Certain things in life are sacred, we know, but sometimes, just occasionally, institutions change and move on – rather than wither and die of stagnation.

I’m not talking here merely about the publishing industry, but also the BMW M5, which for aeons has been powered by a large atmospheric engine that sounds fantastic, revs to the nines and provides enough performance to turn your hair grey.

Until now, that is, because as from September a new M5 will be introduced that boasts, as we well know, a twin-turbo V8 beneath its bonnet. If you ask BMW very nicely indeed and hand over a few more pesos, they’ll even fit yours with four-wheel-drive.

So the world has changed, and the M5 – thank heavens – has changed with it. But why, though, hasn’t BMW gone one further and done away with a petrol engine altogether? Why not go straight to the next stage and fit the new M5 with a stonkingly torquey, pleasingly powerful yet astonishingly clean (read economical) twin-turbodiesel engine, and sod the fact that it won’t sound right or rev particularly well. Why even bother with a twin-turbo petrol engine in the interim?

Here’s why, according to BMW. Or, to be more specific, according to Carsten Pries, head of product management for M Automobile, to whom I put the dreaded question last week.