Oh, and think diesel too. And meet the BMW 535d, on sale in left-hand drive Euro markets in September, the UK shortly after that, and fully Euro 4'd so it can have two fat, straight-out tailpipes and no tax penalty. Yes, the march of the megadiesel takes another long-striding step (would you believe 70mph represents just 2200rpm?), and the results are intriguing.

You are assuming that this is a 3.5-litre engine. Really? Don't be ridiculous: model names, be they BMW's or Mercedes-Benz's or Saab's, have long ceased to be reliable indicators of engine size, even though they purport to be exactly that. Marketing has killed the logic, buyers have been too dim to fight back. So, the 535d is actually a 3-litre, like a 530d. Which also means it's a straight-six.

But doesn't BMW also have a V8 turbodiesel, used in lhd 7-Series? It does, but it's a bit on the heavy side. And that would spoil the all-important weight distribution. Far better, BMW reasoned, to get more out the 530d's engine, especially as it could be done with a mere 14kg weight increase. Power is 272bhp (up from the 530d's 218bhp) and torque is 413lb ft (up from 369) with 391lb ft on offer from just 1500rpm. From a standstill to 62mph takes a meagre 6.6 seconds despite the standard-fit, six-speed automatic transmission, and the maximum speed is capped to 155mph.

That torque is stupendous, enough to have helped a silhouette, carbonfibre-bodied, 535d-engined X5 to fourth place overall in the Paris-Dakar rally (it won the diesel class, of course). But think about the power for a moment. It's more than BMW's new 3-litre petrol engine produces, the 258bhp unit about to slot into the 6-series. World domination by diesel is not such a far-fetched idea.

How, then, does it work? The key point is that two sequential turbochargers (Mazda used a similar idea in the final RX-7) flow better than a single variable-geometry turbo, whose adjustable orifice ensures high gas speeds (and hence high turbo speeds and decent boost) at low engine revs but whose variable vanes muddy ultimate gas-flow. So there's a small turbocharger placed near the cylinder head, where it can react quickly to the flow of exhaust gases, and a larger one further away. A system of valves diverts airflow via one or both turbos according to engine speed.

Now pay attention; I'll be asking questions later. At low engine speeds, exhaust gases spin both turbos but only the small one spins fast enough to compress the intake air. As the engine revs rise, a valve diverts some of the exhaust gas directly to the large turbo, which now spins fast enough to pre-compress the intake air. The small turbo compresses it further.

With revs continuing to climb, that diverter valve is now fully open and the small turbo is out of action. Another diverter valve now lets the intake air, thoroughly compressed by the now rapidly-spinning large turbo, to head straight for the intercooler instead of being caught up in the small turbo. And finally, as the rev ceiling approaches (now 5000rpm in the lower gears), yet another valve opens to let some gas bypass the turbo. This valve also regulates total boost pressure so there's no need for a conventional wastegate.

Other changes over the 530d include a compression ratio reduced half a unit to 16.5 to one, low in diesel terms but better for smoothness and reduced NOx emissions (below 14 to one, however, a diesel becomes very difficult to start.) The extra boost pressure more than makes up for the lower compression ratio when required.

One problem with extracting huge power from diesels is that they can run more roughly. Not this one, though; it idles quietly with an even ticking, and is as creamy as a BMW straight-six should be as it turbo-boosts its way off the line. That boost builds up almost instantly, and is ready to go again after every gearchange so there's almost no pause-and-whoosh effect. The acceleration is just one long, continuous and rather steep curve, best plotted by leaving the transmission to its own automatic devices. That said, the Steptronic manual mode is quick, responsive and smooth should you feel an interactive need.

The engine has a real voice, too, deep and cultured as it extrudes the BMW along the road as if by some giant hydraulic force. Uprated brakes match the pace, and the broad rubber and tight suspension of a Sport chassis set-up ensure tidy torque deployment when the road ceases to be straight. You may want to pass on the test car's Active Steering, though; learning a mental response map to match that of the steering system is arguably more effort than it's worth.

Otherwise, the 535d might just be the most engaging 5-series of the moment, effortless but always interesting. You'll probably have to find another ΂£4000 or so over the 530d's price, but you won't regret a single pound.