It's Porsche's 40th birthday beast

The Porsche 911, one of the world's most famous and highly regarded cars, celebrates its 40th birthday in September with the unveiling of yet another variant, the race-bred GT3 RS.

The covers will come off the lightweight Renn Sport model at the Frankfurt motor show, about two weeks before the 911's birthday cake is cut on September 23.

Just 300 of the street-legal Le Mans racers will be built and a handful have been ordered for New Zealand.

The stripped-down and powered-up 911 is based on the race-ready GT3 and does without air-conditioning, a glovebox and centre console in an effort to reduce weight.

The steel body has also been changed, with the door mirrors, bonnet, engine cover and rear wing made of carbon fibre.

Further weight-saving measures include an acrylic rear windscreen in place of the GT3's heavier glass screen.

Porsche says the changes make the RS up to 45 kilograms lighter than the GT3.

Power comes from a reworked version of the 911 GT3's 3.6-litre flat-six engine. With a larger air collector and new single-mass flywheel, it delivers 280kW (378bhp) at 7300rpm - the same power rating as the standard GT3 but 100rpm lower down the rev range. Peak torque remains unchanged at 385Nm.

Porsche claims the added power and lower weight of the RS helps it shave 0.1 seconds off the 0-100km/h time of the GT3 at 4.4 seconds, but top speed is unchanged at 305kmh.

The suspension in the RS has been lowered slightly and is firmer for more stable cornering. Porsche plans to finish all cars in white, with decals and wheels in either red or blue, like the original 911 RS of 1973, one of the most collectable models.

Much about the 911 has changed since the 1963 original. Most recently, the switch a few years ago from the traditional air-cooled engined to a more conventional water-cooled set-up. But the change did nothing to dilute the wonderful 911 look and flavour and it remains a hugely capable sports car with a horizontally opposed (or boxer) six-cylinder engine mounted behind the rear axle.

The original 1963 model was going to be called the 901, until it was pointed out that French carmaker Peugeot owned all three-number combinations with a zero in the middle.

These days each new 911 body style typically starts as a regular Carrera coupe, before the arrival of such variants as a Cabriolet, Targa (glass roof), Carrera 4S (all-wheel-drive), turbo and wide-bodied, as well as various combinations of each.

By the end of next year, as the current 996 series approaches the end of its life, Porsche will have offered 11 variants on the one basic vehicle.