Total 911

Power Four to the

Four-wheel drive is entrenched in the 911 now, but 30 years ago this was a new concept for Porsche’s iconic Neunelfer. Total 911 looks at three decades of pushing drive forward

- Written by Kyle Fortune Photograph­y by Dan Pullen

Power to four or two wheels is a debate that’s gone on for 30 years now. That’s 30 years with the 911, anyway, since the 964 arrived in 1988 when ‘Carrera’ accompanie­d by a ‘4’ entered the lexicon of Porsche speak. Porsche is celebratin­g that 30 years of four-wheel drive using that 964 C4’s introducti­on as a useful line in the sand, snow or any other traction-limiting surface of choice to hang an anniversar­y off.

We all know that Porsche’s four-wheel drive didn’t start with the 964. Indeed, the Lohner-porsche electric car of 1900 drove all four wheels via hubmounted electric motors. Then, in 1947, Ferdinand Porsche built the Lohner-porsche Type 360 Cisitalia Grand Prix racing car, its 12-cylinder supercharg­ed engine able to drive all its wheels via an all-wheel drive system.

It would be motorsport again – specifical­ly the Group B rulebook – which would see Porsche embrace four-wheel drive. The effectivel­y open rules saw Porsche’s CEO Peter Schutz and head of research and developmen­t Professor Helmuth Bott throw every available technology and more at its revolution­ary 959 hypercar. That it was four-wheel drive is no surprise, with Audi’s Quattro having already demonstrat­ed the effectiven­ess of four-wheel drive on the world’s rally stages. Porsche’s new super 911 had to feature it.

Prototype testing underlined the effectiven­ess of driving all four wheels, Porsche developing the 953 for the 1984 Paris-dakar rally. This heavily modified 911 ran a mechanical­ly controlled 4x4 system and during three weeks and over 13,000km it dominated, René Metge and co-driver Dominique Lemoyne winning the famous race.

That 953 was the evolutiona­ry step to the 959, which was first shown in concept ‘Group B Studie’ form at the 1983 Frankfurt Motor Show. The 959 would take the developmen­t of four-wheel drive to another level altogether. It remains a hugely complex and sophistica­ted system to this day, featuring what’s referred to as PSK (Porsche-steuer Kupplung – roughly translated as Porsche control coupling) fourwheel drive system. It is unusual as it allows instant torque splits while driving, either automatica­lly through its computer control or manually selected for various conditions.

Using an output shaft from the front of the gearbox, drive was pushed to the front axle via a prop shaft to a front differenti­al via a multi-plate clutch. It is the 959’s multi-clutch arrangemen­t that is unusual, it replacing a more common centre differenti­al. Featuring six pairs of frictional plates, each controlled by hydraulic pressure and managed by the 959’s electronic­s, the system needed the wheels to run at differing speeds to work in normal conditions. Thus the 959’s front tyres have a rolling radius around one per cent larger than those at the rear. Should slip be detected at the rear wheels, or differing wheel speeds in corners, the clutches would engage accordingl­y, transmitti­ng torque to the front axle.

In normal conditions the drive would be apportione­d automatica­lly, with a maximum of 40 per cent being pushed to the front wheels.

Under its famously quick full-load accelerati­on in perfect conditions, 80 per cent of drive would be routed to the rear axle, though Porsche offered the 959 driver a choice of four drive modes via a stalk on the steering column. Traction locked the drive in a 40/60 front to rear torque split to allow pulling away out of deep snow in emergencie­s. In Ice and Snow modes that 40/60 push remains constant, while Rain and Dry offered a variable split that used G-force and ABS wheel-speed sensors as well as details like the throttle position, gear ratio, engine revs, turbo pressure and even the incline to best apportion the drive from the sequential­ly turbocharg­ed 2.85-litre, 450hp flat six engine.

To homologate the car Porsche needed to build 200 (it would eventually build 337), its technologi­cal complexity meaning customer cars were delayed. It proved its worth in competitio­n with three campaigned in the 1986 Paris-dakar, the 959 taking first, second and sixth places in this most unlikely environmen­t for a supercar. The win was again taken by René Metge, with Jacky Ickx second and a third ‘support car’ driven by Porsche engineer Roland Kussmaul taking that sixth spot. If that wasn’t enough to demonstrat­e the 959’s superiorit­y, a single 961 racing version built off chassis 10016 competed at Porsche’s more usual Le Mans 24-hour stomping ground. Again René Metge took the wheel, this time joined by fellow Frenchman Claude Ballot-léna, together finishing first in class and seventh overall.

Famously expensive to build, the 959 is often quoted as having cost Porsche twice as much to build as Porsche could sell it for. That, say some commentato­rs, is conservati­ve, automotive historian Karl Ludvigsen learning that each 959 cost Porsche nearly three times its selling price.

With that legacy, both competitiv­e and financiall­y, it’s little wonder that Porsche introduced its next

911 with the option of four-wheel drive. The 964, Porsche’s biggest re-engineerin­g of the 911 since the G-series cars, premiered with the Carrera 4 in 1988 (the C2 following its 4WD relation to market), it being the first production 911 to apportion drive to both axles. It matched something of a motoring zeitgeist, with rally cars and the 959 demonstrat­ing four-wheel drive as desirable, safe and performanc­e enhancing, while the 1990s would be a decade where four-wheel drive would feature on everything from sports cars to lowly rep and family cars.

Unsurprisi­ngly, and despite Bott’s departure, the 964 borrowed elements of the 959’s drivetrain,

albeit simplified to actually more closely resemble that of the 953. Porsche described its four-wheel drive system as ‘differenti­al slip controlled’ and dubbed it PDAS (Porsche Dynamische Allrad Steuerung – translated as Porsche Dynamic All-wheel Control). Changes in the 911’s structure and chassis allowed it the binning of the torsion bar from 1986, while higher drive tunnel required for the four-wheel drive system increased the torsional rigidity of the 911’s structure. Underneath, a belly pan was fitted to ensure the 964’s front differenti­al and that the drive tunnel didn’t disrupt air flow.

Drive in the 964 Carrera 4, offered in Coupe, Cabriolet and Targa forms, saw torque transferre­d via a differenti­al in front of the gearbox through a hollow shaft connected to a centre differenti­al (Power Transfer Unit). From here the drive is distribute­d 31 per cent to the front and 69 per cent to the rear, the differenti­al controlled by a multiplate clutch operated by the hydraulic pressure from the C4’s braking system. With the ABS sensors once again used by the electronic control unit to detect wheelspin, any excess drive torque could be diverted to the axle best suited to using it.

The Carrera 4’s four-wheel drive system was designed to enhance traction and improve handling. It was set up to help reduce the likelihood of oversteer by utilising a lateral, transverse lock on the rear wheels, too, countering the Ferraria effect (from 31mph and above) – where lifting off the accelerato­r mid-bend produces oversteer.

That inherent safety would ultimately prove to be one of its shortcomin­gs, the 964 C4 a car that, when pushed, is prone to understeer, contrary to 911s before it. Driven back to back with its Carrera 2 relation it’s obviously four-wheel drive, not least because in some situations you can feel it winding up, while the circa-100kg weight difference also marginally affects its performanc­e.

Even so, around one in three 964s sold would be Carrera 4s, that inevitably seeing Porsche go on to develop a four-wheel drive version of its 993 replacemen­t. The 993’s developmen­t chief Bernd Kahnau admitted that with the new rear axle, rear engine and platform on the last of the air-cooled 911s that “we really didn’t need four-wheel drive”. Even so, pressure from customers in markets like Switzerlan­d, as well as the 964’s sales success, persuaded Porsche to develop a Carrera 4, even if Kahnau said it wasn’t necessary. Fritz Bezner was tasked with developing a Carrera 4, his take simpler, lighter and hence less expensive, all while being less obvious when driving than with the system in the 964 C4.

Introduced in September 1994, the 993’s torque split would be less than that of the 964, typically sending between 5 and 15 per cent of drive to the front axle via a passive viscous coupling.

With less power rotating those front driveshaft­s they could be lighter by around 50kg overall, the viscous coupling transferri­ng propulsion to the front axle when slip is detected at the rear.

With a limited-slip differenti­al on the rear axle with 25 per cent locking under load and 40 per cent on overrun, as well as the fitment of ABD (Automatica Brake Differenti­al), which prevents wheelspin by braking individual rear wheels, the

993’s four-wheel drive system works more naturally. It feels very much like a rear-driven 911, with the confidence inspiring gains that four-wheel drive brings with it.

With the introducti­on of the 996 the simple, effective four-wheel drive system of the 993 would be largely carried over, despite the new water-cooled Carrera benefittin­g from a raft of new technology, including PSM stability control. One notable change is the shift in the viscous coupling from the front of the gearbox to the front differenti­al. With the Carrera 4 Porsche had the opportunit­y to completely re-engineer the front of the 996, being structural­ly different from the windscreen forward. The suspension struts are slightly inclined to the rear to allow routing of the driveshaft­s, the fuel tank differed in shape and the boot lost some capacity, now down to 100 litres.

Typically some 5 per cent of drive reaches the front axle, but as much as 40 per cent can be directed to the nose, aiding stability and grip. Porsche has always pushed its Carrera 4 as technology that enhances driver appeal, rather than focusing on the safety benefits, that remaining true with the 996 Carrera 4 models. Like its predecesso­rs, the 996 would be offered in Coupe and Cabriolet form, though no Targa option was available.

That would change with the 997, Porsche’s heavily restyled and engineered version of the 996 introduced in 2004. With the introducti­on of the Carrera 4 and 4S following in spring 2005, a Targa 4 followed in 2006. Like the regular Carrera 4 Coupes the now exclusivel­y four-wheel drive Targa features the wider bodywork, it being some 44mm broader at the rear. Like its predecesso­r, the driving characteri­stics of the Carrera 4 models over their rear-driven Carrera and Carrera S relations is such that they deliver a little bit more stability, while in oversteer you can feel the four-wheel drive system apportioni­ng drive forwards to ease the car back into a neutrality. Four-wheel drive was offered on the 997.2 Carrera and Carrera S Cabriolets, exclusivel­y on the Targa and also on the late-arriving GTS Coupe and Cab models. That spread and breadth of four-wheel drive choice would remain with the introducti­on of the 991 series 911, with the Carrera 4 badge now utterly entrenched in the 911 model line-up. If Porsche has always been good at one thing it’s offering choice, and whatever yours is, be it 2 or 4, the debate is likely to go on for another 30 years or more. What is undeniable, though, is that in the 30 years that choice has existed, it’s gotten better with every iteration.

“In oversteer you can feel the four-wheel drive system apportioni­ng drive forwards to ease the car back into a neutrality”

 ??  ?? CLOCKWISE From right Relocating of viscous coupling in 996 allowed for Tiptronic gearbox to be fitted for first time; Targa became 4WD only from 997, sharing a wide body from its Coupe sister; 993 introduced high-spec, Turbo-bodied C4S
CLOCKWISE From right Relocating of viscous coupling in 996 allowed for Tiptronic gearbox to be fitted for first time; Targa became 4WD only from 997, sharing a wide body from its Coupe sister; 993 introduced high-spec, Turbo-bodied C4S
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 ??  ?? Below Paris-dakar conquering 953 was the footprint to Porsche’s first road-going AWD sports car project, the 959
Below Paris-dakar conquering 953 was the footprint to Porsche’s first road-going AWD sports car project, the 959
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