ANY Porsche 911 should be considered a classic. A design that can last three decades and remain competitive without the need for major alteration thoroughly deserves the accolade of classic status. But the Carrera RS 2.7 of 1972 stands out from the Porsche pack and is considered by enthusiasts to be, perhaps, the best 911 yet built. The RS is the most sought-after of the early 911s because it was the fastest and most sporting of the pre-impact bumper cars. Special features are the larger and more powerful engine, pert little duck-tail spoiler and the legendary ''Carrera'' boldly painted across the sides of the car. To the untrained eye, most 911s look alike and, among the hundreds of Porsche 911s seen on Hong Kong's streets, the 1973 RS Carrera, belonging to Matthew Birch, attracts little attention. Few pedestrians give it a second glance, despite the bold Carrera graphics painted in blue against the white body colour. But Porsche enthusiasts stop and stare. This is the ultimate 911 and the car to which most Porsche owners aspire. The rarity and desirability of the RS keeps prices high, and an industry has grown up catering for those who want the Carrera thrills, but cannot afford the steep price tag. The less powerful and cheaper 'S' models are butchered and modified to create fake Carreras. Some of these replicas are made simply to give the owners the higher performance of the RS, but many are traded as the genuine article and can fetch $150.000 more than a standard 'S' with the fraudulent use of the Carrera name. The real thing has become thin on the ground and the rarity has only increased the desirability. Therefore, the invitation to drive Mr Birch's newly imported 1973 RS was not to be missed. Mr Birch, a regular competitor in Supercar racing in the region, imported this immaculate right-hand drive RS from England. The car is in remarkable condition, despite its age. The doors shut solidly and the panels are ripple free and line up perfectly. This RS has clearly been pampered but the car's fine order serves as a reminder of Porsche's fine-build standards; only a little wear in the interior betrays the car's age. The immense power and nimble chassis make a mockery of the tame performance of most modern cars. Stepping on the throttle produces a great howling surge of acceleration, filling the cabin with the wailing scream of the air-cooled flat-six engine and flinging the car past slower traffic. The sound of the radio (one of the few non-original parts) is soon drowned by the high-pitched sounds from the rear engine compartment and, at high revs, the noise is almost deafening. The secret of the car's startling performance is in the lightweight construction. The early 911s are at least 30 per cent lighter than the latest models, making the 210 brake horsepower of the special Nikasil-coated alloy-cylinder motor more effective. The centrally located tachometer spins through its range urgently as the five speeds are selected in the slightly notchy, but precise, gearbox. The RS is not an unmanageable beast, because one of the striking qualities of the car is its smooth, fuss-free running and tractability. My immediate impressions led me to believe the car was going to be tame, and spinning the starter motor for the first time did not raise my hopes. The engine took half a minute to come alive and the whirring of the starter motor is coarse and loud. But once the engine has caught, it idles surprisingly quietly and pulls away from other traffic with no more than a mechanical growl. Unless the driver chooses to rev the engine hard, the noise remains reasonably subdued, giving the throaty whine that is characteristic of air-cooled motors. Neither hard driving, nor heavy traffic pushed the temperature gauge above normal and the light, non-power-assisted steering slides effortlessly through the hands. Mr Birch's RS is that most rare commodity - a usable classic. For hardly half the price of a new Porsche 911, he has a car that has enough performance to make his hair stand on end. Equally, it will transport him down to the shops at weekends or the office during the week.