Mercedes-AMG's new M139 pumps 421 horsepower

I think it’s fair to say that the Mercedes A45 AMG packs the most powerful four-cylinder in a production car. The M133 in the noses of the A45, CLA45 and GLA45 (anything else?) parps up a massive amount of power to which all other aspire.

But being AMG and with a new A45 on the way to take over from the A35 as the top-of-the-A-range, there’s a new engine. Well, a heavily-updated one, anyway – it’s the M139.

Mercedes M139

The M139 is still a 2.0-litre turbo four-cylinder for installation in transverse-engined Mercs. With a fair few changes, the new engine spins up 285kW (387PS).

Big deal, you’re saying.

Yeah, that’s the “basic” version. The S version winds it up to 310kW (421PS). That is…a lot of power.

Torque for the basic comes out at a staggering 480Nm between 4750rpm and 5000rpm while the S has a nice round 500Nm between 5000rpm and 5250rpm. Not a wide torque-band, no, but hey. There’s a lot of it.

The new engine has done an about-face and put the exhaust manifold on the turbo on the firewall side of the engine. That means all the cooling is on the rather more sensible side, out the front where, you know, the cold air is. It also means that the intakes are shorter and there’s less complexity in the pipework.

The new turbo has roller bearings to reduce friction, spinning up to 169,000 more quickly. The new exhaust manifold is also tricksy, to smooth the flow of turbo-turning exhaust and keep the individual cylinders from upsetting each other.

There’s a ton of other detail changes, but they’re not that interesting. Yes, the “One man, one engine” handbuilt philosophy continues in the Affalterbach factory and I daresay you’ll be hearing one of these engines farting in back streets near you before mid 2020.