This is an absolute gem of an engine. It doesn’t seem long ago that the raucous punch of a Volkswagen/Audi TDI motor was an extraordinary achievement, and yet this twin-turbo masterpiece manages to outpace any rivals and offer class-leading refinement.
Choose any barometer of performance and the 123d impresses. It springs from rest to 60mph in just 6.7 seconds, and can dust the fourth gear 40-60mph increment in a paltry 4.1 seconds. In the real world, this is a genuinely fast car.
Factor in the outstanding fuel economy (you should be able to beat 50mpg) along with a minimal environmental impact (just 138g/km of CO2), and it’s easy to see this as the 1-series coupé engine of choice.
Still, the reality is that even using the latest technology it hasn’t got the thrilling rev range of a performance petrol engine, so it’s essential to use the slick-shifting six-speed manual ’box to keep the engine spinning where it works best.
If you’ve ever driven a 1-series hatch on low-profile rubber, you’ll appreciate how well the 123d rides. With the optional M Sport lowered suspension of our test car it translates every camber and twitch in road surface.
But with the jarring slap of run-flat tyres over sharp ridges and potholes that makes a 130i so wearisome wiped from the repertoire, the 123d becomes a capable mini-GT.
With a shorter wheelbase than a 3-series the 123d has a nimble, slightly edgy response to steering inputs. It has a voracious appetite for changing direction and in this sense feels more alert than the 135i.
The downside is that it lacks the poise and rhythm of a 3-series and it’s all too easy to find yourself tackling a sequence of curves with a series of aggressive stabs at the wheel.
For everyday driving it has an ideal level of assistance, but when you want to drive the 123d with conviction you’ll find yourself relying on the considerable grip levels generated to provide the thrills.