14/11/11

Overview: Porsche 911 Carrera

Low mass and high engine output: a straightforward philosophy. It’s not the end of the story of course, but it was high on Porsche’s list of priorities with the new generation of 911s. The new Carrera S manages a power-to-weight ratio of 285PS/t and, with a dual-clutch transmission, an overall 8.2l/100km (34.4mpg) in the NEDC rolling-road tests.

The all-new, steel-aluminium body allows the kerb mass of each model to be reduced by up to 45kg compared with its predecessor, and the 911s are also a little lighter than their competitors, at 1400kg for the standard model and 15kg more for the ‘S’. By comparison, a two-seater Ferrari 458 Italia weighs 1485kg at the kerb, a McLaren MP4-12C, with its carbon-fibre bucket, records a kerb mass of 1434kg, and the 2+2 Jaguar XK — which is perhaps more what Porsche has in mind as a competitor — weighs 1660kg.

The Porsche flat six engine has been substantially developed in the interest of both economy and power. The swept volume was reduced from 3.6 litres to (for the standard Carrera) 3.4 litres. The direct injection system of both engines now uses multi-hole injectors. Internal friction has also been reduced.

The battery is primarily charged during braking and overrun — kinetic energy capture — while the alternator charging load is reduced when the battery is fully charged. A map-controlled thermal management system reduces the time taken for the engine and transmission — they share the cooling system — to reach operating temperature. Pressure losses both in the intake tract and exhaust system have been reduced.

Stop-start is fitted to manual transmission models; Porsche estimates the fuel saving at 0.6l/100km over the NEDC.

The two seven-speed transmissions — manual and dual-clutch PDK — both deploy a fairly long top gear in the interest of cruising economy, though the PDK’s top is considerably higher than the manual box’s, at 0.62 against 0.71 for both the Carrera and the Carrera S — the two versions have identical gearing. The new seven-speed manual transmission reduces engine speeds in top by around 19 per cent. compared with the previous six-gang. Apart from the marked difference in top gear ratios between the new seven-speed manual box and the PDK, there is one other difference: the dual-clutch unit runs with a lower third, at 1.65 instead of 1.55 for the manual. Otherwise the ratios — including that for the final drive — are the same.

A peculiarity of the new manual transmission is the shift gate lock on top gear, preventing the driver from unintentionally landing in top instead of fifth.

One of the functions that Porsche has built into the latest version of its PDK (Doppel­kupplungs­getriebe) transmission might startle traditional sporting drivers: coasting. Driving with the transmission disconnected is anathema to many, because it reduces the degree of control the driver has over the car, but it can save fuel under some circumstances. Bear in mind that even petrol engines cut off their fuelling on the overrun (with the accelerator released) above a certain crankshaft speed — usually in the region of 2000rpm — and you will see that coasting with the engine idling is actually only of use in a limited number of circumstances — in a kind of grey area between cruising and overrun. Nevertheless, Porsche claims that the facility will allow savings of ‘up to one litre per 100 kilometres with a forward-thinking driving style’. The coasting function is triggered by slowly taking one’s foot off the accelerator or by manually changing up if the highest useable gear is already engaged. Coasting is ended by accelerating, braking, or by manually changing down.

Electro-mechanical power steering has been adopted for the new 911. Quite apart from its being more efficient than electro-hydraulic systems, it also facilitates a good many driver assistance functions, as we have seen elsewhere.

The 911’s aerodynamics have been improved over the previous model, helped along by the new car’s greater length. The cooling system of course requires no large air intake underneath the car, so the underbody can be made relatively smooth. The air intake and outlet cross sections have been redesigned so that, notwithstanding the modification of the cooling system to cope with greater engine and braking performance, the Carrera retains its C_d value of 0.29. The Carrera S manages an improvement of 0.01 compared with its predecessor, giving it the same figure of C_d 0.29.

The rear spoiler has been reworked to improve the 911’s active aerodynamics. Depending on the vehicle configuration and sliding roof position, the spoiler extends to different heights and at different angles. Additionally, a flow element located on the leading edge of the spoiler is controlled by means of a pivoting mechanism, adjusting the airflow over the spoiler blade. This reduces the rear axle lift at maximum speed compared with the outgoing model by as much as 80 Newtons. The aerodynamically effective area of the rear spoiler has been widened from 898mm to 1137mm. It is extended automatically at 120km/h and retracted at 80km/h, but can also be activated manually.

Additional aerodynamic improvements were achieved by changes to the air brake spoilers, the exterior mirrors, the engine ventilation and fairings on the front wheels. The cooling fans, which now use brushless motors to save a kilogram in weight, now work on demand to reduce air resistance.

Finally, Porsche has specified low rolling-resistance tyres which, the Company says, grip just as well as the previous items.

Mass

According to Porsche, saving 45kg in kerb mass from one generation of 911 to the next has meant an intrinsic saving of 98kg. This is because the new model’s bodyshell and equipment had to allow for more stringent safety requirements; and also, of course, because the new car has a longer wheelbase.

For the first time, the new 911 Carrera uses a bodyshell constructed from steel and aluminium. The front end, middle and rear of the car are predominantly made of aluminium. The openings and wings are entirely made from aluminium, with the exception of the safety reinforcements in the doors.

The rear side sections with the outer roof frame are of soft deep-drawing steels and the door reinforcements, lower A-pillar and roof cross members are made of high-strength steel and advanced high-strength steels. Ultra-high-strength boron-alloyed steels are used for passive safety, in particular elements such as the inner roof frame and B-pillar. The different materials are joined by rivets and clinched connections, combined with structural adhesives. The dynamic torsional rigidity of the Coupé body has been increased by approximately 20 per cent. compared with its predecessor — Porsche has not quantified this.

Lightweight construction is also applied to body parts, with the use of aluminium, magnesium and plastic trims. For example, the dashboard cross member, centre console cross member and components in the rear seat backrest are made of magnesium.

The new modular roof also offers weight advantages. For the production vehicle without a sliding roof, the roof’s steel outer skin is replaced by aluminium. The new design of optional sliding roof, which is of the electric slide-tilt type, in conjunction with rail-to-rail technology, is slimmer and saves headroom. The materials used for the individual components in the sliding roof module are made from aluminium, steel and plastic.

More mass was saved in the add-on parts in the rear section of the car. The engine cover, for example, has lost more than 60 per cent. The rear spoiler module is based on a light, thin-walled aluminium die-cast bracket which helps to increase body stiffness. The struts are made of steel, aluminium, thermoplastic and fibre-reinforced plastic. The spoiler blade itself features a thermoplastic polymer surface, mounted on a rigid substructure. Despite being significantly wider, the rear spoiler’s redesigned blade weighs 20 per cent. less.

All in all, the total weight saving on the body accessories amounts to around 80kg.