First in Fuel Injection

The revolutionary fuel-injected M198 powered the famous W198 300SL Gullwing introduced in 1954
by Graham Robson
 
This is a story that Mercedes-Benz can – and should – always tell with pride. Not only has it been a technological pioneer for more than a century, but it was the very first concern to apply fuel injection to high performance gasoline engines. And not merely with the legendary 300SL of 1954, but with other and equally significant products Mercedes-Benz led the way where every one of its rivals – every one – trailed behind, playing catch-up.

Photo above (feature image): The M198 fuel-injected engine, shown installed in one of the 300SL roadsters built from 1957 to 1963. Note the elaborate intake manifold used in conjunction with the fuel injection.

In fairness, Mercedes-Benz was not quite an automotive world pioneer, for Bosch supplied the same system in two awful little German road cars called the Gutbrod Superior 600 and then, a year or two later, the Goliath GP700. On the other hand, and along with Bosch, Mercedes-Benz was the first to supply fuel-injected diesel engines in its 260D saloon of 1936. Not only that, but it also produced the DB600 series of inverted V12 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 eventually pushed out to 35.7 liters and up to 2,000 horsepower. A version of that engine, of course, was also planned to power the gargantuan land speed record contender of the late 1930s, but that car was never completed.

Now we come to the 1950s, when the company designed and launched its first new engine in many years – the beautifully-detailed M186 – a 2,996cc (183CID) unit which initially produced 115 horsepower at 4,600 rpm, and ran on conventional carburetors. The genesis of this engine, described in the January-February issue of The Star, would be used as the basis of the first petrol-powered fuel-injected Mercedes-Benz power unit.

From the beginning, it seems, engineers at Stuttgart began to investigate the combination of Robert Bosch’s latest fuel injection system with its own ultra-modern 6-cylinder engine. Although there wasn’t time to perfect a fuel-injection installation in the 300SL race cars of 1952, there seems to have been an intent to do just that for 1953… until the 1953 race program was cancelled and all future engine development was concentrated on the 8-cylinder engines being designed for the new Formula 1 and racing sports car models.

The decision to turn the successful 300SL race car into an ultra-sophisticated road car, however, concentrated the corporate mind wonderfully, especially after the North American Mercedes-Benz importer, Max Hoffman, placed an order for 1,000 of the new cars. So, what were the advantages of fuel injection in the new W198 (which was the 300SL road car’s project name)?

There were two reasons – official reasons, that is: the team thought it could extract much more power that way, and they hoped to make the engine bay installation neater than before. Hindsight tells us that the injection system was more difficult, not easier, to service and maintain than the carbureted types, especially because the cylinder block had to be canted over by 40 degrees to fit neatly (and tightly) into the 300SL’s body shell.

We can, I believe, assume that work on the fuel-injected engines began even before the basic carbureted unit was ever finalized, a conclusion of renowned F1 engine expert Harry Mundy, who analyzed this engine in Autocar magazine in April 1958:

“Preliminary examination of the combustion chambers of the Mercedes 300 series engines might indicate that the layout was conceived primarily with direct injection in mind,” he wrote. “The SL-type engines use an injector protruding into each combustion chamber just below the cylinder head joint top face. This system is acceptable for a sports-car type of engine, but when it was decided to adopt petrol fuel injection on the 300 limousine in 1957, more refined control was necessary.…”

A display version of the 300SL Gullwing, showing the M198 fuel-injected engine installed in the unique tube-frame chassis of the sportscar.

Incidentally, what Mundy did not point out was that the injector nozzles in the 300SL derivative used the same positions that the spark plugs used in earlier types, and that different cylinder head castings were therefore needed to accommodate spark plugs in more conventional positions close to the valve heads.
This was where Ing. E.H. Fritz Nallinger’s engineers were so brave and resourceful. Right from the start, it seems, they chose slant machining of the cylinder block/head face so that the head could be machined totally flat, and right from the start they had envisaged several different induction and exhaust installations surrounding the cylinder block itself.

What was fascinating about the fuel-injection installations in the W198 300SL sports car, and the W189 300d sedan was that they were almost completely different, not only in their placement location, but in their architecture and in their integration into the power unit itself. In the 300SL there seems to have been little effort to reduce costs, or complications – for the Bosch plunger-type injection pump was mounted in what was effectively the underside of the engine (as it was installed at a steep angle in the 300SL engine bay), while the impressive, carefully detailed, and smoothly profiled inlet manifold took up much of the space on the right side of the engine bay.

Incidentally, Mundy reported that in his later discussions of the engine with Rudolph Uhlenhaut:

“When asked if they had evolved a formula by which the optimum [inlet] pipe lengths could be designed for a given engine, Uhlenhaut’s answer was that the development engineers produced the results by trial and error based on past experience, and then the mathematicians evolved a formula which proved the practical findings.…” – which seems like a classic example of post hoc ergo propter hoc reasoning.

Even so, and in spite of the cut-and-try methods presumably employed, the design team not only managed to install what was a bulky engine under a long, low, and sweeping bonnet, but they also managed to produce a great deal more power. On the 300SL, not only was Bosch fuel injection harnessed, but differently shaped pistons (whose crowns incorporated much of the combustion chamber shape) were created to raise the compression ratio from 6.4:1 (in the original M186) to no less than an 8.55:1 ratio, and a camshaft with much enhanced opening periods was also adopted.


No wonder that a great deal of extra power and torque was found, and liberated. How much extra? That information depends on who was handing out facts, who was telling the story, and how the peak rating was to be measured – enough, it seemed, to endow the new 300SL with a top speed of up to 155 mph if the correct overall gearing was adopted. According to the six-page analysis Mundy developed, after spending many hours in Stuttgart working on it, the original M186 had produced 115 brake horsepower (net), while the 300SL derivative produced 215 brake horsepower.

Now for the complications. In the United States, the 300SL’s output was usually quoted at an SAE rating of 240 horsepower, while other European authorities (not this author) sometimes quoted 195 horsepower instead. Who cares? Only the nerds or the number crunchers (those who spend hours in darkened rooms, hunched over computer screens) –  for regardless of exact numbers, the overall effect was still startling.

These drawings allow a look inside the 300SL M198 engine. Quite noticeable is the oblique joint between cylinder head and cylinder block as well as the incorporation of part of the combustion chamber in the piston. Also visible is the placement of the injector in the cylinder block of the engine, with the spark plugs placed in the head. The engine was mounted at an oblique angle to reduce overall height and provide an aerodynamically advantageous flat hood, which meant the injection system was on the underside of the engine.

Variations

That, of course, was just the start, for once the engineering team members had been given time to enjoy themselves like this, it was time to get back to bread-and-butter issues – and to help the company make more money. In 1954, for instance, the company had manufactured 34,500 cars (more than 28,000 170/220 types, 2,000 300s, and 4,400 early Pontons), and had plans to build many more passenger cars in the future.

Nallinger, therefore, set up two different programs. One was to work hard on the smaller “sister six” of 2,195cc (which also made its debut in 1951 on the 220 model). The other was to keep developing, rationalizing, simplifying and – eventually – to use more widely the fuel-injection system that was proving to be such a success on the 300SL, and of course on the W196 F1 car and 300SLR racing sports cars.

As far as the two principal road-car engines – 2,195cc and 2,996cc – were concerned, they were similar, but by no means the same, for the smaller unit had but four main crankshaft bearings, and smaller bore and stroke dimensions. Even so, until the company began to produce many more cars (and much more money) they were the future, and were both to be nurtured.

The history books tell us that fuel injection on the Mercedes-Benz sedan/coupe/convertible ranges was gradually introduced from 1957, this being well before any other mass-market car-maker took the same big step – although Chevrolet did produce Corvette sports cars with a Rochester “Ramjet” fuel-injection system, which was speedily abandoned after only 240 units were produced.

The Mercedes-Benz program was carried out, quite literally, with a cold-blooded and determined intention – both on the part of Mercedes-Benz, and of Robert Bosch, the favored supplier – to make the system a mass-market, as opposed to a niche, product: It succeeded for Mercedes-Benz, originally the market leader, became the standard setter thereafter.

As far as the 2,996cc inline-six engine was concerned, the first road-car application came in 1957 with the launch of the W189/300d sedan, which was closely followed by the cabriolet. At a casual glance, the major visual difference was that the engine was mounted in an upright position in the more spacious engine bay of the sedan/cabriolet, and possessed a conventional wet-sump lubrication system; this allowed the inlet manifold to be simplified, too.

By juggling what they already knew and what they had learned – very rapidly indeed – from the 300SL, engineers provided the 300d with a modified version of the 300SL plunger-type injection pump installation and the same 8.5:1 compression ratio pistons, all allied to a very mild setup of camshaft timing, and the same, smaller-diameter, valve sizes of the 300S model.

It was refined, at once ingenious and practical, yet it delivered a peak power output of 160 horsepower (net)/180 horsepower (SAE gross), which helped to deliver a top speed of more than 110 mph. Expensive? Of course. Capable? Of course. Even so, in five years – 1957 to 1962 – 3,077 of these 300d cars were sold.

Fuel injection on the smaller (2,195cc) inline-six then followed in 1959, for the first on the fintails, but for the big 2,996cc engine there was one more important update to follow. Two years after the first fintail sedans were introduced – and remembering that there were 4-cylinder and 6-cylinder, gasoline and diesel versions of this car already on sale – it was time to introduce a big new, and up-to-date “flag ship” derivative: the W112/300SE range.

Compared with the original postwar 300 types, the new W112 fintails, coupes, and cabriolets not only had an all-steel unit-body chassis, but were at once more modern, more stylish, and with much modern engineering hidden away. Somehow or other, the massive 6-cylinder engine was squeezed into a standardized engine bay: A longer-wheelbase type – the extra space being concentrated in the rear passenger area – would follow two years later.

The big change was to the architecture of the 6-cylinder engine, which was always handicapped by its great weight. For the 300SE car, which weighed a massive 440 pounds less than the original 300 built in 1951, some of this saving – 88 pounds, no less – was achieved by giving the engine a light-alloy cylinder block, featuring steel press-fit dry cylinder liners, though there was no change to the existing bore, stroke, or camshaft layout.

Think what a weight reduction like this could have done, you might say, for the performance and handling of the 300SL sports car. But, then, hindsight is a wonderful thing, and from what is known this was never done. Compared with the 1957 sedan installation, by the way, there had been no increase in peak power output – it was still quoted as 160 horsepower (net), although this would climb to 170 horsepower in 1964 – even though the compression ratio was lifted to 9.0:1.

Mercedes-Benz was still proud of this engine, especially because it would propel the latest 300SE at between 112 mph and 115 mph, And well they might be proud, for the engine would remain in production until 1967. From 1965 to 1967, the last iteration of all for the seminal 2,996-cc power unit was in the original W108 300SE – after that a new generation of large 6-cylinder engines was already on the way.

By that time, development of this particular 6-cylinder engine was at its peak (and what a peak!) – for the engineers had already progressed to a new layout, a new design philosophy, and a new performance level completely. Beginning with the gargantuan 600 range in 1963, the company moved firmly into the V-8 era.