Not so long ago the words “5.5-litre V8” and “fuel economy” wouldn’t have appeared in the same sentence but with large doses of Formula 1 tech and fuel-injection magic Mercedes has managed it in the just-launched two-seater SLK 55 AMG.And not a turbocharger in sight. Image gallery.AMG, the high-performance arm of Mercedes-Benz, has created what it claims is “an exclusive combination of advanced technologies” that combines the engine’s ability to run on four of its eight cylinders when not under full load, a high compression ratio of 12.6:1, a fuel shut-off when stationary and the highest fuel-injection pressure of any petrol engine (200 bar) to achieve an average 8.4 litres/100km.Exhaust emissions average 195g/km.'THE TOP PERFORMER'They’re all history-making figures given the V8’s ability to produce 310kW/540Nm and a 0-100km/h time of 4.6 seconds; the previous car made “only” 265kW/510Nm. The SLK 55 AMG is, quite simply, the most powerful production SLK yet created.Mercedes says the car is “the top performer” in terms of fuel-efficiency versus output and should lay to rest for ever the archaic term “gas-guzzler” when talking about high-performance sports cars. Indeed, the automaker believes the SLK 55 AMG “will meet the highest expectations of sports-car lovers”.The cylinder shut-off system in particular, newly developed by Mercedes-AMG, disables cylinders two, three, five and eight at less than 3600rpm; it’s the same system as used in 560kW F1 cars to conserve fuel during races, for example through slow corners, under pace-car supervision and during pit-stops.Cylinder disabling will, however, only occur if the driver selects transmission mode "C" (for Controlled Efficiency). The instrument cluster reports when shut-off is active but, even when it is, 230Nm of torque still lurks beneath the driver’s right foot.FAST CHANGERMercedes says the V8 comes in at or below the fuel-consumption of its competitors but asserts than none can match either the output or torque of the SLK 55 AMG – and above all, they do not use large-displacement, eight-cylinder engines.The change from four to eight-cylinder power is not perceptible even though all 16 inlet and exhaust valves are closed by 16 hydraulically compensating elements and a complex oil supply system in the cylinder head. The switch, Mercedes says, takes only 30 milliseconds with the engine running at up to 7600rpm.These are the engine specs:V8 quad-valve displacing 5461cc.Bore x stroke: 98x90.5mmRated output: 310kW at 6800rpmRated torque: 540Nm at 4500rpmMaximum engine speed: 7000rpmCompression ratio: 12.6:1Fuel supply: microprocessor controlled direct injection.All of which is the tech stuff; what counts is how the car goes and that, frankly, is devastating - as a 200km pre-lunch run on some empty roads around the Western Cape proved.The car is astounding in the way it delivers acceleration, braking and cornering. The power is delivered through a seven-speed speed-shift transmission with full auto or manual sequential changes, the latter through F1-style paddles on the sport steering wheel. The changes executed through an automatic double-declutch system that simultaneously activates flaps in the quad exhaust to deliver a soundtrack that makes the high-end audio redundant.Spend another R35 000 over the R975 000 base price and AMG will throw in a performance package with a stiffer suspension, AMG rear differential lock, front composite brake discs and a three-spoked AMG Performance steering wheel finished in nappa leather with Alcantara inserts in the grip areas.R25 000 more will add a “driver’s package” to raise the top speed from 250 to a howling 280; bragging rights are cool, a weekend in jail is not. Standard with the car are distinctive AMG alloy rims, AMG body-styling with a rear boot-lid spoiler and two pairs of plated tailpipes.MEANT FOR THE TRACKThe auto transmission has three modes - Controlled Efficiency (C), Sport (S) and Manual (M) and road grip is enhanced with “torque vectoring brakes” (the relevant rear wheel is braked in extremis to keep the tail tucked in under hard cornering).Braver drivers can invoke the ESP (electronic stability programme) in three modes: "ESP ON", "SPORT Handling Mode" and "ESP OFF" – the last meant for the track with an expert at the wheel. Do not, as they say, try this at home...High-performance bi-xenon headlights with their own cleaning system and a new grille design lead the way into a long bonnet with the cabin set well back along the body. The cabin ambience is pure signature AMG with the centre console finished in brushed aluminium and the sports seats finished in Nappa leather with AMG badges.Mercedes’ Airscarf neck-warmers add cool-morning comfort when the roof is down; up, the car becomes a coupe. Options here are a panoramic vario-roof in tinted polycarbonate or (for the first time) a glass roof that can switch from dark to transparent electrically.Let the power of the sun be your guide...MAINTENANCE PLANOvercoming the SLK’s inbuilt stability will take some doing but, should you manage it, the car comes with a new, fibre-reinforced roll-over bar, two-stage driver and front passenger airbags, headbags, thorax airbags, seat belt tensioner and belt-pressure force limiters.As with all Mercedes-Benz cars, the SLK 55 AMG comes standard with a six-year or 120 000km maintenance plan.It also comes with a classy cachet, delivers a supreme driving experience, adds upper-crust luxury to the cut-and-thrust of sheer performance and (at least compared to other sports cars) helps to save the world from asphyxiation.Good on yer, Merc. Find out more about the SLK range.