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Mercedes Benz 6.9 450SEL

Quick, Quiet, and Comfortable

Modern Motor, September 1975

MERCEDES-BENZ has ignored the problems imposed by the fuel crisis to answer the pleas of faithful 6.3 owners - by offering them a replacement for the out-of-production big gun. The engine, enlarged to 6.9-litres, pushes along a super-sophisticated version of the 450SEL.

Though many people were sounding the knell for big cars in the wake of the oil crisis, the 450SEL, then the highest development of the Mercedes Benz S series, was considered so outstanding that it was voted Car of the Year by the European jury of which I was a member in 1973.

450SEL 6.9
It looks like any other 450SEL, but those 6.9 badges tell a different story. It goes much harder - and rides much better, thanks to a new "all-absorbing" hydro-pneumatic suspension.

Even then Daimler Benz was developing an even more advanced model, the 450SEL 6.9 which marks a further big advance in comfort, performance and safety. It is the highest achievement of a group of dedicated engineers who in good time and bad continue the search for perfection. In some respects it sets standards which are not equalled by any other car in the world.

It is the successor to the fast and impressive 300SEL 6.3 which had achieved an output of 6,526 when production ceased in 1972, and it gives very much the same high performance with still greater safety and much greater comfort. In presenting the new model Professor Hans Scherenberg, Daimler Benz technical director, said that following the introduction of the 450SE and 450SEL in 1973 the energy crisis focused attention on other developments such as the 240D and the five-cylinder 240D 3.0 - but owners of the 300SEL 6.3 had made it quite clear that they still awaited a modern successor and so the company decided to go ahead with this new top model.

It will be made at the rate of 60 a month, of which one third will go to the German market, one third to the United States and one third to other export markets.

The 6.9 is distinguished from other S class models by its different engine, different suspension, and even more luxurious equipment. The engine is a development of the 6.3, with the same stroke but bore increased from 103 to 107mm and it has new DB-designed hydraulic tappets. There is also transistor ignition with no contact points and new thinner head gaskets made of Ferrolastic, which eliminate the time-consuming job of tightening the cylinder heads periodically. These and other developments have made it possible to extend the intervals between service on the new model to 15,000kms.

450SEL 6.9
The new 6.9-litre V8 fitted to the 450SEL 6.9 is only marginally bigger than the 4.5 - so there is still room under the bonnet for the hydro-pneumatic suspension equipment.

To fit the bigger engine into the existing S-Type car, dry sump lubrication has been adopted, with a separate oil tank. The 6.9 also marks a break with recent Mercedes practice of using electronically-controlled injection. The Bosch system used on the 6.9 is simpler, probably more reliable, and certainly easier to service. Instead of measuring air temperature and pressure it responds to the actual air flow in the engine. The air draws down a flap which directly regulates the fuel flow to the injectors which squirt fuel continuously into the air stream just ahead of the inlet valves. There is, of course, thermostatic adjustment of the mixture for cold starting. With a capacity of 6,840cc the new engine produces 210kW (286bhp) DIN at 4250rpm and reaches a maximum torque 55Nm at 3,000rpm - this immense torque makes it possible to use an axle ratio of 2.65:1, giving a reduction in engine speed which is one of the big factors in the relaxed comfort of the 6.9 when travelling at high speeds. A limited-slip differential cuts down wheelspin and makes it possible to use the performance on wet roads.

But the other big factor in comfort and safety is the hydro-pneumatic all-independent suspension system. Instead of coil springs and dampers, each corner of the car rests on a hydraulic strut connected by a flexible pipe to a separate sphere containing the gas which is compressed by the oil. Sensors on the anti-roll bars at front and rear regulate the oil flow to maintain a constant ride level and so a constant ride frequency whatever the load. In addition the driver has a control which will increase the ground clearance by 4cm for travel over very rough roads. The handling seems very little affected with this in use but the ride becomes slightly harder.

On the normal setting this suspension is quite fantastic. Rushing along over bumpy mountain roads at speeds up to 160km/h you see the bumps coming, but simply do not feel or hear them. The lack of road noise is almost uncanny. Like the 4.5-litre model, the 6.9 has anti-dive front suspension to prevent the nose dipping under heavy breaking, and an anti-squat linkage at the rear. The combined effect is a serene, level ride in all conditions - and, of course, it works in conjunction with the self-levelling feature to keep the headlamp beams correctly aimed at all times.

I only realised fully what a great advance the 6.9 is when I drove the 6.3 which had been restored to new condition over the same roads. With its terrific performance and air suspension I thought the 6.3 was one of the world's greatest cars when it first appeared, but trying to follow a 6.9 round tight corners I was sliding and sawing at the wheel while the other driver sat relaxed and went around as though on rails. Road noise and vibration was much higher and when travelling very fast over pot-holes and loose gravel the 6.3 needed constant corrections to hold it on course where the 6.9 sailed ahead as though it was on a perfectly smooth road.

Though the new engine weighs about 50 kg more than the 4.5 V8, the extra weight on the front wheels is not apparent in driving the car. Yet the suspension is far softer and the Mercedes engineers have managed to combine this softness with a very strong resistance to roll without producing the irritating roll rock that you get with steel springs and stiff anti-roll bars.

With its stronger structure and extra safety features, such as reinforced doors, the 6.9 is appreciably heavier than the 6.3 at a kerb weight of 1,935kg but acceleration is practically the same. It accelerates from 0 to 100km/h in 7.4sec, 0-150km/h in 18sec, and has a maximum speed of 225km/h. On one slightly downhill stretch of autobahn I wound it up to 238km/h on the speedometer - and even then the tachometer was only showing 5,200. At 100km/h the engine is turning over at only 2595 rpm.

Transmission is automatic of course, with the excellent Mercedes central selector which allows instant shock-free changes with a fingertip between second and top. Average fuel consumption is quoted as 14 to 24 litre/100km according to average speed.