BMW doesn't make a 323d. Too many diesels in the line-up already, the story goes - too expensive and risky to try and squeeze another engine into the near-invisible gap between the 320d and the 325d. So, if you want BMW's lovely bi-turbo four-pot in your life, you'll have to make do with the 123d.
Unless, that is, you go to Alpina, who will whip you up a 3-Series with the 2.0-litre diesel out of the 123d, with a revised engine management and a new exhaust system. This is good.
We like the stats too. 214bhp. 0-62mph in 6.9 seconds. 152mph top speed. Yes, that's Golf GTI-beating performance from a car that'll return 52mpg. That's nearly 150bhp per tonne. From a diesel. Simply astonishing.
The D3 doesn't feel like a quick diesel though. In fact, it doesn't feel like a diesel at all. Despite a massive 331lb ft of torque, there's never a genuine gut-punch from the turbos. Instead, the rev needle skips to the red line with petrol-like vim, a fizzy responsiveness that should convert even the staunchest diesel-haters. Leaving aside such freakshows as Audi's giant V12, this might just be the greatest diesel in the world.
The D3 rides better than any current BMW too. Alpina has ditched the 3's hard-riding run-flat tyres - though hasn't added a spare wheel, leaving you instead with a handy can of foam - and fiddled slightly with the suspension set-up, and the result is a beautifully judged ride over any surface.
In fact, it's good enough to make you wonder why Alpina didn't go a bit further with the D3. The new spoilers, steering wheel and seats, the engine and handling, put the car right at the sporty end of diesel, and make the unchanged gearbox and steering feel a little lax by comparison. Yes, revising them would have pushed the price up, but the D3 is being imported in such small numbers - the UK'll get under 100 each year - that interested punters would surely have paid the premium.
Which raises the bigger question: should BMW make its own 323d? It'd clearly create a bit of a headache for the marketing team, but we'd then invoke TG's Altruistic Principle: is it morally acceptable to have an engine this good in just one car?