The turbocharged 2.0-litre unit uses many of the technical solutions already developed for Mercedes-AMG’s twin-turbocharged 4.0-litre V8 petrol engine and it will be offered in two distinct power outputs. In the new standard A45 4Matic+ and CLA45 4Matic+, the M139 develops 387bhp at 6500rpm and 354lb ft of torque between 4750rpm and 5000rpm. That’s 85bhp and 59lb ft more than the A35 and CLA35.
In its highest state of tune, with 2.1 bar of turbocharger boost pressure and water injection for its twin-scroll turbocharger, the M139 provides the A45 S 4Matic+ and CLA45 S 4Matic+ with a class-leading 415bhp at 6750rpm and 369lb ft between 5000rpm and 5250rpm.
This marks the new AMG motor out as the world’s most powerful four-cylinder series-production engine. It also represents an increase of 39bhp and 18lb ft on the outgoing M133 engine of the previous A45.
Despite giving away 28bhp and 15lb ft to the A45 S 4Matic+, the standard A45 4Matic is claimed to be just 0.1sec slower to 62mph, at 4.0sec, and its top speed is limited to 155mph.
Both new A45 4Matic+ models have the same combined cycle fuel consumption and CO2 emissions, at 34.0mpg and 189g/km. The new AMG engine comes mated to a standard eight-speed dual-clutch gearbox and a newly developed multi-plate clutch 4Matic+ four-wheel drive system. This features an AMG torque control function that separates electronically controlled clutches within the rear axle to vary the amount of drive sent to each individual rear wheel.
As a result, they can be configured to send the majority of drive to the rear wheels via a new Drift mode function, which is included in an optional AMG Dynamic Plus package, when the car’s stability control is deactivated and the gearbox is set to manual mode. Overall, the new driveline offers up to six driving modes: Comfort, Sport, Sport+, Slippery, Individual and Race – the last of which is standard in range-topping S models and optional as part of the AMG Dynamic Plus package in standard models.
Further changes made to the standard A45 include a new performance brake package. This features 350mm front and 330mm rear discs in combination with four-pot and one-pot calipers respectively in the standard A45 S 4Matic+. The A45 S 4Matic+ Plus gets larger, 360mm front and 330mm rear discs, with red painted six-pot front and one-pot rear calipers.
AMG has also introduced a re-engineered steering rack with a variable ratio and two different software maps depending on the driving mode. The steering rack itself is mounted on a new carrier, which is claimed to increase precision.