The Mercedes-Benz 450SEL 6.9 is a high-performance version of the S-Class luxury sedan. It was built on its own assembly line by Daimler-Benz AG (now DaimlerChrysler) in Stuttgart, Germany and based on the long-wheelbase version of the W116 chassis introduced in 1972. The model was generally referred to in the company's literature as the "6.9", to separate it from the regular 450SEL.
The 6.9 was first shown to the motoring press at the Geneva Auto Show in 1974, and produced between 1975 and 1981 in extremely limited numbers. It was billed as the flagship of the Mercedes-Benz car line, and the successor to Mercedes-Benz's original high-performance sedan, the 300SEL 6.3. The 6.9 also has the distinction of being among the first vehicles ever produced with optional electronically-controlled anti-lock brakes, first introduced by Mercedes-Benz and Bosch in 1978. The 6.9's successor - the top of range 560 SEL - continued the 6.9's remarkable self-leveling hydropneumatic suspension as an extra-cost option.
[] Special features
The 6.9 was the first Mercedes-Benz to be fitted with the company's new hydropneumatic self-levelling suspension system, unlike the 600 and 6.3 which employed air suspensions. The new system was similar to one developed by Citroën in 1955. Using a combination of fluid-filled struts and nitrogen-filled pressure vessels or "accumulators" in lieu of conventional shock absorbers and springs, the system was pressurized by a hydraulic pump driven by the engine's timing chain. Compared to the new Mercedes-Benz system, Citroën's was belt-driven, exactly like a conventional power steering pump; failure of the Citroën system thus might mean loss of suspension. In the event of a hydraulic failure on the 6.9, every unit was shipped with hard rubber emergency dampers that served as temporary springs and allowed the car to be driven. The special hydraulic fluid required by the system was stored in a tank inside the engine compartment. Not only was the system totally self-adjusting, ride height could be altered by a dash-mounted push-pull knob under the speedometer that raised the car an additional two inches (50 mm) for increased ground clearance. NHTSA decreed this feature illegal in the US market, but it could be enabled simply by removing a limiter at the tank-mounted control valve.
The suspension system gave the 4200 pound (1900 kg) car the benefits of a both a smooth ride and handling that allowed it, in the words of automotive journalist David E. Davis, to be "tossed about like a Mini." The car also featured a model W3B 050 three-speed automatic transmission unique to the 6.9 and a standard ZF limited slip differential both for enhanced roadholding performance on dry pavement and enhanced traction in inclement weather.
Four-wheel disc brakes and four-wheel independent suspension were standard across the W116 model range.
[] The M-100 power plant
The engine was a cast iron V8 with single overhead camshafts operating sodium-filled valves (as are found in piston-driven aircraft) against hardened valve seats on each aluminium alloy cylinder head. Each hand-built unit was bench-tested for 265 minutes, 40 of which were under full load. Bosch "K-Jetronic" electromechanical fuel injection was standard at a time when fuel-injected cars were uncommon. As in all Mercedes-Benz automobile engines, the crankshaft, connecting rods and pistons were forged instead of cast. In non-US trim, the 6.9 L (6814 cc or 417 in³) power plant was conservatively rated at 286 hp (213 kW) with 405 ft·lbf (549 N·m) of torque helping to compensate for the 2.65 to 1 final drive ratio necessary for sustained high-speed cruising. The North American version, introduced in 1977, was only slightly less powerful at 250 hp (186 kW) and 360 ft·lbf (488 N·m) of torque due to more stringent emissions control requirements. In the interest of both engine longevity as well as creating some extra space under the hood, a "dry sump" engine lubrication system was used. Originally developed for use in race cars as a way to prevent foaming of the engine oil by the crankshaft which in turn would create a serious drop in oil pressure, the system circulated twelve L of oil between the storage tank mounted inside the right front fender and the engine as opposed to the usual four or five L found in V8s with a standard oil pan and oil pump. As a result, the engine itself had no dipstick for checking the oil level. Rather, the dipstick was attached to the inside of the tank's filler cap (accessible from the engine compartment) and the oil level was checked with the engine running and at operating temperature. The dry sump system also had the benefit of extending the oil change interval to 12,500 miles (20,000 km). This, along with hydraulic valve lifters which required no adjusting and special cylinder head gaskets which eliminated the need for periodic retorquing of the head bolts made the 6.9 nearly maintenance-free for its first 50,000 miles (80,500 km), requiring little basic service other than coolant, minor tune-ups, oil changes and replacement of the air, fuel, oil and power steering filters.
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