The car's bodywork also offers some unusual extras. Otherwise identical to the basic 1.8-litre 155, the Silverstone has a little black tongue sticking out underneath the front bumper and a wing sitting 4in above the back of the boot lid.
These add-ons are not fitted to save people who like to adorn their cars with 'go-faster' bits the trouble of doing the work themselves. And they certainly do not make the car go faster: in fact, they make the Silverstone marginally the slowest Alfa Romeo 155 you can buy. But they were not fitted for the benefit of the customer. The adjustable black tongue, the rear wing and the metal plates in the boot (which fit on to the wing and push it an extra 4in up in the airflow) are there to help Alfa Romeo win the British Touring Car Championship. Or at least, they were.
On television, the BTCC looks like a sport of thrills and spills. But these are short, dramatic interludes in what is essentially a game of chess played by lawyers and spies. The rules are intended to create a racing series for cars based on showroom models, with minimal changes to the bodywork - except, of course, for the addition of slogans, logos and splashes of colour. Rules, however, are there to be bent; and with the Silverstone, Alfa Romeo bent them backwards.
To have a car approved for the BTCC races (a process called 'homologation' ) manufacturers must produce a written specification for their racing version, and prove that 2,500 road-going cars of a similar specification have been built and are for sale to the public. Alfa Romeo's homologation papers for the Silverstone specified that the aerodynamic devices were adjustable: the rear wing could be raised or lowered, and the black tongue could be moved forwards or backwards.
The closing date for homologation was, appropriately, 1 April. Up to and including that date, Alfa Romeo's racer only appeared in public with the rear wing in the low position. On 2 April, for the first practice of the season, at Thruxton, out came the plates, up went the wing, off went the car: Gabriele Tarquini, Alfa Romeo's lead driver, was fastest and won the race - as he did the next four rounds of the BTCC series. On 16 April, the road-going Alfa Romeo 155 Silverstone (with those same plates inside the boot) went on sale to the public.
Alfa Romeo's rivals in the BTCC were dismayed. One accused the Italian team of 'taking the piss'; but another, the Ford driver Paul Radisich, admitted that 'rules are rules - and if you can get away with it, you do'. Alfa Romeo got away with it until the rules were changed. First the RAC, which controls the British series, decided that 'parts supplied with a car but not fitted to it do not form part of the completed car', so the wing plates were outlawed. Alfa Romeo obediently lowered its rear wing. Then, in late May, the RAC decided that the tongue, known as a 'splitter', was also illegal.
Ford's Team Mondeo made the complaints against Alfa Romeo. To back up its second case, it bought a road-going 155 Silverstone, which it offered in evidence. The RAC agreed that the moveable splitter had been approved; but the 21 rivets supplied to attach it in position were separate parts - like the wing plates. The splitter was legal, the rivets were not. This time, Alfa Romeo did not withdraw its splitter. It withdrew from the BTCC.
Only Alfa Romeo knows exactly how effective the aerodynamic devices are. But the splitter, ostensibly designed to divert cooling air to the brakes, also smooths the airflow underneath the car, sucking it down on to the ground; and the further it protrudes, the better. Similarly, the rear wing, which forces the rear of the car down, is more effective when lifted up into 'clean' air, away from the turbulence caused by the car's bodywork - although the cars continued to win without the raised wing.
The effectiveness of the splitter will be easier to judge this weekend, when the 155 Silverstones will race, at Silverstone, in the British Grand Prix meeting. The international body for motor sport, the FIA, became involved in the dispute last month; and on 9 June it resolved the issue by deciding that the splitter was legal, but no longer is. Tomorrow, the Alfa Romeos, back in (and leading) the BTCC series, will race without splitters; paradoxically, mid-season homologation allows the BMW and Renault cars to appear with wings for the first time.
If you were to visit an Alfa Romeo dealership this weekend, however, you would still find the steel plates and nuts and bolts in the boot of the road-going version of the 155 Silverstone. The handbook would not tell you what they are for, nor where to fit them. But it's all a bit academic now, anyway.