The switch from M135i to M140i was one of the more substantial mid-life facelifts of recent times, but you could be forgiven if it passed you by.
On paper the 3.0-litre turbo six swelled from 240kW/450Nm to 250kW/500Nm, which shaved 0.2sec from the claimed 0-100km/h time (now 4.6sec for the auto, 4.8sec for the manual) and allowed the badge to be changed to give the marketing team something to talk about.
In reality, the changes run much deeper for the M140i uses a completely new engine. The B58 replaces the N55, which has served duty since 2009 in everything from the M135i to the 740i to the X5.
While both are nominally three litres in capacity, the B58 is slightly larger at 2998cc versus 2979cc and is part of BMW’s new modular engine family, which includes the B38 1.5-litre turbo triple and B48 2.0-litre four. Furthermore, the block is identical between B58 petrol and B57 diesel engines; this reduces production complexity and saves BMW money.
The all-aluminium crankcase is stronger thanks to its closed-deck design and the cylinder linings use an arc-sprayed coating to save weight and reduce friction. Emissions and fuel consumption are also further reduced thanks to a very short warm-up phase and heat encapsulation, which allows the engine to retain heat for up to 36 hours and therefore endure fewer cold starts.
On the performance side the twin-scroll turbocharger is bigger and integrated into the exhaust manifold to improve response (the exhaust gases only have to travel a very short distance) while the new water-to-air intercooler is integrated into the intake plenum.
According to various BMW blogs this is going to make life very difficult for tuners – you can’t just whack a bigger intercooler on, for instance – but as Dr Ian Malcolm said in Jurassic Park, “life finds a way”. On initial acquaintance the B58 feels a little too smooth and refined – like a sickly sweet dessert, too much of a good thing – but there is some aggro to be found.
It has an extraordinary powerband; maximum torque is delivered from just 1520rpm, yet meaningful power continues to be produced all the way to the 7000rpm redline. It proves the benefits of less boost/more capacity compared to rivals like the Audi RS3 and Mercedes-AMG A45 which offer even more power but not the same bandwidth.
In a standard M140i the extra 50Nm torque produced by the B58 is almost overkill; the standard open diff can sometimes struggle to apportion so much grunt on anything but a perfect surface. That said, the broad spread of power makes it a breeze to performance test as even just walking it off the line results in meaningful acceleration.
At Bang For Your Bucks this year a standard automatic M140i more or less matched its claim by hitting 100km/h 4.65sec and continued on to a 12.76sec quarter mile at 180.53km/h. But our manual long-termer managed to beat this, clocking 4.58sec to 100km/h at Heathcote on the Pirelli P Zero runflats.
Once rolling the auto’s shorter gearing and instant shifts gave it the slight upper hand, the manual managing a 12.82sec quarter mile at 179.30km/h. It wasn’t even particularly difficult. With a cleanish strip and the LSD ensuring both wheels are turning equally it was merely a case of getting the car moving with around 2000rpm dialled up and then changing gear as quickly as possible.
Too many revs would result in excessive wheelspin so there’s the possibility of a faster time on a cleaner surface, though probably only a tenth or so. On the road this translates into massive performance available at virtually all times.
Want to see more of our BMW M140i long-termer?
It might sound like an odd comparison, but the M140i’s big engine/small car characteristics give it a distinct muscle-car vibe – the B58 definitely feels like the defining feature of the car.
About the only aspect that grates is the contrived, almost apologetic nature of the pops and bangs on the overrun, which feel at odds with the otherwise silky smooth nature of the boosted inline six. Nobody’s perfect.
Fuel this month: 11.63l/100km
Distance this month: 1042km total 2151km
Like: Performance to burn
Dislike: Contrived pops and burbles
Favourite moment: Besting BMW’s 0-100km/h claim