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Two of 2010’s most exciting cars were the Porsche 911 GT3 R Hybrid racer, and the Porsche 918 Spyder concept car that previewed Porsche’s hybrid successor to the Carrera GT supercar.

What do you get when you mix the two together? Answer: the new Porsche 918 RSR, just unveiled at the 2011 Detroit motor show.

It’s not a simple case of sticking on a set of slicks and a big wing though: the conventional hybrid system of the 918 Spyder doesn’t work with racing, so it’s been abandoned for the GT3 R Hybrid’s flywheel set-up. Outgoes the rear-mounted lithium-ion battery pack and electric motor that allowed zero emissions running – and for the concept to squeak through official emissions testing with a super-clean 70g/km.

The 918 RSR still has two nose-mounted electric motors like the 918 Spyder, each driving a front wheel, but these are now linked to the flywheel set-up. Under braking the two motors acts as generators, charging the flywheel accumulator that sits alongside the driver in the cockpit – brave, as the rotor that stores the energy spins at up to 36,000rpm. At the push of a button, this energy can then be fed back to the two 75kW motors, aiding acceleration and overtaking, but also helping reduce fuel consumption too. In the case of the RSR, the twin motors have a torque vectoring function too, helping to improve agility.

Don’t worry, there’s still a big, petrol-fed V8 mid-mounted in chassis, but it’s seen a spot of development since the 3.4-litre race engine from the LMP2-spec RS Spyder found its way into the 918 Spyder. Rather than the 918 Spyder's 493bhp and a 9200rpm red line, the RSR’s 3.4-litre V8 produces 555bhp at 10,300rpm. Together with 201bhp from the electrics, that gives the RSR a grand total of 756bhp.

Other changes in the switch from Spyder to RSR include the abandonment of the seven-speed PDK transmission for a paddleshift version of the RS Spyder’s six-speed sequential ‘box, and the obvious addition of a roof – to aid driver changes at pit stops the doors now swing up and out. The interior abandons the touch-sensitive centre console from the 918 Spyder for a simple set of toggle switches, and there are screens for the door mirror-replacing cameras.

Exterior bodywork additions include a front lip spoiler, front spats and a rear wing to increase downforce levels, while slats atop the front wings reduce air pressure in the wheelarches, and there’s a 917-aping fan atop the V8 engine. Finally there are 19-inch centre-lock wheels shod with Michelin slicks.

Porsche is keen to point out that the 918 RSR is just a technical exercise for the company, but who knows what race regulations the ACO might approve for Le Mans in the future...

CLICK HERE for more Porsche 918 RSR videos and the live show unveilling

Porsche 918 RSR Live unveiling at the Detroit auto show

By Ben Pulman