Mercedes-Benz comes out fighting with the 355bhp A 45 AMG (w/VIDEO)

It is Mercedes-AMG’s first foray into the ‘compact’ class, and they’ve gone straight to the top by fitting the world’s most powerful four-cylinder engine currently in production.

With 355bhp, 332 lb-ft (450 Nm) of torque and a specific ‘power density’ of 177bhp per litre, the AMG 2.0-litre four-cylinder turbo engine packs a bigger punch than Audi’s similarly configured S3 (296 bhp) and BMW’s turbocharged six-cylinder M135i (315 bhp).

But Mercedes hasn’t just whacked up the boost on one of its everyday four-cylinder engines. This is an engine which is hand-built according to the traditional AMG philosophy of “one man, one engine”, bearing an AMG engine plate with the signature of the fitter responsible.

It’s also an efficient engine – complying with the 2017 EU6 emissions standard and returning a stunning 40.9 mpg and 161 g/km of CO2 emissions.

Such capabilities would be impressive in a one-off special, but this is a mass-produced engine, one designed to survive in the custody of drivers who never check the oil, drive a little too enthusiastically when cold and fill its tank with cheap petrol when they can find it. While Mitsubishi FQ400 owners might have enjoyed more power, the efficiency and reliability of the A45 AMG’s four-cylinder powerplant is without peer in its class.

The A45 AMG’s engine revs to 6,700 rpm and accelerates from zero to 62 mph in 4.6 seconds. Top speed is an electronically limited 155mph and thanks to AMG’s 4MATIC all-wheel system, its driver can unleash its performance in almost any weather condition.

In fact, that’s the only ‘fly-in-the-ointment’ that stands out in an otherwise gold-star performance. The A45 AMG’s all-wheel drive system switches to front-wheel drive in normal conditions, and when the traction and dynamics require it splits the torque between front and rear axles ‘up to’ a ratio of 50:50.

Mercedes say the variables influencing the power distribution ratio are vehicle speed, lateral and longitudinal acceleration, steering angle, speed difference between the individual wheels, selected gear and accelerator position, but that still leaves the impression that any power oversteer antics will be off the menu.

Mercedes-AMG has developed a compact power take-off unit (PTU) for delivering torque to the rear axle. They’ve developed a novel oil circuit which is fully integrated into the seven-speed DCT sports transmission, and saves around 25kg over conventional all-wheel drive systems.

An electro-hydraulically controlled multi-disc clutch integrated with the rear axle differential detects slip at the front axle, then instantaneously presses the discs together and channels the torque to the rear. To ensure a fast response, the hydraulic pump remains permanently engaged.

Another innovative feature on the A45 AMG is the AMG Speedshift DCT seven-speed sports transmission. Flange-mounted on the transversely installed AMG 2.0-litre four-cylinder turbo engine, the system uses software adopted directly from the SLS AMG with double-declutching and race-start functions.

The AMG DCT sports transmission boasts spontaneous gear shifting without any interruption in tractive power, making for near instantaneous gear changes. To aid such determined driving it features a unique “Momentary M mode”, in C and S mode, this enables the driver to activate manual mode (“M”) without having to remove a hand from the steering wheel, by pressing the “up” or “down” shift paddle once.

With the gearbox in Momentary M mode, the driver can shift manually in all the automatic driving programmes without control being returned to full auto (“D”). Mercedes say the shift times in manual mode “M” and in Sport mode “S” are as sporty in character as those of the SLS AMG GT, which should be fun finding out in due time.

A third transmission mode, “C” (Controlled Efficiency) offers tangibly softer transmission and engine characteristics for a more fuel-efficient and comfort-oriented driving style.

As you’d expect of an AMG product, there are plenty of suspension settings to play with. Mechanically, the A45 AMG features three-link front suspension with stiffer steering knuckles and totally new elastokinematics. More rigid bearings are employed in the lower link plane leading to higher camber stiffness and enabling higher cornering speeds.

A four-link rear axle is optimised as per the front, with more rigid bearings and the subframe is now rigidly connected to the body. As you’d expect, tuned spring/damper units and larger stabilisers provides for high lateral acceleration and reduced body roll in fast corners.

Mercedes has thrown all the usual electronic acronyms at the A45 AMG, with a three-stage ESP system featuring Curve Dynamic Assist. These are integrated with the control modules in the 4MATIC all-wheel drive system, so choosing “ESP SPORT Handling” mode results in more torque biased to the rear axle and a more responsive driving style.

One more piece of AMG-tech worth mentioning. To keep the A45 AMG’s potential under wraps around town (or when cruising on a motorway) an exhaust flap continuously adjusts the flow of gases to ensure the most peaceful sound. When the throttle is opened, the twin AMG exhausts release a full and fascinating sound which befits the AMG name, while during downshifts and when upshifting under full load, there’s throaty sound which adds to the fun.

While we’ve become used to the anti-lag systems of turbocharged four-cylinder cars in WRC, Mercedes-AMG has developed its own solution to the challenge of small-capacity turbo lag. By manipulating the exhaust flap restriction and fuel injection mapping, the system makes use the exhaust gas back pressure, exhaust gas temperature and exhaust gas impulse. A swifter build-up of torque right from the lower rev range is said to eliminate turbo lag and improve fuel consumption and exhaust emissions.

For AMG fans there’s a whole toy-box of details to pore over in the A45 AMG’s spec, both inside and out. A selection of which include:

• AMG Aerodynamics package: AMG rear aerofoil, additional flics in front apron, larger front splitter
• AMG carbon-fibre exterior mirror housing
• AMG Exterior Carbon-Fibre package: front splitter, side sill panel inserts and trim on rear apron in genuine carbon fibre
• AMG rear aerofoil
• AMG Exclusive package
• AMG multi-spoke light-alloy wheels fitted with 235/35 R 19 tyres, in two variants: painted in titanium grey with high-sheen finish or in matt black (235/40 R 18 tyres are standard)
• AMG Night package: privacy glass, black anodised waistline trim strip, black tailpipe trims in chromed finish, radiator fins in silver chrome, high-gloss black painted finish for front splitter, exterior mirrors, side sill panel inserts and rear apron trim
• AMG Performance exhaust system incl. tailpipe trims in specific design
• AMG Performance suspension with tauter spring/damper tuning
• AMG Performance steering wheel and E-SELECT lever, electronic key with AMG emblem
• AMG Performance seats
• AMG wheel bolt covers in black with hub caps in central-locking look (for standard AMG light-alloy wheels only)
• Red painted callipers

You get the picture, and no doubt you’ll be able to spec a A45 AMG up to stupid levels, but at a rumoured £36,000 when it goes in sale this summer it could well be one of the cars of 2013 (provided it can get a look-in past the McLaren P1, Ferrari F150, Alfa 4C…).

For US customers wondering whether the A45 AMG is headed stateside, look out for the same configuration in the new CLA 45 AMG. Hatchbacks are apparently not your thing (don’t blame us, speak to Mercedes), so your compact class AMG will come in sedan form.