These things are important (they are to me anyway). When it comes to litres, certain number combinations just seem right. Being a bit of a Jaguar fan, 3.8 has an almost spiritual resonance and 4.2 isn't far behind, while 2.9 makes me wince, for obvious reasons. Similarly, Porsche fanatics register something approaching sexual arousal should you whisper the words two-point-seven into their ear.

Expanding the V8 from 4.0 to 4.2 litres isn't the only change to the XK coupe range - there's a new six-speed autobox too, latest-generation driver aids, and a raft of cosmetic tweaks, the most obvious of which are the new wheel designs - but naturally it's the new engine that gets us fired up. Last month we tried the 400bhp supercharged version in the XKR, and very fine it is too. Now it's the turn of the 300bhp normally-aspirated XK8.

It's a full six years since the XK8 appeared. It felt great back then, fast and refined, a real Jaguar, though some of us thought the styling was a bit anodyne. In fact it's aged beautifully. I'm not a fan of Lawson-era Jaguars generally, but they got this one absolutely right. The XKR we featured last month, with its racing green paintwork and Ben Hur-spec 20in wheels, looked simply spectacular. Even this more humble coupe, despite its rather polite blue hue, looks pretty darn good for a car which, by modern standards, is well into its twilight years.

Those optional 19in alloys, slicked with supercar-issue rubber - giant Pirelli P Zeroes, 245/40 fronts, 255/40 rears - obviously help. Unfortunately the appeal diminishes drastically when you drive out of the showroom. Yes, they give the XK8 tarmac-leeching levels of grip, but there's a high price to pay.

You expect the supercharged XKR to ride stiffly (it does). The ostensibly smoother, cruisier XK8 ought to slip along serenely. But although the ride does get more fluid at speed (it simply creams fast A-roads) on slower stuff and especially around town it hobbles along disjointedly, musclebound, like Schwarzenegger trying to glide like Gene Kelly. The very worst potholes actually send a shimmy through the structure and you realise that, in the business of torsional rigidity, six years is a lifetime.

So it improves as you up the pace, and the grip, as we've mentioned, is prodigious. It rolls a fair bit in tighter turns, but it's the lack of connection that really disappoints. Right from the start, XKs have always had a pretty numb helm, and this one's no exception. What makes it worse is that the wheel itself, with its half-leather half-plastic-wood rim, is a tactile disaster, and your other main point of contact with the dynamics - the seat - conspicuously fails to embrace your body (surely seats should be engineered, not styled).

But just as the ride gels the faster you go, so the handling. Take a leap of faith, throw it in a bit harder. When you get it up on tiptoe, get it moving around, start to play with the slip angles, you find you can trim your lines with the throttle. It's still got that old Jaguar poise; you just had to look that bit harder for it. But how many XK8 drivers will drive it like that? My guess is that rather more will miss the fabulously supple ride that's been a hallmark of generations of Coventry cars.

The engine, though, is a total joy. The V8 has always had a good spread of torque; now there's just more of the stuff, and it's everywhere - the mid range is swollen with meaningful urge, and as the revs rise the power just keeps flooding in - now it really races for the red line, pulling hard all the way. Better still, at high revs there's a soulful V8 howl that's masked in the more potent XKR by the supercharger. According to Jaguar, the extra cubic centimetres have helped slice a good half-second off the 0-60 time and it really does feel that quick. The new six-speed 'adaptive' gearbox helps too. When you're on it, it snicks around between ratios almost frenetically.

The engine has given the XK8 a necessary shot of viagra, but it's also highlighted a number of, erm, shortcomings - the remoteness, the lack of stiffness in the shell, the so-so ride if you opt for the sporty wheels. And yet the basic XK8 still has substantial appeal. It still does the effortless wafting thing, slices through roundabouts with a smear of lock, fires you onto the straights. There's the terrific A-road pace, and the inspirational sound of the non-blown V8. It's flawed but it's a real Jaguar, and a 4.2-litre Jaguar too.