The Porsche is 199mm shorter, 132mm narrower and 7mm taller than the BMW, but still has a pleasing appearance almost two years after this latest incarnation arrived.
The i8 features a lightweight carbonfibre-reinforced plastic body and is underpinned by a chassis made almost exclusively from aluminium. But unlike the boxy i3, the new i8 has a plug-in hybrid drivetrain consisting of both a conventional petrol engine and a pair of electric motors.
BMW says the i8 weighs 1485kg, which is impressive given that there’s 200kg of lithium ion batteries running down the centre of the cabin. The aluminium-bodied Porsche tips the scales at 1430kg in PDK form.
Getting into the i8 isn’t easy. It requires a good deal of gymnastic dexterity to fold yourself between the lower edge of the scissor door and the substantial slab of carbonfibre sill. The 911, on the other hand, is a cinch to enter. Its conventional doors, relatively narrow sill and generous door openings are part of its enduring everyday appeal.
Swing your legs down into the i8’s footwell and you’ll find an interior that’s every bit as new world as the exterior, with deeply dished seats, a futuristic dashboard and a heavily sculptured fascia. And the Porsche? Its cabin lacks the sheer modernity of the i8, but there is nevertheless a satisfying completeness to it despite myriad buttons and switches.
When it comes to packaging, there is little to separate the two. Each provides seating for up to four, although the rear seats in both are really for emergency use only. As for luggage capacity, the i8 comes out ahead. The 154 litres under its glass liftback-style tailgate is 19 litres more than offered by the 911.
All up, the i8 boasts a combined 357bhp at 5800rpm and 420lb ft at 3700rpm. Just how much of this is available at any given time is dependent on the state of charge of the batteries and the drive mode that is chosen: Eco Pro, Comfort or Sport. Alternatively, you can select E-mode although it is only available when the batteries have enough charge.
Eco Pro and Comfort are entirely suitable for urban driving or cruising at a steady clip. Provided that the batteries are sufficiently charged, the electric motor within the front axle is automatically engaged. With 128bhp at 4800rpm and 184lb ft the moment that your foot brushes the throttle, it provides the i8 with zero-emissions front-drive running for limited distances up to an 80mph top speed.
Read our first drive review of the BMW i8 plug-in hybrid sports car.
The turbocharged 1.5-litre, three-cylinder petrol engine mounted at the rear engages when you give the throttle a decent nudge or when the energy in the batteries is depleted to a preset level. The engine is a more highly tuned version of the unit first unveiled in the new Mini Cooper.
It boasts the highest specific output of any current BMW engine, with 228bhp at 5800rpm and 236lb ft at 3700rpm – all channelled through a six-speed automatic gearbox to the rear wheels, essentially giving the i8 part-time four-wheel drive.
A second electric motor, mounted at the rear, acts as a generator to integrate the operation of the combustion engine with the front electric motor. There’s a brief interruption as the three-cylinder unit springs to life and begins to send power to the rear wheels, but it does not impede progress in any great way.
With the petrol engine and two electric motors working together in either Eco Pro or Comfort mode, the i8 feels powerful, but not explosively so. Peg the throttle at a standstill and the driveline delivers the sort of performance that you’d expect of a turbocharged six-cylinder engine, albeit allied to an odd, synthesised three-cylinder thrum.
The Porsche’s driveline lacks the complexity of the BMW’s, but it is not shamed for power. The 911’s naturally aspirated 3.8-litre flat six engine, mounted behind the rear axle, nominally delivers 395bhp at 7400rpm. Our test car is a US-spec model, though, and receives a power kit as standard, upping it to 424bhp.
It can’t match the BMW’s torque, though. With 325lb ft at 5600rpm, it gives away a considerable 95lb ft. In standard guise, it’s all sent through a seven-speed manual gearbox to the rear wheels, but our test car has the optional seven-speed PDK dual-clutch auto with better standing start qualities and superb shift quality.
Read our comparison of the Porsche 911 Turbo S and the Jaguar F-type R coupé.
The initial impression is that the i8 offers a very different driving experience from any traditional BMW sports car. There is an uncharacteristic lightness to the controls and a faint whine from the electric motor under load but, for the most part, the impression is of outstanding mechanical refinement.
The electric motor drives through a two-speed automatic gearbox, providing instantaneous responses and the sort of standing start acceleration to allow the i8 to challenge a well sorted hot hatchback away from the traffic lights in electric mode.
Drive it in isolation over a flowing back road without too many tightening second-gear corners and you may never doubt the dynamic ability of the i8. The steering is precise, if a little light, while turn-in is urgent and it happily accepts increasing lateral forces without awkward body movements.
With its electric motor and combustion engine channelling their combined reserves to all four wheels, there is plenty of traction and the brakes are powerful, albeit lacking in ultimate pedal feel. And besides some annoying patter at the front over pockmarked surfaces, the ride is quite compliant for a car of such sporting pretensions.
And boy, does it go when you put your foot down on the exit of a bend. On smooth roads, it is capable of keeping the rear-engined 911 honest, which speaks volumes for the effectiveness of its high-tech driveline. The Porsche has nothing like the same amount of low to mid-range torque; nor does it possess anywhere near the initial accelerative force out of corners.
But the 911 is not gutless. Above 4000rpm, its naturally aspirated engine provides prodigious shove. But by then, the i8 has already benefited from its initial spurt of electric propulsion and is away down the road. BMW claims 0-62mph in 4.4sec, compared with 4.5sec for the Porsche. However, a constant, linear surge of energy right from the outset makes the i8 feel considerably faster.
Read Autocar's full road test review of the Porsche 911.
The i8 would appear to tick the boxes that buyers of modern-day sports cars are likely to demand, but not if engagement and driving entertainment are your priorities. Seek out the final degree of dynamic finesse and you’ll discover one central weakness: a lack of front-end grip.
The new i8 rolls on 20-inch wheels, albeit only 7.0 inches wide at the front and 7.5 inches at the rear. They are shod with 195/50 tyres up front and 215/45s at the back and, with their limited contact patches, they lack for sheer purchase. The result? Early breakaway as you enter second-gear corners, followed by incessant understeer on a loaded throttle. As a result, the blinking yellow stability control system light is a constant companion on winding roads.
By contrast, the fluidity of the 911 is almost enough to seal this contest in one. It is so pure and involving that it makes the i8 feel stifled and artificial. The Porsche displays a lovely balance that prompts you to keep pushing at the sort of cornering speeds that begin to trouble the BMW, and there is a compelling completeness to the whole car, even as you approach the limits.
The Porsche’s steering is one of the best electro-mechanical systems fitted to any current production sports car. It is deliciously involving and perfectly weighted, and far more engaging than the light and over-servoed rack of the BMW.
And then there’s the rest of the 911’s absorbing dynamic character: its superb body control at any speed, the outright grip it generates, the terrific poise when loaded through a quick corner, the traction on the way out, the well-controlled ride and the way that its electronic driver aids allow you to dabble in a spot of tail-out action. Even if you put the i8 on its optional wider and stickier tyres, I still doubt that it could match the 911.
So the Porsche takes the dynamic contest – and by quite a margin. The tacit handling of the Carrera S is intoxicating on the right road.
However, the BMW is not disgraced. The i8 is one of the most intriguing sports cars to challenge the establishment in a very long time, and it is a clear winner in an urban environment, where its ability to run in electric mode and strong low-end torque make it wonderfully refined and amazingly frugal.
In the end, you can make a strong case for owning either one of these cars. If you’re an enthusiast and treasure driving, choose the 911. The Porsche is clearly more entertaining on a back road and the better sports car.
But if you’re keen on the futuristic appearance and are more obsessed with technology than dynamic appeal, go with the i8. The BMW’s blend of coolness and complexity makes it a compelling statement. Think of it as the world’s most accomplished urban sports car.
Price £94,845 (including government grant); 0-62mph 4.4sec; Top speed 155mph; Economy 135mpg (combined); CO2 49g/km; Kerb weight 1485kg; Engine 3 cyls in line, 1499cc, turbo, plus electric motor; Power 357bhp at 5800rpm; Torque 420lb ft at 3700rpm; Gearbox 6-spd auto (petrol), 2-spd auto (electric)
Porsche 911 50th Anniversary Edition PDK
Price £103,251; 0-62mph 4.2sec; Top speed 187mph; Economy 32.5mpg (combined); CO2 205g/km; Kerb weight 1430kg; Engine 6 cyls horizontally opposed, 3800cc, twin-turbo, petrol; Power 424bhp at 7500rpm; Torque 325lb ft at 5750rpm; Gearbox 7-spd dual clutch automatic