The new Mercedes-Benz SLK 55 AMG is the most powerful SLK of all time. Its V8 engine combines low consumption figures with the superior power delivery and the typical sound experience which only a large-displacement eight-cylinder engine can provide. The newly developed AMG 5.5-litre V8 engine featuring cylinder shut-off combines the ultimate in performance with the lowest possible fuel consumption. It produces peak output of 310 kW (422 hp) and maximum torque of 540 Nm. The fuel consumption of 8.4 litres/100km and CO2 emissions of 195 g/km are around 30 percent lower than those of its predecessor. The SLK 55 AMG accelerates from zero to 100 km/h in 4.6 seconds, and reaches a top speed of 250 km/h.
The new M152 is based on the AMG M157 5.5-litre V8 biturbo engine presented in 2010, which powers the E 63 AMG, CLS 63 AMG, S 63 AMG and CL 63 AMG. Numerous design features are identical, such as the displacement, bore/stroke, distance between cylinders, start/stop system and direct petrol injection technology. Distinguishing features versus the M157 include the new intake air ducting, new cylinder heads, the modified valve drive, an adapted oil supply system and an optimised crankcase.
The high-tech M152 powerpack enthrals with a combination of advanced technologies which is unequalled anywhere in the world. No other petrol engine features direct injection at a pressure of 200 bar, spray-guided combustion and piezo injectors, all in conjunction with map-controlled cylinder shut-off, an all-aluminium crankcase with “spectacle” honing, four-valve technology with continuous camshaft adjustment, a high compression ratio of 12.6 : 1, a start/stop system and generator management – while revving to a maximum speed of over 7000 rpm. All of these measures lead to high thermodynamic efficiency, which is a precondition for high fuel efficiency and low exhaust emissions. Moreover, deciding in favour of a large-displacement, naturally-aspirated eight-cylinder unit with cylinder shut-off resolves the conflict of aims between high output/torque delivery and low fuel consumption figures.
It is the AMG Cylinder Management cylinder shut-off system in particular – completely newly developed by Mercedes-AMG – that significantly improves efficiency: cylinders two, three, five and eight are cut off under partial load, which lowers fuel consumption considerably. In similar form this technology is also used in the approx. 750 hp V8 engines used in Formula 1. As efficiency also plays a major role at the pinnacle of motor racing, two or four of the eight cylinders are cut off e.g. when cornering at slow speed, in the Safety Car phases or for pit stops. With a fuel consumption of 8.4 litres per 100 km and CO2 emissions of 199 grams per kilometre (NEDC combined), the naturally-aspirated AMG V8 engine comes in at or below the level of its competitors, however these are unable to match either the output or torque of the SLK 55 AMG – and above all, they do not use large-displacement eight-cylinder engines.
The cylinder shut-off function is available over a wide engine speed range from 800 to 3600 rpm if the driver has selected transmission mode “C” – Controlled Efficiency. The AMG main menu in the instrument cluster informs the driver whether cylinder shut-off is active, and whether the engine is currently running in four or eight-cylinder mode. No less than 230 Nm of torque is still available in four-cylinder mode – enough power to ensure plenty of acceleration in most driving situations. As soon as the driver has a need for more power and leaves the partial load range, cylinders two, three, five and eight are activated. The switch from four to eight-cylinder operation is immediate and imperceptible, leading to no loss of occupant comfort whatsoever. At an engine speed of 3600 rpm the activation process takes no more than 30 milliseconds.
This is made possible by intelligent interaction within the highly effective engine management system, with 16 hydraulically compensating elements and a complex oil supply system in the cylinder head. The compensating elements are integrated into the cylinder head, and keep the intake and exhaust valves of cylinders two, three, five and eight closed when cylinder shut-off is active. At the same time their fuel supply and ignition are deactivated. This not only enables the load-change losses of the four deactivated cylinders to be reduced, but also increases the efficiency of the four remaining cylinders. This is because the operating point is transferred to the higher load range. The compensating elements are compact and lightweight, allowing taut valve train operation and engine speeds up to 7200 rpm.
For the first time AMG is using a sports exhaust system featuring an exhaust flap on each side. Each of the two rear silencers has a flap which is variably activated on a logic-controlled basis depending on the driver’s power requirement and the engine speed. At low loads and engine speeds below 2000 rpm the flaps remain closed. This causes the exhaust gases to cover a longer distance and flow through an additional damping element, so that the engine sound is pleasantly subdued and irritating frequencies are effectively suppressed – resulting in a harmonious sound impression especially in four-cylinder operation when cylinder shut-off is active.
When the driver accelerates, the flaps open at an angle of 15, then 30 and up to 50 degrees so that some of the exhaust gases cover the longer, acoustically dampened distance and some the shorter distance. This produces a sonorous engine note. Under full load at higher engine speeds in eight-cylinder mode, both flaps are fully opened so that the occupants are able to enjoy the decidedly muscular sound typical of an AMG V8. In short, they are left in no doubt about the performance potential of the 310 kW (422 hp) naturally aspirated AMG eight-cylinder engine. The two-pipe AMG sports exhaust system with two chromed twin tailpipes has a pipe diameter of 65 millimetres from the manifolds to the pre-silencer. The diameter is reduced to 60 millimetres from the pre-silencer to the rear silencers.