SLR McLaren Powertrain Details
Engine and transmission: A V8 powerplant with motor racing in its genes
An abundance of power and high-tech features from the world of motorsport - these are the defining characteristics of the V8 engine in the Mercedes-Benz SLR McLaren Roadster, with its 5.5-litre displacement, a cylinder angle of 90 degrees, screw-type supercharger and a crankshaft supported by five bearings. Which all means that the V8 power unit developed by Mercedes-AMG fits perfectly into the high-performance concept of the new open-top Gran Turismo.
The key data at a glance:
From a mere 1500 rpm, the SLR powerplant delivers in excess of 440 lb-ft of torque, rising to over 515 by 2000 rpm. The maximum of 575 is available from 3250 rpm and remains constant over a broad rev range up to 5000 rpm. The sublime torque delivery is coupled to the agile responsiveness of a sporty engine whose high-performance character is never in any doubt: a peak output of 626 HP says it all. The SLR is thereby endowed with what is currently one of the most powerful engines to be found in a series-produced sports car, and achieves class-beating performance figures as a result.
Power from screw-type supercharger
To ensure good cylinder charging, the engine has a belt-driven supercharger between the two banks of cylinders featuring two screw-shaped aluminum rotors that are Teflon-coated to keep friction losses to a minimum. Its innovative technology allows a significantly higher charge pressure than conventional belt-driven chargers because the two rotors can attain a top speed of around 23,000 revs per minute, forcing air into the intake system of the 5.5-litre powerplant at a maximum overpressure of 0.9 bar. This means that they compress around 1850 kilograms of air into the eight combustion chambers every hour - which is as much as 30 per cent more than the figure achieved by rival charging systems.
In order to maintain the greatest possible efficiency at all times, the AMG engineers created an intelligent engine management system which regulates the operation of the screw-type supercharger according to engine speed and load. Consequently, the charger is only called into action when it is really needed. Nevertheless, the system ensures that maximum power is available the instant it is summoned by a squeeze of the accelerator. This prompts the electronics of the engine management system to trigger an electromagnetic coupling, which immediately activates the supercharger that is driven by a separate poly-V-belt. Because the charger delivers its output in fractions of a second, even the most perceptive driver will not notice the non-supercharged phases. The charger system's air recirculation flap is opened under partial throttle conditions to help reduce fuel consumption.
Two intercoolers with separate water circuit
In addition to the supercharger, the electronics also monitor all other variables relevant to the engine - including both the powertrain management for the pedal-sensitive drive characteristics and the implementation of spontaneous interventions by the transmission or ESP®. The electronics also ensure optimum control of the water circuit for the intercooling system, as efficient intercooling is essential for maximizing output. This is because cold air is denser than warm air and therefore contains significantly more oxygen for combustion. In the Mercedes-Benz SLR McLaren Roadster's V8 engine, two separate intercoolers are responsible for this key task - one for each bank of cylinders. This ensures that loss of pressure is very low.
The engine-mounted intercoolers operate on the highly efficient principle of an air-to-water heat exchanger: after being compressed and hence heated by the supercharger, the air is cooled down via the system's own, separate water circuit - making the process independent of the ambient temperature. This enables the V8 to deliver its maximum output and torque spontaneously at any time.
However, the tremendous output of the 8-cylinder engine in the new SLR Roadster not only demands effective cooling of the combustion air, it also results in an overall increase in engine cooling requirements. The engineers made allowance for this by incorporating generously proportioned cooling air inlets and outlets as well as a powerful 850-Watt suction fan.
Crankcase with dry-sump lubrication
The Mercedes-AMG GmbH engineers also applied their extensive experience in the world of motorsport and in the design of high-performance engines to other details of the SLR powerplant. The entire engine housing, for example, and the enclosed bottom section of the crankcase are cast in aluminum. Each crankshaft is finely balanced and is supported by five bearings made from a high-endurance material for lasting transfer of the immense forces generated by the supercharged engine.
The pistons are forged and are therefore only manufactured in very small numbers. Like the forged lightweight connecting rods, they are precision gauged and weighed then allocated to the individual engines in such a way as to produce minimal mass balancing tolerances. The pistons slide in extremely durable, wear-resistant and friction-optimized barrels made from a special compound which is otherwise only used in motor racing. Dual oil injection ensures efficient piston cooling.
The design of the powerplant's oil cooling system also draws on the experience gathered at the race track: a sophisticated dry-sump lubrication system with an oil capacity of around eleven liters combines with a five-speed oil suction pump and a two-speed oil pressure pump to maintain reliable lubrication under the full spectrum of driving conditions. An important secondary effect of the dry-sump lubrication technology, which is usually only found in racing cars, is that it reduces the engine's overall height, allowing it to be installed lower down and producing a low centre of gravity that favors dynamic handling.
In line with the customary practice at Mercedes-AMG, each SLR engine is manufactured by hand. In order to attain the very highest quality, the principle of 'One man, one engine' is applied. This means that each powerplant is the responsibility of one AMG engineer, who carries out the entire engine assembly process – from installing the crankshaft in the engine block and assembling the camshafts and the supercharger, right through to the cabling.
Powerful high-pressure pumps in the tanks
The SLR's high-performance engine draws its fuel from two interconnected aluminum tanks. In order to keep the centre of gravity as low as possible in the interests of exceptional handling dynamics, the tanks are installed low down on the left and right in front of the rear axle. The tanks, which have a total capacity of 97 liters (of which twelve liters form the reserve), are equipped with two integral high-pressure fuel pumps. These are controlled by the engine management system and ensure that the fuel supply matches the engine speed and load. Here once again, a highly effective solution was developed that is specific to the SLR: one pump operates constantly while the second only cuts in as required.
Four metal catalytic converters for efficient emissions control
Secondary air injection and dual ignition give the engine the prerequisites required for low exhaust emissions. A sophisticated exhaust system complements these measures: the cascade-design two-pipe system consists on both sides of a close-coupled firewall catalytic converter plus, in the same housing, a main catalytic converter with a special precious-metal coating. The state-of-the-art metal design allows extremely thin walls, resulting in a very low exhaust backpressure. Thanks to its cutting-edge engine management and emission control technology, the V8 engine in the new SLR meets the stringent EU4 emissions limits as well as the current limits.
On each side of the vehicle, the catalytic converter housing opens into a rear silencer which flows into two stainless-steel tailpipes with a diameter of 60 millimeters just behind the front wheels. These side pipes - a further reminder of the 1950s SLR racers - pave the way for a smooth-surfaced underbody, which is instrumental in giving the new Mercedes-Benz SLR McLaren Roadster its first-rate aerodynamic properties. The silencers are folded several times into precisely calculated acoustic sections which produce the Roadster's distinctive thrilling engine sound. The volume of each silencer is 19.6 liters.
Manual transmission program for shift times worthy of a racing car
Likewise designed for high performance, the five-speed automatic transmission was developed by Mercedes-Benz and has been used successfully in several exceptionally powerful models. It has been specially optimized for very high torque and also offers the driver the option of choosing between different shift characteristics. The transmission transmits the engine's power via a finely balanced aluminum and steel powertrain to the differential and the rear axle.
It includes the Speedshift system that was developed by Mercedes-AMG for particularly sporty motoring and also offers a wide range of functions in the new SLR Roadster. Drivers can decide, for example, whether to leave the gearshift work to the automatic transmission or to shift gears manually. They are also able to select the shift speed, which determines how sporty the gearshifts are. A rotary switch in the centre console offers a choice of three program settings: "Manual", "Comfort" and "Sport". The activated program is indicated in the instrument cluster display by the letters "M", "C" or "S".
"Comfort" and "Sport" are automatic shift programs which, as the names indicate, place the emphasis either on comfort or on a more sporty drive. In "Manual" mode, on the other hand, the driver can select the five gears either using the shift paddles on the steering wheel or the selector lever's Touchshift function. When "Manual" is selected, there is also the option of choosing between three shift-speed levels for fine-tuning a sporty driving style:
The higher the level, the shorter the shift and response times inside the automatic transmission, and hence the shorter the gearshifts themselves.