Inside AMG’s outrageous M139 2.0-litre four
415bhp from a 2.0-litre four? How the hell did AMG manage that? By Ben Miller
Try explaining to AMG’s CEO Tobias Moers that an engine’s peak power output is ‘just a number’ – the man can barely contain his pride in the fact that AMG’s new M139 motor is, at 415bhp, the most powerful turbo four-cylinder engine in production. What’s more, the competition aren’t even close. The four in BMW’s 135i taps out at 302bhp. Honda’s Civic Type R engine manages 316bhp. Okay, so both those cars undercut the AMG A45 S’s punchy list price (£50,570) but the truth is that with the same basic set of tools – four cylinders, a 2.0-litre displacement and a turbo – AMG’s engine is some 30 per cent (or 50bhp per litre) more powerful.
So, how has AMG done it? Key to the M139’s eciency and potency is its orientation – described by Moers as ‘a fundamentally new’ unit, the four is rotated 180° relative to its predecessor, so that the exhaust manifold and turbo now sit at the back of the engine, nestled up against the firewall, and the intake system sits at the front of the engine. The advantages? Far shorter, more direct and more ecient plumbing for both intake and exhaust, together with a lower bonnet for less drag.
But power is nothing without control, and AMG’s worked hard to prevent the M139 being a laggy, unresponsive hand grenade of an engine. The fat twin-scroll turbo’s turbine is fed by two independent ducts from the manifold, so that the stream of turbocharging pressure is more consistent across the engine’s firing order, boosting torque and the engine’s ability to respond to your right foot’s bidding. To that end, the M139’s turbo also spins on expensive but low-friction roller bearings – a detail borrowed from AMG’s V8s.
Boosting power and durability further are a heavy-duty cooling system (the turbo is air-, water- and oil-cooled), a superrigid, race-inspired crankcase design and forged aluminium pistons running in bores coated with an F1-developed lowfriction coating. And of course, like its V8s, each M139 four is lovingly assembled by a single AMG technician.