Audi's numbering system makes no sense at all!. I mean OK, what the numbers mean has now been explained, but otherwise i'm no clearer as to the reasoning behind it. Do we really need a naming system to indicate how much power the car has?, presumably the buyer knows, so it is either to shame buyers of lower powered cars into buying more expensive and powerful variants, or to enable buyers of more those more powerful versions to show off!. I don't know if is still the case, but you used to be able to specify the car (BMW and Mercedes, not sure about Audi) 'de-badged' from new. Very unlikely to happen i know, but If i was buying one, that is what i'd do.

BMW (2-litre 330 for example) and Mercedes (1.5-litre C200) are no better. With both makes, the numbers used to denote capacity, but no longer: what the purpose of the change was I have no idea.

As for the BMW and Merc current system, i think it is probably because folk are used to exactly what you say, that of the number being for the engine capacity. Someone buying a 330 knows they are getting something with a big meaty engine, pretty near the top of the range for power. But with down sizing, a smaller engine can easily equal the power (and exceed the torque) of a large n/a unit. However, if they badged a 330 as a 320, uninformed buyers would probably be put off thinking they were getting a lesser car!.

– matching the acceleration of free fall…

That's the depreciation.

A neighbour got a new A2 with a 30 badge. When I suggested that with 3 litres it should go well he told me the engine was far smaller and he had no idea what it stood for. Looking at it if you square the designation and device by 10 you get near the bhp. Perverse to say the least!

I see the TDi badge has disappeared from all VAG cars. I wonder why?

No, I don't think that's correct.

Just this morning I saw a new shape 19 reg Audi Q3 with 40 TDi on the back. This is the 178BHP 2.0 diesel.