The skeletal Atom you see here is the wildest one yet, and by some margin. To keep pace with the current crop of more powerful trackday machines, Ariel has bolted a Jackson Racing supercharger onto a 1796cc Honda Civic Type-R engine and produced a unit that develops 275bhp (up from 190bhp). With an all-in weight of 500kg, it's guaranteed to provide some serious shove.

As is the 220bhp Audi turbo engine now fitted to the Westfield XTR4 in place of the Suzuki Hayabusa bike unit. This makes the character of the car very different. The rev-hungry Suzuki unit was incredibly loud, had an electric throttle response and little torque, which is almost the exact opposite of what we have now. Throttle response is still sharp, and there is very little lag, but the soundtrack has almost completely disappeared, with the waste-gate whoosh almost the loudest noise it makes. If our Trackday Car of the Year competition was judged by residents living near Bedford Autodrome, the XTR4 would be the clear winner.

The Atom wouldn't. The combination of a VTEC engine with a blower is a scream in more ways than one. But the thrust is almost unbelievable and the experience of being fired towards the horizon is amplified by the fact that the Atom has no skin. The gearbox is superb, but it needs to be as the revs pile on so quickly you're constantly grabbing for the next ratio. The surge of power is relentless - intense doesn't even begin to explain it.

Cornering was always the Atom's Achilles' heel, but Ariel's boss, Simon Saunders, has been working hard on this and the damping problem has now been resolved. The Atom is now considerably more faithful and the excess of power allows you to adjust your line accordingly.

On-limit handling in the Westfield is also a little less edgy and the extra torque allows for more gratuitous powerslides. There's plenty of grip, the brakes are strong and the balance is good. It's not as chuckable as, say, a Caterham, but the way it deals with the high-speed stuff is impressive.

The Westfield has also benefited from developments to the driving position, with the pedal box not as offset to the left as it once was. The XTR4 is considerably more comfortable and easier to drive slowly now, making road use much more feasible. On the track, feeling comfortable in the car allows you to wring its neck without feeling you are wrestling with it, so it's a shame that the gearbox isn't as slick or as snappy as the bike unit that went before. There is a small dog-leg between first and second and between third and fourth that requires concentration to make a quick shift. As the laps pass it becomes more familiar, but it is never satisfying and you never feel at ease with it.

The road/track compromise is a difficult one to get right and this Westfield was a little soft, with more roll than is ideal for the track, but of course it is possible to alter the settings. The XTR4 is fitted with cross-ply Avon ACB10 tyres and, as a result, on the road it skips and tramlines a little. The Avons are excellent dry-track tyres, but a radial tyre may be the best choice for road use.

The Atom wears Dunlop Formula-R rubber and feels much more suited to road work. It's still extreme (it requires a helmet - and a wetsuit if it's raining), but it's more like an aero-screened Caterham than a race-track refugee; a car you would use at weekends, rather than just to drive to a trackday.

Despite improvements, the XTR4 is still flawed, though with a better gearbox and different tyres things could have been different. But the Atom is better to drive on the road, and it also out-performs the Westfield on the track. It may not look like a Le Mans racer, but it provides more thrills and greater rewards for the driver. After five years of development, the Atom has come of age.