The new M186, however, was ruthlessly practical in its intentions. By adopting the flat-head face, more space was liberated for large valve heads – and by staggering the line of the inlet from the exhaust valves, it allowed them to be enlarged. And this is where the rationale for using fingers between the single overhead camshaft and the heads of the valve became clear. By using just one finger shaft/pivot, and a single camshaft, different-length fingers could complete a very elegant engineering layout in the head itself.

Designed in 1951 for the new W186 chassis or the 300 sedans and cabriolets, the M186 engine was extremely innovative, including the design of the valve train, with a single overhead cam actuating intake and exhaust valves with two different lengths of levers pivoted on a single offset shaft.


The remainder of the layout was conventional, although it was quite clear that rugged reliability, as opposed to ultra-high output and sporty behaviour, were considered important. As noted, in the 20-year life of the engine as manufactured, no production car used anything other than the 2,996cc size, and a study of drawings suggests that there was little space for increasing the bore or stroke. In fact, by arranging very little water cooling to surround all the bores and designing the sturdy wall thickness, the team specifically arranged that this be so.
 
The crankshaft itself was a massive forged-steel component with seven main bearings and eight counter weights, while a single-stage chain from the nose of the crankshaft drove the overhead camshaft.
 
Were there any built-in drawbacks? The engineering team thought not, especially on original-type engines; they designed spark plugs to be mounted in the cylinder block itself, to one side of the wedge-shaped combustion chamber. They did admit, however, that because the engine was mounted vertically (conventionally) in the engine bay of the new W186 sedan – and as a consequence of the direction in which the inlet ports were aligned – downdraft carburetors would have to be employed. All of which meant – not only was this new engine quite bulky, it was also tall.
 

The first application of the M186 engine was in the W186 chassis 300 limousines.


A Truly Versatile Engine
 
Except for its clever cylinder head, combustion chamber, and machining features, the new M186 engine was a study in orthodoxy and well suited for the big and luxurious 300 model series for which it was intended. We must not, of course, consider that 115 horsepower from a 3-liter engine was behind the trend of the day – even then, a 4.5-litre 6-cylinder Rolls-Royce engine probably produced no more than 130 horsepower (RR never admitted to anything), while Cadillac claimed 160 horsepower (almost certainly overstated, as all such American outputs were in those days).
The first M186s were the A-Types, with twin downdraft Solex 32 PAJ80 carburetors. Original 300s with curb weights of approximately two tons, the 115 horsepower just barely reached 100 mph. But by the time this became known, more than 50 cars were produced each week. And engineers were already doing what they enjoyed most – sitting back in their offices and working out ways to get more – and more – out of their new baby.
 
The next version – and this was a major advance – was to provide super power for the gorgeous and super-stylish 300S. This S-Type derivative used three Solex downdraft 40 PBJC carburetors, a much higher compression ratio – 7.8:1 – a different camshaft grind, and produced no less than 150 horsepower at 5,000 rpm. With a top speed of approximately 120 mph, these were splendid, prestigious machines, though they were priced so highly that only 760 cars – Roadsters, Cabriolets and Coupes – were ever delivered.
 
The engine would even serve as the powerplant for the first 300SL race cars of 1952, assembled largely from off-the-shelf components. However, to reduce the engine’s height under the low hood of the race car, the engine had to be tilted well to the left of the engine bay by 45 degrees. To produce the power required for racing, the engine was much modified, originally to use twin Weber carburetors, and later to use triple Solex units similar to the set-up on the 300S. The result was that there was about 171 horsepower on tap (240 SAE horsepower), and the engine was race reliable for days at a time, even in the challenging altitudes and circumstances of the Panamericana in Mexico. That was quite enough to make the 300SL the supreme sports-racer of the year.
 
As history would prove, the engineers had more than met their design brief with this engine. Sturdy, straightforward to produce, and capable of continued improvement at little cost, the M186 would power the elite cars in the range for 16 years.
 
 

Total production of cars with this engine: 27,302