Detroit 2011: Porsche unveils 918 RSR

It looks brilliant – the closed cockpit making it look shorter and more purposeful than the 918 Spyder – and it's got cool orange stripes, and bears the number 22, which was the winning number of the Porsche 917 K that won Le Mans 40 years ago with an average speed of 138mph, not beaten until Audi did it last year. Also, the fan sits over the engine, exactly like the one on the 917's flat-12.

The 918 RSR uses a V8 from the RS Spyder racer, tuned for 563bhp at 10,300rpm (!). There are two electric motors for the front wheels – they're independently powered to give torque vectoring to improve cornering – and max power is 767bhp when the driver pushes a button to activate the electric motors. It's a six-speed paddleshift transmission for the V8.

The electric energy comes from a flywheel in the ‘passenger's seat' linked to a motor/generator to turn electric energy into flywheel energy and vice-versa. The flywheel spins to 36,000rpm.

By contrast the road 918 has a battery hybrid system instead of the flywheel. Batteries allow fairly large quantities of energy to be stored; it can do 15 miles of gentle driving on its batteries before the engine starts. The batteries can also be charged from the mains.

But, explains TopGear magazine Man of the Year Wolfgang Duerheimer, the RSR uses a flywheel because, crucially, the energy can be got into and out of the flywheel faster than it can be with batteries, so the electric kick out of a corner can be bigger. Also, the flywheel is lighter than batteries. Top Gear/BBC