When Ferry 'Butzi' Porsche started it all, he needed just one kind of car - a small, two-door lightweight sportster - running on one kind of fuel, petrol, to realise a fantastic world-changing dream.

Look at Porsche now. Butzi's iconic rear-engined baby, the jewel-in-the-crown 911, is surrounded by family.

Today Porsche is as much about a small mid-engined roadster and coupe and two platform-sharing, front-engined large vehicles - a SUV and, soon, a sedan. It's also a diesel brand and, from next year, will have hybrid petrol-electric technology.

Cayenne caused a lot of fuss, but even cynics would have to admit it has performed a big job for its maker. Within three years, it was outperforming even Porsche's own supposedly optimistic expectations with more than 100,000 sales. Not a lot? Look at it this way. Back in 1996, when 911 was the only model, Porsche's total annual sales came to one-fifth that.

Still, even after moving so much metal, Porsche does seem to walk a tightrope. Every new model - the Panamera being a classic example - tends to carry some level of risk.

The economic downturn has highlighted a financial vulnerability that seems to have become positively precarious in the wake of the thwarted play to swallow Volkswagen. Now it seems VW is set to control Porsche.

Does it seems a touch ironic that, concurrent with the release of a movie of the brand's story, 'The Porsche Way', it was also seeking emergency aid from a German bank to service a debt of nine billion Euros?

In the grand scheme, the decision to implant a diesel engine into the Cayenne might seem minor stuff, but it's not. The sports ute might have been a huge earner, but it hit the market entirely with petrol engines, which goes completely against the emerging trend.

But Porsche reckoned diesels were beneath its image; they didn't rev and sound as Porsche engines should. It stuck to that line even when the Cayenne Turbo topped Europe's hit list of large, fuel-gorging SUVs.

Now, at last, Cayenne has the engine it always deserved. Or has it? With proliferate performance, hang the cost, always being a Porsche thing, it does seem odd that the one brand that supposedly always puts kapow ahead of all else has settled for a 'mere' 3.0-litre V6 from Audi. Why not the Q7's V8 or V12 or the VW Touareg's V10?

The rationale for starting small has never been fully explained. I suspect it's as much to do with engineering constraints (the V6 might the only engine that immediately fits under that sloping bonnet) being weighed against this generation Cayenne only having a year of life left. Maybe next time ...

As is, the V6 is a really good toe in the water even if, in pure performance terms, this vehicle will test the mantra that all Porsches are performance kings.

By diesel SUV standards, it's not bad, with 0-100kmh in 8.3 seconds. It's one of the few sports utes with a Sport function to liven the engine and transmission responses up. Still, even when the thing is lunging, there's nothing here to raise huge excitement among the 911 fan base.

But that's okay because it reminds Cayenne drivers to take stock about what it is they are really driving. Yes, we know what the first letter of SUV stands for, and we know that Porsche builds sports cars. But, just from looking at it, you know that if you want a 'sports' car in the true blazing sense, you don't want something like this.

No, the big draw is that it is a big, useful vehicle whose engine can muster a massive 550Nm of torque - more than the $50,000-dearer 4.0-litre V8 Cayenne S generates - and a level of frugality that no petrol Porsche of any size can come close to matching.

With a claimed average of just 9.3 litres per 100km, it consumes 28 percent less fuel than the most frugal petrol Cayenne, also a V6, but of 3.6 litres' capacity. With 1000kms' running claimed from its 100-litre tank, it has almost three times the range of the V8 Turbo.

That's astounding and important too because, contrary to popular opinion, the old saw about "if you can afford to buy a Porsche, you can afford to feed it" is not a given everywhere, anymore. Or at least, not outside of OPEC member countries.

All that torque makes it a great tow vehicle, and it'll hang 3.5 tonnes off the hook, no worries. Porsche has played around with the engine management and transmission settings. Hit the Sport button and the changes are set significantly quicker, while the throttle response is also sharpened.

Or you can just let the diesel get on with the job, which is just as satisfying in its own way. The muscle builds up from such low revs - you're getting it all by 2200rpm - and despite it being, um, rather 'porky; at 2240kg (70kg more than the V6 petrol model) it's not concerned about shifting that mass.

This also means the steering wheel-mounted gearchange buttons that come with the six-speed Tiptronic auto seem a bit superfluous. It's just as happy going through the gears in its own good time.

The characteristic diesel chatter on start-up aside, the other giveaway is the rev counter that reaches to just 4600rpm.

Handling-wise, a mega-911 the Cayenne ain't. But driven with proper consideration to its size and weight, it's well above the class-average. The diesel is nicely composed and the added weight of the engine and associated changes barely affects the balance. It holds the road with confidence and the tyres do a great job - and, of course, with all four wheels constantly sharing the workload traction is very rarely an issue. Off-road? Well, I suppose you could ...

The only thing missing is a proper engine note. Light the fire and instead of a chest-beating bellow there's, well, a lot of grumbling. Seems not even Porsche can make a diesel sound sporty.

A Porsche that's cheap to run is a new experience, but don't imagine that means it's cheap to buy. The diesel costs $136,000, but to take it as tested means spending more.

Options here included 21-inch alloys which, with arch extensions add $7800, front-seat heaters ($1210), roof rails ($1850) and Bluetooth prep ($1400). So - ka-ching - that's $148,835, danke.

The interior is unremittingly fancy, with dark plastics and black leather upholstery relieved by aluminium highlights. It has Porsche's trademark siamesed instruments, a screen-based system for the audio/trip computer functions, toggle switches for the air conditioning and terrific sound from the audio system.

The power-adjustable driver's seat is long, narrow and firmly bolstered, with excellent lumbar support. The wheel adjusts for height and reach. Split sunvisors are useful. Thick pillars restrict vision, particularly to the side.

They call it a five-seater, but realistically only with four aboard are optimum comfort levels achieved by every occupant. The back seat has adequate space for two, but an upright backrest and short and firm cushion cuts erode comfort. Three child-restraint anchors are fitted on the back of the seat. Boot space matches the opposition.

The Cayenne diesel doesn't shape up entirely well against its opposition - an Audi Q7 with the same engine costs less, and then there's the BMW X5 - and, in performance terms, has only the petrol V6 in its sights. The standard V8 and tarmac-melting Turbo remain in another league.

Even so, it's still the right car for Porsche and it's highly likely the next one will present an even stronger argument for going to the dark side.

What this means to you: Is Porsche uncomfortable about introducing a diesel? There's not even a badge to give the game away.