AMG reveals world’s most powerful four-cylinder

When it debuts in the new Mercedes-AMG A 45 hot hatch and CLA 45 sports sedan later this year, Affalterbach’s all-new M139 turbo four will outpower the M133 engine in the outgoing A 45 by up to 30kW.

More importantly, its outputs will eclipse those of high-performance four-cylinder engines like the 225kW/450Nm 2.0-litre in the upcoming BMW M135i xDrive, the 269kW/430Nm 2.5-litre boxer in the Porsche Cayman and the 257kW/440Nm 2.3-litre in the old Ford Focus RS (which makes up to 276kW/500Nm in Mountune form).

For further comparison, it even tops Audi Sport’s 294kW/480Nm 2.5-litre five-cylinder turbo engine in compact all-wheel drive competitors like the RS 3 and TT RS.

For the first time, AMG’s turbo four will be available in two states of tune – standard and S. Apart from the A 45 hatch and CLA 45 sedan, it will also power the GLA 45 crossover – but not the B-Class, upcoming GLB-Class SUV or A-Class sedan (which will remain topped by the A 35), all of which are based on Daimler's front/all-wheel drive MFA compact-car platform.

Australians will have access to all these models, except the CLA Shooting Brake wagon.

Matched exclusively with 4MATIC all-wheel drive and what's expected to be a new eight-speed dual-clutch automatic transmission, the highly under-square (long-stroke) transverse-mounted four-pot engine is basically all-new except for its 83.0x92.0mm bore x stroke dimensions.

In standard form, it outputs 285kW (387hp) at 6500rpm, 480Nm over 4750-5000rpm and revs to 7200rpm in standard form – up from 280Nm/475Nm in the M133 engine of the current 45 AMGs.

In S guise, the 1991cc donk thumps out 310kW (421hp) at 6750rpm (up from 6000rpm) and 500Nm over 5000-5250rpm (up significantly from 2250-5000rpm).

That gives it a record specific power output of 155kW (211hp) per litre, which is more than the 147kW/200hp figures achieved by many (non-turbo) 1000cc four-cylinder superbikes. If BMW M’s new 3.0-litre inline turbo six matched the M139's specific output, it would knock out 465kW instead of the still-sizeable 375kW seen in the new X3 M and X4 M.

Other noteworthy numbers include a massive 2.1 bar of turbo boost pressure, which equates to no less than 30psi in the old money and is up from 1.8 bar in the old M133, and a still relatively low 9.0:1 compression ratio – up from 8.6:1.

Mercedes-AMG hasn’t revealed the new engine’s 0-100km/h performance potential (because its first application hasn’t been formally announced) but insists its “carefully balanced torque curve” benefits from ‘torque shaping’ tech to offer linear delivery with little to no turbo lag and “immediate response”.

“With this configuration, the AMG engineers have achieved a power delivery similar to that of a naturally aspirated engine for an even more emotive driving experience,” said AMG in a press release. “The engine developers were also able to realise a dynamically increasing torque curve in the lower engine speed range, thereby improving agility.”

Given the extra power and torque, and the fact the outgoing A 45 could hit 100km/h in 4.2 seconds (claimed; close to it in the real world – especially in the wet – despite plenty of turbo lag), the next A 45 could be the first hot hatch to dip below the 4sec 0-100km/h mark.

Underlining its newness, the M139 has been flipped 180 degrees in its engine bay, with the exhaust manifold and turbocharger at the back of the engine against the firewall and the intake at the front.

Controlled by an electronic wastegate claimed to give more accurate response -- especially at partial throttle loads -- than the pneumatic unit it replaces, the all-new twin-scroll turbo adopts roller bearings like Mercedes-AMG’s 4.0-litre biturbo V8, reducing friction and allowing it to spin up to 169,000rpm even faster.

Combining direct and manifold injection for the first time, the 16-valve DOHC M139 employs CAMTRONIC valve timing adjustment for the exhaust camshaft and a dual-injection system including 200-bar piezo injectors and manifold solenoid valves.

Engine weight remains around the same at 160.5kg. Perhaps most importantly when it comes to durability, the M139 employs a chill-cast aluminium crankcase for the first time, allowing the closed-deck design withstand combustion pressures of up to 160 bar.

Within it is a forged steel crankshaft operating forged aluminium pistons with new piston rings within patented mirror-like Nanoslide cylinder coating technology, which was first seen in the AMG M156 and the Mercedes F1 engines and is claimed to be twice as hard as grey cast-iron liners.

There’s also an on-demand electric water pump, baffle plates designed to reduce oil sloshing in the sump, electronic alternator management, ECO idle-stop function with fast restarting, gliding function and, for some markets (but not Australia), a petrol particulate filter.

Finally, boreholes near the combustion chambers are designed to better cool the cylinder-head, which houses slightly repositioned fuel-injection nozzles that make way for 2mm-larger exhaust valves, improving engine breathing and turbo performance.

“We already set the benchmark in the segment with the preceding engine,” Mercedes-AMG chairman Tobias Moers said. “This fundamentally new four-cylinder presented us with the challenge of doing even better and we succeeded with a number of sometimes-revolutionary solutions.

“With the M139, we have once again impressively demonstrated the engine expertise of Mercedes-AMG. Not only is the output per litre unrivalled for a turbocharged engine, the high level of efficiency also demonstrates that the internal combustion engine still has further potential.”

Mercedes-AMG M139 vital stats:
Type: 2.0-litre four-cylinder turbo-petrol
Displacement: 1991cc
Bore x stroke: 83.0 x 92.0mm
Power: 310kW (421hp) at 6750rpm (S); 285kW (387hp) at 6500rpm (standard)
Torque: 500Nm at 5000-5250rpm (S); 480Nm at 4750-5000rpm (standard)
Max engine speed: 7200rpm
Compression ratio: 9.0:1
Turbocharger: Single twin-scroll with roller-bearing compressor and turbine wheels
Max charge pressure: 2.1 bar (S); 1.9 bar (standard)
Mixture formation: Third-generation combined direct and manifold injection