Turbo world domination is in hyperdrive with the AMG range. Will the SLK55 be the last of the atmospherically-combusted AMG V8s?
With the SLS bowing out and the C63 AMG and C63 S new for 2015 with turbocharged 4.0-liter V8s, the list of unboosted AMG models is now down to just a handful. Sure, the CLK63 is technically on the books – but we would be surprised to see this old girl renewed for 2015.
On the other hand, a new SLK is still more than a year out. The SL-Class is most likely to benefit from an AMG-GT-style makeover for 2016, which leaves the SLK55 on deck for a 2016.5 or 2017 model-year-redesign.
Why celebrate this fact? As a nimble coupe-cabrio, the SLK is the most chuckable sportscar of the range. So enjoying its naturally-aspirated bellow from the front is sure to be savoured.
Balancing the growl from the front, the SLK55 AMG has AMG sport flaps in the exhaust for a dual-mode howl, plus a Performance pack with stiffer springs and a limited-slip differential in back.
The new turbo AMG models all yell loudest from the back, but this SLK55 is one to love face to face as well….
2015 Mercedes-Benz SLK55 AMG
THE NEW SLK 55 AMG – TOP MARKS IN ALL DISCIPLINES
Affalterbach – The Mercedes-Benz SLK 55 AMG is the most powerful SLK of all time. Its newly developed AMG 5.5-litre V8 engine featuring cylinder shut-off combines the ultimate in performance with the best possible fuel consumption. With a peak output of 422 hp and maximum torque of 540 Nm, the SLK 55 AMG outperforms all of its direct competitors. Its fuel consumption of 33.6 mpg on the combined cycle and CO2 emissions of 195 grams per kilometre are around a 30 percent improvement of that of its predecessor. High levels of driving dynamics are ensured thanks to the AMG sports suspension with Direct-Steer system, Torque Vectoring Brakes and 3-stage ESP®, as well as the AMG high-performance braking system.
Forming part of the third-generation of the SLK launched in March, the new top-of-the range AMG model already demonstrates its individuality even at first glance: distinctive AMG light-alloy wheels, specific AMG bodystyling with AMG spoiler lip and two chrome-plated twin tailpipes at the rear complement the classic roadster proportions, while adding a markedly athletic touch. The expressive design of the SLK 55 AMG is both eye-catching and unmistakable.
The SLK 55 AMG is the perfect embodiment of the new AMG brand claim of “Driving Performance”: in the compact high-performance roadster segment there is nothing to compare to its combination of tangible driving dynamics, superior sportiness, unique high-technology and low fuel consumption. The SLK 55 AMG accelerates from 0-62 mph in 4.6 seconds, and reaches
a top speed of 155 mph (electronically limited).
Key figures at a glance: *
SLK 55 AMG
Bore x stroke
98.0 x 90.5 mm
12.6 : 1
422 hp at 6800 rpm
540 Nm at 4500 rpm
Engine weight (dry)
Acceleration 0 – 62 mph
* provisional figures; ** electronically limited
The world’s most fuel-efficient V8 engine combines the lowest consumption figures with the superior power delivery and the typical sound experience which only a large-displacement eight-cylinder engine can provide.
In line with its “AMG Performance 2015” strategy, with the new V8 unit Mercedes-AMG is meeting its promise to continue to reduce both the fuel consumption and emissions of new models over the next few years – while reaching new heights with the central brand claim of “Driving Performance”. At the same time, AMG is also demonstrating its social responsibility with the M152.
New naturally aspirated eight-cylinder closely related to the V8 biturbo
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.
Exclusive combination of advanced technologies
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 improved fuel consumption figures.
AMG Cylinder Management
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 33.6 mpg and CO2 emissions of 195 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.
Valves closed, fuel delivery and ignition deactivated
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 7,200 rpm.
All-aluminium crankcase with Silitec cylinder liners
The crankcase of the new AMG 5.5-litre V8 engine features a diecast all-aluminium construction. The low (dry) engine weight of just 187 kilograms is the result of uncompromising lightweight construction methods. The bearing covers for the main crankshaft bearings are of grey cast iron, and are bolted
to the crankcase, as well as on the sides, for high rigidity.
The piston rings are carbon-coated to minimise internal friction and optimise the wearing characteristics. Cast-in Silitec cylinder liners ensure low friction for the eight pistons. So-called “spectacle” honing is another measure to reduce friction and therefore fuel consumption: in this sophisticated and complex process, the cylinder liners receive their mechanical surface treatment when already bolted in place. As honing would no longer be possible with the cylinder head installed, a jig resembling spectacles is bolted to the crankcase. The cylinder liners distort as if the cylinder head were in place, and are only then given their mechanical surface treatment. As a result, any static distortion of the cylinder liners caused by tightening the cylinder head bolts can be completely eliminated.
Other measures to optimise efficiency:
weight-optimised cast aluminium pistons with special piston skirt coating
special ventilation holes in the crankcase
oil pump with electrically controlled pressure stage
separate oil pump for cylinder shut-off as a controllable, low-friction vane cell pump
engine cooling on the particularly efficient cross-flow principle
electronically controlled fuel pressure, fully variable and demand-related between 100 and 200 bar
The forged crankshaft of high-grade 38MnS6BY steel alloy rotates in five main bearings, has eight counterweights and has been optimised with respect to torsional rigidity, inertia, low rotating masses and a long operating life. A two-mass viscous damper mounted at the front reliably eliminates vibrations. Each connecting rod journal on the crankshaft carries two forged, cracked connecting rods.
Four-valve technology with variable camshaft adjustment
Perfect charging of the combustion chambers is ensured by large intake and exhaust valves, of which there are four per cylinder. The exhaust valves, which are subject to high thermal loads, are hollow and sodium-cooled. Four overhead camshafts operate the 32 valves via low-maintenance, low-friction cam followers. The infinitely variable camshaft adjustment within a range of 40 degrees on the intake and exhaust sides depends on the engine load and engine speed, leading to high output and torque values. This also results in consistent idling at a low speed. Depending on the engine speed, valve overlap can be varied for the best possible fuel/air supply to the combustion chambers and efficient removal of the exhaust gases. The variable camshaft adjustment is carried out hydraulically via four pivoting actuators. These are electromagnetically actuated and controlled by the engine control unit. The camshafts are driven by three high-performance silent chains, which have considerable advantages in terms of noise comfort compared with cylinder roller chains.
Controlled Efficiency start/stop function as standard
The Controlled Efficiency start/stop function further contributes to the low fuel consumption. This system is standard equipment on the SLK 55 AMG, and active in the fuel economy transmission mode “C”. Once the driver comes to a halt e.g. at a red traffic light, the V8 engine is automatically switched off. Once the brake pedal is released or the accelerator is depressed, the engine is immediately restarted and the car is able to move off quickly. Intelligent technology guarantees a comfortable and immediate starting procedure: a crankshaft sensor which recognises the direction of rotation registers the resting position of all eight pistons. For an automatic engine start, the cylinder with the most favourable piston position receives an injection of fuel into its combustion chamber. The precise piezo-electric injectors greatly assist this process, as they make particularly fast starts possible. The M152 always starts in eight-cylinder operation.
A framed “ECO” symbol in the AMG main menu shows the driver that the Controlled Efficiency start/stop function is active. When cylinder shut-off is active, the driver is informed by the symbol “ECO4”, while “ECO8” stands for eight-cylinder operation. In the two more performance-oriented driving modes “S” (Sport) and “M” (Manual), the start/stop function is always deactivated. If required, the driver can also switch it off while in “C” mode as well. In this case neither a “start/stop” nor an “ECO” symbol appears.
Generator management helps to save fuel
A generator management system makes a further contribution to fuel economy: whenever the V8 engine is on the overrun or when braking, kinetic energy is used to charge the battery rather than being wasted as heat in the usual way. In all other operating modes a combination of onboard network and generator management enables the generator to be kept at a low voltage. This reduces the load on the engine and enhances fuel efficiency in city traffic with its frequent overrun and braking phases.
New AMG sports exhaust system with integrated exhaust flaps
For the first time AMG is using a sports exhaust system featuring an exhaust flap on each side. This technology resolves the conflict of aims between an emotional sound when driving in a sporty manner and a more discreet engine note in the partial load range. The main focus of the sound design was on achieving a perfect synthesis of perceptible dynamism and the long-distance comfort that is a hallmark of Mercedes.
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 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.
Effective emissions technology with new catalytic converter boxes
Low exhaust emissions, compliant with country-specific standards and a characteristic engine sound – the requirements for the exhaust system of the new, naturally aspirated AMG 5.5-litre V8 engine were complex. The SLK 55 AMG complies with the current Euro-5 emission standard, as well as meeting all the requirements of the US market (LEV-III standard, On-Board Diagnosis II).
Air gap-insulated manifolds ensure a rapid catalytic converter response. For efficiency and to save space, this concept has a tandem catalytic converter housing on each side of the vehicle: adjacent to the firewall, two thin-walled ceramic substrates are grouped into each housing. This solution makes the previous, additional underbody catalytic converters unnecessary. The two ceramic substrates differ to ensure rapid and efficient emissions control: the front one is coated with palladium, while the rear one has a bimetal coating of palladium and rhodium. One lambda sensor per row of cylinders is located in front of each catalytic converter housing, and there is a lambda diagnostic sensor between each of the two thin-walled substrates.
Highly variable: the AMG SPEEDSHIFT PLUS 7G-TRONIC
Power is transferred to the rear wheels by means of the AMG SPEEDSHIFT PLUS 7G-TRONIC automatic transmission which features three transmission modes: “Controlled Efficiency” (C), Sport (S) and Manual (M). The automatic double-declutching function when downshifting as well as the brief, precisely defined interruption of ignition and injection when upshifting under full load shorten shifting times and increase emotional appeal when adopting a sporty driving style. A double turbine torsional damper with centrifugal pendulum which adapts to engine speed helps to minimise vibrations in four-cylinder operation and enhances the impression of comfort experienced by the vehicle occupants.
Perfect combination: highly dynamic driving style and comfortable cruising
The AMG sports suspension with Torque Vectoring Brakes and AMG Direct-Steer system provides both impressive lateral dynamics as well as agile handling. Looking for a highly dynamic driving style or comfortable cruising? The new SLK 55 AMG is able to offer both. Independent suspension struts with stiffer spring/damper tuning, torque strut bearings on the front axle featuring higher rigidity, stiffer stabilisers all round as well as a more negative camber on the rear axle result in high cornering speeds and a low body roll in fast S-shaped bends.
In critical cornering conditions, the Torque Vectoring Brakes produce a defined rotational movement of the vehicle about the vertical axis in fractions of seconds, through selective brake actuation at the rear wheel on the inside of the bend. This results in the SLK 55 AMG steering into the bend precisely and under full control, without any compromises where dynamism is concerned. Complementing this feature perfectly is the AMG Direct-Steer system: it enhances handling and agility compared with the standard steering, whilst also reducing the amount of physical effort required when parking. A new power steering pump helps to improve fuel consumption. The reason behind this: the power assistance for the steering only requires energy when the driver is actually steering. In addition, the AMG Direct-Steer system has a parameter function adapted to match the AMG sports suspension. This results in optimum response from the central position, outstanding road feel and dynamic handling in all driving situations.
The AMG-exclusive 3-stage ESP® gives individual control strategies at the touch of a button: the ESP® button in the centre console allows the driver to choose between “ESP ON”, “SPORT Handling Mode” and “ESP OFF”. The AMG high-performance brake system has internally ventilated and perforated brake discs on all wheels, measuring 360 x 36 mm at the front and 330 x 22 mm at the rear. The system is characterised by outstanding responsiveness, short stopping distances and excellent fatigue strength.
Optional AMG Handling package for even greater driving dynamics
The optionally available AMG Handling package enhances the vehicle’s sportiness even further. It comprises an AMG Performance suspension with stiffer tuning, an AMG rear axle differential lock, composite brake discs at the front and a three-spoke AMG Performance steering wheel finished in nappa leather with Alcantara® inserts in the grip areas.
Unmistakable: uniquely styled design and appointments
Classic roadster proportions with typical AMG design characteristics as well as a hint of the SLS AMG – these are the striking visual highlights of the new SLK 55 AMG. The AMG designers have complemented the exciting basic forms of the long bonnet, the greenhouse positioned well to the rear and crisp rear section with typical AMG styling at the front, sides and rear. The new AMG front apron has large air inlets and AMG-specific LED daytime driving lights. Inspired by the SLS AMG super sports car, the wide radiator grille features a large, centrally-positioned Mercedes star. In the case of the SLK 55 AMG, the slats in the radiator grille and the lower cross strut on the front apron are painted in a high-gloss black finish. Darkened clear-lens headlamps and darkened bicolour tail lamps also come as standard exclusively on the top-of-the-range V8 model. The Intelligent Light System (ILS) is also available as an optional extra.
The vent grilles with chrome-plated fins in the front wings are not only reminiscent of the SLS AMG, but also evoke memories of the famous Mercedes-Benz 300 SL Roadster from the 1950s. The “V8 AMG” lettering beneath the grilles points to the top-of-the-range eight-cylinder model from the SLK model series. AMG-specific, distinctively styled side sill panels continue the design of the front apron through to the new rear apron. With its own unique shape, black diffuser insert with three fins as well as the two chrome-plated twin tailpipes of the AMG sports exhaust system, it gives a distinctive finishing touch to the vehicle. The spoiler lip on the boot lid, painted in the vehicle body colour, reduces lift at the rear axle and thereby increases driving stability at high speeds.
Inspired by the SLS AMG: high-quality, stylish interior
Functional, stylish, high-quality: the interior design of the SLK 55 AMG is also reminiscent of its bigger brother, the SLS AMG Roadster. Associations with the super sports car are evoked courtesy of the four air vents in the style of jet engines, as well as the centre console finished in brushed aluminium. An optional addition to the centre of the dashboard is an AMG-specific analogue watch featuring an IWC design. The AMG Performance steering wheel, with its flattened top and bottom sections, also boasts metal trim, aluminium shift paddles and grip areas finished in perforated leather. The clearly-arranged AMG instrument cluster is positioned perfectly in the driver’s field of vision and features four circular instruments, distinctive dials and the AMG main menu. The switch for the transmission modes C, S and M is also located in the centre console.
The AMG sports seats with individual seat upholstery layout and optimised lateral support come as standard with the upholstery combination ARTICO man-made leather/black fabric and high-quality AMG badges in the backrests. Nappa leather and Exclusive nappa leather are both available as optional extras. As in the previous model, the AIRSCARF neck-level heating system is also available as an optional extra. This creates a flow of warm air around the neck and shoulder areas of the occupants from a vent in the front of each head restraint.
Magical: the panoramic vario-roof with MAGIC SKY CONTROL
The standard vario-roof opens and closes in less than 20 seconds, transforming the open-top Roadster into a closed-top Coupé with rigid roof at the touch of a button. Alternatively, a panoramic vario-roof in tinted polycarbonate can also be chosen. The third variant is a world premiere – the panoramic vario-roof with MAGIC SKY CONTROL. This glass roof switches to light or dark as required at the press of a button. When light it is virtually transparent, offering an open-air experience even in cold weather. In its dark state the roof provides welcome shade and prevents the interior from heating up when the sun’s rays are very intense. In other words: a feel-good atmosphere at the touch of a button
The frames of the three roof variants are made of magnesium. This makes each roof around six kilograms lighter than in the previous model, giving the car a lower centre of gravity – which benefits its agility. A comfortable alternative to the standard draught-stop is the new AIRGUIDE pivoting draught-stop. It consists of pivoting transparent plastic layers which are attached to the reverse of the roll-over bars. This is the world’s first draught-stop system that enables different comfort levels to be selected for two occupants, as the driver and passenger can individually pivot the units to the centre of the vehicle in a flash, thus taming turbulent air flow from the rear. The advantage: no installation or removal effort required.
More individuality: designo and the AMG Performance Studio
Designed to meet the needs of those with even more individual tastes are the optional appointments “designo leather” and “designo Exclusive leather”, offering eleven single-tone and ten two-tone upholstery variants. With designo the AMG sports seats feature a specific design which distinguishes them from the standard sports seats. Available matching elements include “AMG carbon-fibre trim” and “designo trim in black piano lacquer” for the centre console. The designo Alcantara® windscreen frame, seat belts in silver or red and also five attractive paint finishes – including two matt paint finishes – round off the stylish appearance perfectly.
Other attractive optional appointments are also exclusively available for the SLK 55 AMG from the AMG Performance Studio:
AMGmulti-spoke light-alloy wheels, painted in titanium grey with a
high-gloss finish, with 235/40 R 18 (front) and 255/35 R 18 (rear) tyres
AMGmulti-spoke light-alloy wheels, painted in matt black with a
high-gloss finish on the rim flange, with 235/40 R 18 (front) and
255/35 R 18 (rear) tyres
AMGHandling package (AMG Performance suspension, AMG rear axle differential lock, composite brake discs on the front axle, AMG Performance steering wheel with Alcantara® inserts in the grip areas).
AMGdoor sill panels, illuminated in white using LED technology
AMGPerformance steering wheel with Alcantara® inserts in the grip areas
Exceptional position: modern assistance systems on board
The SLK 55 AMG sets new standards when it comes to safety: a whole host of the latest assistance systems support the driver and have helped to secure the new AMG Roadster’s exceptional position in its segment. Standard equipment includes new, fibre-reinforced roll-over bar, two-stage driver and front passenger airbags, headbags, thorax airbags, seat belt tensioner with degressive belt force limiters, NECK-PRO crash-active head restraints, automatic child seat recognition, three-stage ESP®and also ATTENTION ASSIST drowsiness detection.
Further assistance systems are available on request:
DISTRONIC PLUS: this proximity control system automatically maintains a set distance from the vehicle ahead, braking the SLK 55 AMG to a standstill if necessary and also accelerating the Roadster again.
The Intelligent Light System (ILS) provides five lighting functions (cornering light function, country mode, motorway mode, active light function and enhanced fog lamps), which are activated depending on the driving conditions
Speed Limit Assist is able to detect speed limit signs using a camera on
the inside of the windscreen, and indicate the detected speed limit in the instrument cluster or central display.
The anticipatory and reversible PRE-SAFE® occupant protection system is able to activate protective measures for the car’s passengers as a precaution. The aim is to prepare the occupants and the vehicle for an imminent collision so that the seat belts and airbags are able to fulfil their protective function to the full during an impact.
The PRE-SAFE® Brake detects the acute danger of a rear-end collision and prepares for autonomous braking. The system is active at speeds of between 30 and 200 km/h when moving vehicles are detected in front
of the car.
Three generations: the SLK made by AMG
The new SLK 55 AMG is already the third generation of the exclusively dynamic Roadster model made by AMG. It all started in 2001 with the SLK 32 AMG. Its AMG V6 supercharged engine with a displacement of 3.2 litres produced 354 hp and maximum torque 450 Nm – as such, the SLK 32 AMG
was the most powerful vehicle among the competition.
At the end of 2004 this was followed by the SLK 55 AMG. With its AMG 5.5-litre V8 engine, which developed 360 hp and 510 Nm of torque, it also assumed the leading position in its segment. No other vehicle in this class offers a large-displacement eight-cylinder engine – and Mercedes-AMG has remained consistently true to this philosophy with the successor model.
Tom Burkart is the founder and managing editor of Car-Revs-Daily.com, an innovative and rapidly-expanding automotive news magazine.
He holds a Journalism JBA degree from the University of Wisconsin – Madison. Tom currently resides in Charleston, South Carolina with his two amazing dogs, Drake and Tank.
Mr. Burkart is available for all questions and concerns by email Tom(at)car-revs-daily.com.