The engine is actually made by Yamaha - a specially-designed 4.4-litre unit that's been mounted transversely into the XC90's engine bay. To aid the shoehorning process, the cylinder angle has been reduced to 60 degrees from the customary 90, and various ancillary units, such as the alternator, have been bolted directly onto the engine itself. Looks promising, but the noise was going to be the first test. Turn a Volvo key and you hardly expect to hear the deep-throated rumble of a V8, but it's here alright, and sounding muscular. Indeed, this is something that Gothenburg's engineers have worked particularly hard on; while the American market demands an omnipotent grumble from the engine bay, Europeans favour a subtler mechanical soundtrack. Both need pleasing, but you get the feeling the Americans were uppermost in the minds of the engineers. The result is a pleasurable, throaty resonance when you pile on the revs, but a reassuringly discreet presence while cruising on the open road.
Once you get out there and crank it up, the XC90 V8 is no slouch. Give it the boot and the 315bhp engine will launch you from 0-60mph in less than seven seconds on the way to an eventual top speed of 130mph. The Americans won't want to hear this, but this is a Volvo with proper performance guts.
Wading through rush-hour Phoenix traffic, the XC90 behaves with the dexterity of a car half its size. Using the new six-speed Geartronic gearbox in manual mode gives you the instant power delivery you need in order to duck and dive with the best of them, and it's surprisingly responsive. Once you're free of the traffic, it's a nimble car, which feels every bit as stable as an X5 when being chucked into bends and a lot more responsive than the gargantuan Range Rover. While there's plenty of feedback through the wheel, however, it never quite forgets its sensible roots.
Once out on the open road, there is nothing for it but to stick it back into 'auto' and cruise to the back country - real Clint Eastwood territory in these parts. No surprise that the XC90 is a great car for long-distance road trips (big, comfortable and user friendly), but I can't resist ducking off onto some back roads, and this is where it gets interesting.
Having crossed the spectacular Roosevelt Lake Bridge, we find ourselves on a dirt track with a sheer drop on one side - there's no room for error here. While the XC90 might be more familiar as a mum-mobile, it handles impressively well on the rough stuff, too - even steep, off-camber climbs are a cinch. Slippery sand eventually gives way to good old tarmac, though, and I have great fun flinging the big fella round mountain hairpins, shamelessly enjoying the echo of the V8 from the surrounding rock faces.
But here's a thing - even such hooligan behaviour fails to blunt one of the big surprises of this V8 - its impressive fuel economy. The quoted combined figure of 21.7mpg even compares favourably with the smaller 3-litre, six-cylinder T6 model (22.2mpg). The figures are partly due to the Geartronic 'box, where sixth acts as an 'overdrive' - saving fuel and cutting road noise. On the motorway at 70mph-plus, the engine pulls less than 2000rpm.
When you check out the visual changes to the V8, there's not a lot to distinguish it from its less powerful siblings. Aside from the rather unassuming V8 badges on the boot and front grille, the only giveaways are the twin exhausts at the back (one either side) and some chunky 18-inch alloys. It's a study in Swedish understatement.
Inside, it's business as usual: loads of legroom, heated leather seats, cruise control and electric everything, plus typical Volvo practicalities like foldaway picnic tables and cubby-holes galore. The sound system is pretty nifty, too, with an in-dash six-CD stacker making the most of the Albert Hall acoustics.
The question on everybody's lips is whether we're going to see the XC90 V8 in the UK. That's a question both Land Rover and BMW are going to be taking a real interest in, and they will take little comfort from the fact that the official answer is no. After all, the prestige SUV market is booming, and Volvo, which has the brand, now at last has the top-end product, too.
When you also consider that Gerry Keaney - Volvo's Senior Vice President of Marketing - makes no secret of the fact that he'd like to see the V8 on British roads, you've got the makings of a proposition.