The Mercedes-Benz since 1945
Volume 3 - The 1970s

by James Taylor

A new flagship
The 450SEL 6.9

By 1974 the trend away from the most powerful and thirsty cars was well established, yet the very next W116 to reach the showrooms seemed to go against everything which the fuel crisis represented, for it had the biggest engine seen in any postwar Mercedes-Benz. In fact, the 450SEL 6.9 which appeared at the 1975 Frankfurt Motor Show had undoubtedly been held over until public reaction to the fuel crisis had stabilized. By the beginning of 1975, the trend back towards big engines was already discernible, however, and Stuttgart's directors authorized production of the new car.

The 450SEL 6.9 was inspired by the same philosophy which had lain behind the 300SEL 6.3, the model which it replaced after a gap of three years. In other words, it coupled the largest and most powerful passenger car engine with the bodyshell of the S-class flagship saloon to produce a Q-car, an ultra-high-performance saloon which looked scarcely different to the standard S-class model. The extra weight of the W116 bodyshell as compared to the superseded W109, plus the power-sapping Federal emissions-control regulations, had meant, however, that even the 6.3-litre V8 of the low-volume 600 limousine was not man enough for the job. Stuttgart considered that there was no point in building the car at all if it could not at least equal the performance of the old 300SEL 6.3, so the engineers went for more power and torque in an enlarged version of the big V8.

As early as 1969, 6.9-litre versions of the W100 had been tried experimentally in examples of the 300SEL 6.3 intended for competition use, and the Daimler-Benz engineers already knew how far they could go and still keep the engine both reliable and docile at low speeds. For the W 116, therefore, the W 100 engine was bored-out from 103mm to 107mm, which took the cylinder capacity up to 6,834cc. In order to fit this relatively tall engine beneath the low W 116 bonnet, a dry-sump lubrication system was specified.

However, more power and torque were not the only aims of the 6.9-litre engine, and on it were tested several features which would later find their way into the lesser models. Thus, it became the first passenger car engine from Stuttgart to use Bosch's new mechanically-controlled K-Jetronic fuel injection system, which was not only cheaper to make than the electronically-controlled D-Jetronic, but also easier to adapt to meet varying emissions-control standards. Similarly, a good deal of thought had gone into servicing requirements, and the factory claimed that their new engine should run for up to 50,000 miles without any more than filter, oil and plug changes. On the earlier V8s, the rocker arms in the valve-gear needed periodic re-adjustment to maintain the correct clearances, but in the 6.9-litre engine, oil was fed under pressure to the fulcrum points of these rockers to give them a self-adjusting function similar to that of hydraulic tappets. A side benefit, of course, was that the engine ran more quietly. Finally, a new type of head gasket made by Reinz-Repa eliminated the need for retorquing the heads after the initial running-in period.

In the European 450SEL 6.9, the engine put out 286bhp (36bhp more than its 6.3-litre parent) and a staggering 4051b/ft of torque at 3,000rpm. Driving through the W3B 050 three-speed automatic transmission to a very high final drive of 2.65:1, and putting its power down to the road through a ZF limited-slip differential and even fatter tyres than on the 41/2-litres, this gave precisely the kind of performance which was wanted. Daimler-Benz claimed a very conservative 140 mph top speed, but a number of motoring journalists enjoyed themselves to the full in finding out that the actual top speed was much nearer 150 mph! In that respect, the 450SEL 6.9 out-performed the old 300SEL 6.3, but it was so much heavier that not even all that torque could keep standing-start times at their former levels. Even though 0-60mph in under 7-1/2 seconds sounds quick, the 300SEL 6.3 had been a whole second quicker; and the detoxed Federal cars which arrived in 1977 were, of course, even slower.

Front 3/4 The best way to distinguish between a 450SEL 6.9 and an 'ordinary' 450SEL from the front is by looking at the tyres. The big-engined car was shipped with wider 215/70 VR 14 rubber.

The extra weight of the 450SEL 6.9 was not only attributable to the heavier W116 bodyshell, however. The car was further distinguished from its lesser brethren by a sophisticated new suspension system which Stuttgart was considering for future use in other models, and indeed it did appear as an option for certain models of the 1979 W126 S-class range. The new suspension was a departure for Mercedes-Benz in that it was a hydro-pneumatic system similar in principle to that used by Citroen for many years. In place of the coil springs of the standard W116, it had a pressurized cylinder filled with nitrogen over each wheel and oil-filled struts which incorporated gas dampers. An engine-driven pump kept the nitrogen system under pressure and topped up by a reservoir which ensured that the system still worked after the engine had been switched off. The system was automatically self-levelling, and ride height could be adjusted even while the car was in motion to give up to 1-1/2 inches of extra ground clearance. Where it scored over the air suspension used in the W 109s was that it was less prone to suffer from leaks or faulty balance-valves; and where it scored over the superb Citroen system was in its retention of the handling qualities offered by the standard W116 suspension in combination with a more supple ride. Like the suspension, the braking system with its 'stepped' master-cylinders was also under consideration for future use. The 'stepped' arrangement consisted of tandem master-cylinders with different bore sizes. The idea was that if the front hydraulic circuit failed, the different bore diameter of the cylinder serving the rear brake lines would automatically compensate by increasing the braking effort there, and thus the loss in braking efficiency would be minimized.

rear 3/4
From the back, the badging made it easier to tell;
but some customers ordered their cars without badges!

With all this sophisticated and limited-production technology on board, the 450SEL 6.9 was a fearsomely expensive car (nearly as costly as a Rolls-Royce Silver Shadow saloon), but nevertheless it sold quite well at a steady 1,500-2,000 units a year until it was withdrawn in 1979. It was a particular favourite with those wealthy clients who thought they needed an armour-plated car, as its awesome reserves of power and torque ensured that it remained a rapid machine even when heavily-laden with armour plate. Not surprisingly, it was very often found without identifying badges on the boot-lid.

Under the bonnet Under the bonnet of a 450SEL 6.9. The engine was only marginally bigger than the 4.5-litre V8, leaving room for the hydro-pneumatic suspension equipment. Nevertheless, there was not much space wasted in there!

In the meantime, development had continued on the other S-class models. The 4-1/2-litre engine was fitted with K-Jetronic injection from October 1975, shortly after the arrival of the 6.9-litre, and the 3-1/2-litres were brought into line in January 1976. For 1976, equipment levels had been upgraded across the range, with items like a cruise control, rear headrests, heated seats and a self-seeking radio with stereo tape deck becoming part of the specification. The 280SE was introduced to North America for the first time, no doubt to offset the trend against big-engined cars. Then for 1977 came improved air conditioning units with a larger evaporator and increased blower volume, while the 450SE disappeared from the American market and the frontal appearance of the remaining US models (the 280S excepted) was tidied-up as the foglights were recessed into the bumpers. Yet the most important 1977 changes were to the emissions-control equipment of North American cars, and they demonstrated how Stuttgart was getting to grips with the problems of US legislation. Since 1975, Californian cars had been fitted with catalytic converters in the exhaust manifolds, and now that the rest of America also required such an installation, the converters were resited under the floor with a heat shield, thus reducing under-bonnet temperatures and increasing the life-span of the converters. There was a new vacuum transducer for exhaust gas metering, and on the V8 engines the air injection pump was replaced by a pulsating air induction system which connected to secondary passages in the cylinder heads. Not only did it reduce power drain, but it was also quieter in operation than the air pump had been.

Engine The 450SEL 6.9 engine, featuring dry-sump lubrication, hydraulic valve clearance adjustment, and mechanically operated fuel injection.

Engine exposed
The M100 6.9-litre engine exposed. Based on the earlier 6.3-litre V8, the extra displacement was found by increasing the cylinder bore from 103 to 107mm, the 95mm stroke remained unchanged.

Reprinted from The Mercedes-Benz since 1945, Volume 3 - The 1970s, by James Taylor
Editor: Motor Racing Publications, Ltd 1986

materials provided by:
Giovanni Verzoletto