Road Test: Bentley Continental GT 4.0 V8 2dr Auto

Oddly, driving a supercar on the track never feels as fast as it should. Driving on a real road, between a blur of trees and hedges, a supercar always feels scary-quick - if it doesn't, it's not firing on all cylinders. But out on the wide-open tarmac of Silverstone, the impression of speed is diminished. Tucked up inside a helmet, you can't even hear the engine properly. So, after a few laps in a prototype of the new Bentley Continental GT V8, I'm impressed but not quite blown away.

Apparently, though, I've been getting along at a fair clip. At the end of the relatively short Wellington Straight, hauling off the speed for Brooklands (the left-hander, not the Arnage-based Bentley coupe), I'm told the brake discs have been glowing orange. To be fair, it wasn't me who was doing the driving. Oh, I was, in the literal sense of sitting behind the 'wheel, but it was Juha Kankkunen, coaching from the passenger seat, who was really in charge. I just steered, accelerated and braked when and where he said. When it's a four-time world rally champion, you don't bother having your own useless opinion.

So, flat-out, it actually is fast, and I believe the 188mph top speed claim. It also shows a gain over the W12 in the handling department. It turns into corners more convincingly, even on a greasy, damp, wintry surface. It's more eager to be shifted off the straight-ahead by a steering input.

Between them, the front wheels are carrying 25kg less than they do on the W12. Not a lot, I agree, but the suspension rates and geometry have been recalibrated to take maximum advantage. It's a heavy car, knocking on for 2,300kg, but the way it refuses to roll, and the general feeling that you can push it without it snapping back, mean it's big fun. The adaptive dampers keep the body motions from turning any kind of drunk, and the four-wheel-drive system lets you take liberties. The differential pushes extra effort to the rear tyres once you're on the gas, and trimming the line with the throttle is a nicely connected experience. The ESP is beautifully calibrated, too - should you have some style, it doesn't cramp it, but should you need saving from yourself, then it obliges. And the lighter nose means this new Continental staves off understeer unless you've turned up with ridiculous haste at a properly tight corner.

But to totally learn an engine, you need road driving, so I potter out under the Silverstone barrier and head east. On a track, your needs are simple - power at big revs and quick gearshifts. On the road, you need progressive response, smooth and prompt shifts, power across the rev range, subtle pick-up at traffic speeds, and the general feeling that the engine is talking to the transmission and both are listening hard to the driver.

While we're on the subject of listening, we want a good sound. Oh, and fuel economy, where the V8 is supposed to show its benefit over the W12. Working all those characteristics into a powertrain - especially one still in prototype stage before the April launch - is a whole lot harder than making one that's simply good for tear-arsing round a track. This is a very complicated engine.

First off, on the road, there's always all the performance you need. The torque push (487 lb ft, only six per cent less than the W12) arrives in all its glory down at 1,700rpm and carries on to 5,000rpm, so in any of the lower gears, the rate of acceleration feels pretty much as strong wherever you are in that band. But, of course, there are eight speeds in the transmission, so those sort of single-gear crescendo runs are really just an exercise to see what the engine is made of, and to listen to its noise change as it goes.

That noise is wonderful. At low revs, a rocking old-school V8 gurgle, and then, with some power on, a nicely naughty rumble with a hard edge definitely audible beneath the buttered surface. That growl kicks right in as you howl 500bhp towards the red line, with a bit of cheeky crackle on gearshifts when you're hurrying. It's deliberately different from the German-saloon exhaust of the related engines in the new Audi S6 and S8. Good. It's got a real Bentleyness to it. The chassis is the best of British on the road, too: tight control but a surprisingly plush ride, even on B-roads.

Along with the new engine, the V8 gets an eight-speed automatic transmission instead of the six-speed in the W12. More ratios mean it's more often in exactly the right one, and because each shift moves less of a jump, they turn it smoother and faster. Response to the lovely solid metal finger paddles is super-quick when you feel like getting involved and DIYing it. Cleverly, the new transmission is programmed to slip into neutral when you're sitting at rest with your foot on the brake, to save fuel. Lift off, and it engages first without you noticing.

That's just a tiny shard of all the cleverness in this powertrain. The engine is a brand-new V8 developed with Audi but in a different spec. Slightly less power than an S8 but more torque, for instance. The big aim was efficiency: in 2008, Bentley announced it would have a powertrain delivering the same performance as that year's W12, but with 40 per cent less fuel used and CO2 emitted in the official test cycle. This is it.

How? Nearly half of the efficiency gain is by reducing the size and cylinder count, while direct petrol injection - and the higher boost pressure that allows - brings the power back up to Bentley level. In search of fast boost response, the engine uses a pair of small twin-scroll blowers. For the same reason, the exhaust is fed through the centre of the V rather than the outboard side of the banks, keeping the tubing short to the centre-mounted turbos.

The eight-speed transmission also plays a big part in the efficiency gain, and the third biggest contributor is another clever engine technology. Variable displacement they call it, but actually what's happening is that the engine can switch between two displacements. Little electric actuators slide the cams of four of the cylinders away from their valves, so the valves stay shut and no fuel burns.

That sounds innocuous, but, crikey, look what it means. A Continental GT, a car that we've always known as a 6.0-litre 12-cylinder, now comes as a V8 that will at times run as a 2.0-litre V4. But you know what? You never notice it. Bentley decided it wouldn't be fitting to have a little ‘eco' light burning away when it's in V4 mode, so for all I know, it wasn't even actuated on the car I drove. But assuming they weren't flat-out lying and it was operating, it works wonderfully well. The engine idles as a V8 to stay smooth, but on a light throttle, the V4 mode operates between 1,500 and 3,000rpm. Which covers the majority of driving when you aren't on a hoon. Brian Gush, the chief engineer, had a four-cylinder light fitted to his development car and cruised the 30 miles past Crewe on the M6 from Stafford junction to Sandbach at 80mph without it ever blinking out.

Making the valves lock shut and open again is quite the fancy piece of engineering, but there's more. An engine running on four has completely different vibes and exhaust pulses from a V8, so to make sure you never notice, extra flaps reconfigure the exhaust, and active engine mounts control the vibrations.

Add all of those technologies together, and you get a car that does what's promised, saving its buyers tax and getting green brownie points. Two small troubles. The tech is expensive, so the V8 is only about 10 per cent less to buy than the W12, and it has less equipment, so if you spec the interior up to match the W12, the price comes very close. Second, while it's impressively economical when wafting along on something resembling the official test, when you're enjoying all 500bhp in this 2.3-tonne car, you get (or I got, on roads in Northamptonshire and Oxfordshire) something worse than 16mpg.

The W12 would have drunk more. That engine continues as is (it'll get direct injection and the eight-speed later). The V8 has slightly different styling, including a new lower front bumper, black grille, new wheels, figure-eight tailpipes and black lower rear diffuser. Subtle, but visible if you look. It's not the poor relation. It's the better car for fun on your favourite road, and when you're stuck at normal-traffic speed, it'll go far further on a tank.