The new V8 engines from Mercedes-Benz use many of the same technologies that were developed for the company's recently introduced V6s. These include three valves per cylinder, phase-shifted dual ignition, friction optimized cylinder liners, and an active service system.

The removal of one exhaust valve per cylinder reduces the surface area of the exhaust ports by about 30% compared to a four-valve engine, considerably reducing heat losses in the exhaust flow. Subsequently, the catalytic converter reaches operating temperature after a cold start about 12 s earlier. Instead of cast iron cylinder liners, the engines feature a low-friction aluminum-silicon alloy that contributes to 50% lower piston ring stress. With the Active Service System (ASSYST), which monitors not only oil level but also contamination, oil-change intervals are extended to an average 22,500 km (14,000 mi.).

Now, with the new 5.0-L S-Class V8 engine, Mercedes-Benz has become the first European automotive manufacturer to offer cylinder cut-out. It is available as an option on the S 500. According to Gerhard Doll, Cylinder Cut-out Project Manager, the technology provides a fuel savings with no impairment of the performance potential and refinement of the engine. During part-load operation, the system deactivates four of the eight cylinders, and brings them back into operation when greater performance is required.

The S 500 V8's cylinder cut-out system deactivates all 12 valves of the four cylinders.

When the system shuts off four of the eight cylinders, the S 500's fuel consumption in the New European Driving Cycle (NEDC) is reduced by about 7%. Even greater savings are possible in other driving situations. At a constant 120 km/h, consumption is cut by around 13%, and at a steady 90 km/h, by as much as 15%.

Mercedes explains that, in conditions such as city traffic, out-of-town driving, or when traveling at constant speed on the motorway in the mid-speed range, an eight-cylinder engine suffers due to impaired volumetric efficiency and high charge-cycle losses. In some cases, the large-displacement engine has to be throttled to the point that cylinder pressure at top dead center is roughly half that of a four-cylinder engine. Lower cylinder pressure means lower fuel efficiency, in turn leading to high consumption. Briefly shutting off four of the cylinders eliminates this inherent drawback of a V8 power unit. In four-cylinder-mode, volumetric efficiency is greater and the quantity of exhaust gas recirculated into the engine can be increased for additional efficiency.

As soon as the engine drops into part-load operation, the electronic engine management system deactivates cylinders two and three on the right cylinder bank and five and eight on the left. This is done by deactivating the corresponding intake and exhaust valves and cutting off the fuel supply to those cylinders.

The transition between modes of operation is made imperceptible because the electronic engine management system briefly alters the throttle valve position and adjusts the ignition timing to prevent any sudden jump in torque. Automatic cylinder cut-out is operational between 1000 and 3500 rpm in third, fourth, and fifth gear with the automatic transmission. This corresponds to a speed range of 40 to 160 km/h.

To stop actuation of the valves, the link between the valve and the camshaft is interrupted. To achieve this, the one-piece roller-type rocker arm is replaced by two special arms on each cylinder, which are referred to by engine development engineers as the primary and secondary arms. The role of the primary arm is valve control, while the secondary arm follows the cam lift. When the engine is in eight-cylinder mode, the two arms are linked by a locking piston. Automatic cylinder cut-out breaks this link hydraulically by forcing the locking pistons against special return springs by means of electromagnetic shift valves. Valve actuation on that cylinder then ceases. To reactivate the cylinders, the hydraulic pressure acting on the pistons is released, via electromagnetic shift valves, and the springs return them to their original position, where the primary and secondary arms are locked.

Control of the hydraulic valve by the engine management computer ensures that the changeover always takes place at the proper time and the valves are shut off in the correct sequence. To prevent the deactivated cylinders from cooling down during the cut-out phase, the exhaust valves are always closed immediately after a power stroke. The mix therefore remains in the cylinder after combustion, keeping the cylinder walls warm.

When the engine is required to produce power from all eight cylinders again, the system actuates the exhaust valves first. These open, allowing the exhaust gas to leave the cylinder; a fresh air/fuel mixture then enters the chambers again, and the combustion process starts once more.

Engineers have also modified the exhaust system to ensure smooth transitional operation of the power units. An exhaust valve in the mixing tube, downstream of the underfloor catalytic converters, closes immediately after the cut out of the four cylinders. This lessens the high-pressure waves occurring in the exhaust-gas system in four-cylinder mode and the resulting tendency of the engine and transmission assembly to oscillate. This acoustic trick is said to also favorably affect exhaust system exit noise.

The alterations to the engine are said to not affect overall torque characteristics. However, power output drops slightly from 225 kW (306 hp) to 220 kW (299 hp) in the S 500.