The New Mercedes-Benz SLS AMG Roadster: Perfect open-top performance - V
Engine and power transmission: Naturally aspirated eight-cylinder engine with transaxle dual clutch transmission
Eight cylinders and a displacement of 6.3 litres in a front-mid-engine arrangement, a high-revving concept, dry sump lubrication and a performance-oriented intake and exhaust system: opening the almost two-metre long bonnet of the SLS AMG affords a view of an outstanding high-performance engine. With a peak output of 420 kW (571 hp) at 6800 rpm and 650 newton metres of torque at 4750 rpm, the AMG 6.3-litre V8 unit delivers first-class performance figures. The Roadster accelerates from zero to 100 km/h in 3.8 seconds, reaches 200 km/h in 11.3 seconds and has a top speed of 317 km/h (electronically limited). The DIN kerb weight of 1660 kilograms produces an extremely favourable power-to-weight ratio of only 3.95 kilograms per kilowatt or 2.9 kilograms per hp.
The use of specific fuel economy measures has enabled a fuel consumption of 13.2 litres per 100 kilometres (NEDC combined) to be achieved. Power transfer in the SLS AMG is by a new, seven-speed AMG dual clutch transmission located at the rear axle in a transaxle arrangement. It is rigidly connected to the engine by a torque tube.
The combination of a high-revving concept and a large displacement achieves the best of both worlds: exhilarating flexibility accompanied by high torque at low engine speeds. The new AMG V8 already delivers 545 newton metres to the crankshaft at 2500 rpm, with the maximum torque of 650 newton metres on tap from 4750 rpm – more than any other naturally aspirated engine in this output and displacement class. In short, the new AMG eight-cylinder guarantees dynamic acceleration, rapid intermediate sprints and sheer driving pleasure at the highest level, or relaxed, day-to-day cruising when preferred.
Key data at a glance:
Optimum cylinder charging thanks to newly designed intake air ducting
The newly calculated, more flow-efficient design of the intake air ducting leads to a reduction in pressure losses, and the magnesium intake manifold has precisely configured resonance tube lengths. Eight velocity stacks with a length of 290 millimetres and a diameter of 51.5 millimetres supply the combustion chambers with fresh air. Behind the air filters with a volume of just under 9500 cubic centimetres there are two throttle flaps with a diameter of 74 millimetres. Their electronic adjustment is practically instantaneous: it takes only 150 milliseconds to open the two throttle flaps to their maximum. The result is exhilarating responsiveness. Two hot-film air mass sensors located behind the air filters supply the engine electronics with the necessary temperature and density data for the intake air.
One special characteristic of the V8 engine is its sophisticated valve drive, which borrows features from the powerful AMG racing engines. The 32 valves in the cylinder heads are operated by bucket tappets. Their space-saving design allows a stiff valve train and therefore high engine speeds with large valve opening cross-sections, which in turn benefits output and torque. The large intake valves have a diameter of 40 millimetres, while their opposite numbers on the exhaust side measure 34 millimetres. In contrast to a racing engine, the valve drive is maintenance-free with hydraulic valve clearance compensation.
Four continuously variable overhead camshafts
All four overhead camshafts are continuously variable over a range of 42 degrees. Both the intake and exhaust camshafts are adjusted as a function of engine load and engine speed, ensuring extremely high output and torque values as well as smooth idling and especially low exhaust emissions. Depending on the engine speed, the valve overlap can be varied to ensure an optimal supply of fuel/air mixture to the combustion chambers and efficient venting of the exhaust gases. The variable camshaft adjustment is controlled electrohydraulically, and monitored by the engine control unit.
Distinctive exhaust note with two exhaust headers
Optimised performance and an emotional sound signature: these were the requirements when developing the sports exhaust system. Exhaust headers with precisely coordinated tube lengths – a further technology transfer from motor racing – ensure a significant increase in output and torque by improving the gas cycle. The large pipe cross-sections of the newly developed twin exhaust system effectively reduce the exhaust counterpressure. Two relatively small pre-silencers on the underbody and a large, transversely installed main silencer at the rear help to ensure optimum weight distribution.
The AMG specialists attached particular importance to the sound signature of the eight-cylinder power unit, and exhaustive tests were carried out to achieve a synthesis of immediately perceptible dynamism and the long-distance qualities typical of a Mercedes. Great attention was also paid to the elimination of irritating noises, while accentuating the frequencies perceived as pleasant and typical of a sports car. Especially when the soft top is down, the engine note accentuates the emotionally appealing character of the SLS AMG Roadster. At the same time it was of course necessary to comply with the legal requirements – for example when measuring passing noise.
The two exhaust tailpipes integrated into the rear apron emit a characteristic eight-cylinder sound which tells every car-lover: here comes a genuine AMG. Two ceramic catalytic converters optimised for counterpressure and located at the firewall directly by the exhaust header, plus two metal catalytic converters on the underbody, provide effective exhaust gas aftertreatment and ensure compliance with all current and future limits such as EU 5, LEV 2 and U‑ LEV. The new AMG engine also easily meets specific requirements for the US market, such as Onboard Diagnosis II and lambda sensor diagnosis.
Dry sump lubrication to keep the centre of gravity low
One major criterion when formulating the technical concept for the SLS AMG Roadster was the installed position of the engine. The requirement was for the engine to be positioned low down and well to the rear, so as to lower the car's centre of gravity significantly and achieve a balanced weight distribution between the front and rear axles. The solution found, namely to install the eight-cylinder power unit as a front-mid engine behind the front axle, combining it with a transmission in a transaxle layout, led to an optimum weight distribution of 47 percent at the front and 53 percent at the rear.
The much lower installed position of the V8 engine is the result of a changeover to dry sump lubrication – which eliminates the usual oil sump. The dry sump lubrication system of the M159 consists of a suction pump, a pressure pump and an external 5-litre oil tank located in front of the engine. All in all, 13.5 litres of engine oil circulate around the complete system. The oil scavenging pump sucks the oil directly from the crankcase and cylinder heads, delivering it to the external oil tank at a maximum rate of 700 litres per minute. This efficient oil scavenging further improves the efficiency of the engine.
The reciprocating pressure pump conveys the oil from the external oil tank back to the engine, ensuring effective lubrication even under the high lateral acceleration forces that usually occur on racetracks. The demand-related control of the oil pressure pump is based on the engine speeds programmed into the control unit, as well as the characteristic maps for temperature and engine load. This leads to a reduction in the engine's power dissipation, and a considerable reduction in fuel consumption. Two large coolers in the front wheel arches, each with a blower fan integrated on one side, ensure that the engine oil is efficiently cooled.
Another innovation used in the engine is the so-called "gas-spring principle", where the air compressed beneath the piston as it moves down the gas-tight crank chamber is prevented from escaping. On the upstroke the rebound energy is almost completely returned to the pistons. This principle prevents flow losses. Another positive effect is the much greater rigidity of the crankcase. The gas-spring principle has enabled fuel consumption to be reduced by 1.5 percent – while increasing output by 7 kW (9.5 hp).
Outstanding strength and optimised lightweight construction also for the engine
The eight forged pistons, which are together 0.5 kilograms lighter than cast pistons, reduce oscillating masses and improve both responsiveness and flexibility. Optimum cooling of the severely stressed piston crowns is by means of pressure-controlled oil-spray nozzles in the crankcase. As another specific weight-saving measure, there are no steel liners locating the main crankshaft bearings. The crankcase of the M159 is wholly of aluminium, and around 4 kilograms lighter than its counterpart in the M156. To reduce weight even further, aluminium bolts are mainly used in the M159. This saves another 0.6 kilograms or so compared to steel bolts. The dry weight of the V8 engine in the SLS is 205 kg, producing a first-class power-to-weight ratio of 0.36 kg/hp.
As is usual in motor sports, the crankcase is a rigid bedplate design of closed-deck construction. The engine block and cylinder heads are of cast aluminium-silicon alloys (AlSi7 and AlSi17) which represent the state of the art in terms of weight, thermal and mechanical resistance and long-term strength. As in the M156, the engine specialists at Mercedes-AMG use a particularly advanced process for the eight cylinder walls to produce a tribologically optimal surface, the so-called NANOSLIDE technology. The advantages are extremely low friction and wear accompanied by outstanding long-term durability. A considerably harder surface is achieved compared to conventionally coated cylinder walls.
Cooling module of weight-optimised design
The coolant is cooled by a large cooler module located behind the radiator grille. The condenser for the air conditioning and the cooler for the steering servo fluid are also integrated into this module. As a particular bonus, this weighs around 4 kilograms less than conventional components – while improving efficiency at the same time. A large suction fan located directly behind the radiator conducts the hot air away as required.
Efficient control unit for generator management
The Bosch ME 9.7 AMG control unit also has a generator management function as a further system that helps to save fuel. Sensors monitor the charge status of the vehicle battery, and reduce the output of the generator if the battery has sufficient charge. The system uses the overrun phases of the engine to recharge the battery, braking energy being converted into electrical energy by recuperation.
Careful combination of efficiency-enhancing measures has enabled an NEDC combined fuel consumption of 13.2 litres per 100 km (premium petrol) to be achieved. This is an outstanding figure, as the new SLS AMG is after all one of the most powerful cars in the super-sports car segment.
Engine production at the Affalterbach plant
The new V8 powerpack for the SLS is produced in the ultra-modern AMG engine shop, on the "One man, one engine" principle. This means that a single technician assembles the engine by hand from start to finish – as witness the high-quality AMG engine plate bearing the signature of the technician responsible. The design of the AMG eight-cylinder is also a feast for the eyes, starting with the new, titanium grey cover of the magnesium intake manifold with its two prominent AMG logos, and the optional carbon-fibre engine compartment cover that frames the high-performance power unit so attractively.
AMG seven-speed dual clutch transmission with RACE START
The ideal partner for the powerful, naturally aspirated eight-cylinder engine is the AMG SPEEDSHIFT DCT 7-speed sports transmission – a dual clutch transmission with seven gears, four driving modes and a RACE START function. The advantages of this technology, which is once again derived from motor racing, include highly responsive gearchanges without any interruption in tractive power, a tailor-made control strategy and improved gearshifting comfort. Specifically configured for the high-revving nature of the AMG 6.3-litre V8 engine, the AMG SPEEDSHIFT DCT 7-speed sports transmission provides optimum ratios with only small rpm changes. In short, the new transmission gives the driver even more dynamic acceleration for maximum driving pleasure.
AMG DRIVE UNIT with four driving modes for maximum pleasure
The central control point for the AMG SPEEDSHIFT DCT 7-speed sports transmission and all dynamic control functions is the AMG DRIVE UNIT. The rotary electronic control for selection of the four driving modes and activation of the RACE START function is located to the left of the E-SELECT shift lever.
The dual clutch transmission allows upshifts under full load both in automated and manual mode, where the gears can be shifted using the AMG shift paddles on the steering wheel. There is a choice of four driving modes for maximum driving pleasure and comfort: "C" (Controlled Efficiency), "S" (Sport), "S+" (Sport plus) and "M" (Manual). In Controlled Efficiency mode the high torque already available at low engine speeds is used to shift up to the highest viable gear. The resulting dethrottling effect on the intake side lowers fuel consumption, with accompanying benefits in terms of engine noise. Controlled Efficiency also means comfortable gearchanges and "soft" accelerator characteristics for relaxed cruising.
In "S" mode the transmission remains in each gear to a higher speed, downshifts are faster and gearshifts are performed around 20 percent more rapidly than in "C" mode. Switching to "S+" mode shortens gearshift times by a further 20 percent, while "M" is the most sporty mode of all: the AMG 6.3-litre V8 engine now responds with even more "bite", and the AMG dual clutch transmission changes the gears another 10 percent faster – a total reduction of 50 percent compared to "C" mode. In "M" mode the transmission performs gearchanges in less than 100 milliseconds. The upshift indicator integrated into the AMG instrument cluster informs the driver when a manual gearchange is advisable: seven LEDs coloured white, yellow and red indicate the necessary action. The faster the engine speed approaches the maximum of 7200 rpm, the more LEDs light up.
Best possible traction thanks to the RACE START function
The RACE START function delivers maximum dynamism: in order to use it, the driver has to activate the ESP® Sport function while the vehicle is at a standstill, and press the brake pedal with his left foot. Having preselected the RACE START program using the rotary switch, the driver is presented with a confirmation message in the AMG central display. After confirming the RACE START function by pulling the "Up" shift paddle once, all he has to do is depress the accelerator fully and take his foot off the brake. The optimum start-up engine speed is set fully automatically, and the SLS AMG accelerates away with flawless traction – all the way up to top speed if required. The driver does not need to shift the gears manually, as the DCT transmission performs lightning-fast gearchanges.
Automatic double-declutching function for even more emotional appeal
In the "S" (Sport), "S+" (Sport plus) and "M" (Manual) modes, the automatic double-declutching function is active. Every manual or automatic downshift is accompanied by precisely metered double-declutching – incrementally from "S" through "S+" to "M". This not only adds to the driver's emotional experience: the load-free downshift minimises load-change reactions, which pays dividends – particularly when braking into a bend on the racetrack – and also enhances safety in the wet or on ice.
The AMG dual clutch transmission excels with compact dimensions and a low weight of only 136 kilograms, including the differential - thanks to aluminium construction. The absence of a conventional torque converter improves efficiency and makes a significant reduction in fuel consumption possible. A mechanical, multi-plate differential lock configured for sporty performance at the physical limits is also integrated into the transmission housing.
Torque tube with carbon-fibre drive shaft
The torque tube turns the AMG SPEEDSHIFT DCT 7-speed sports transmission and the V8 engine into a fixed unit. The engine and the transmission installed in a transaxle arrangement at the rear axle are firmly connected with very low flexural and torsional flexibility, and support each other. This has decisive advantages in terms of handling dynamics and driving comfort, as this sophisticated solution makes it possible to eliminate much more free play from the entire powertrain. The 1.64-metre long torque tube consists of a one-piece sand-cast aluminium housing, and weighs less than 25 kilograms. A drive shaft rotates at engine speed inside the housing. As in the Mercedes-Benz C-Class DTM racing touring cars, the shaft is made from carbon-fibre. One of the key advantages of this high-tech material is that despite its high strength, the shaft tips the scales at only 4.7 kilograms. This means that the 1.71-metre long drive shaft, which is required to transfer no less than 650 newton metres of torque from the engine to the dual clutch transmission, is around 30 percent lighter than a steel version would be.
A specially developed torsional damper is also active within the torque tube. This reliably eliminates resonances and vibrations, thereby optimising ride and noise comfort.
* Official photos and details courtesy of Mercedes-AMG GmbH *
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