Last month we contemplated the likelihood of further high performance developments reaching the public from the abruptly curtailed careers of the Group B rally "supercars." The most important development strand has been that of all-wheel-drive. Now J.W. looks at some popular commercially available systems and a few lessons taught by their competition counterparts.

The British public is now offered a 20 car choice in 4WD, excluding the oft-road machinery of the Land Rover/Nissan Patrol school. The arrival of Ford with 4x4 versions of Sierra and Granada-Scorpio increased British sales of all 4WD cars from 2723 in the first four months of 1985 to 4967 this year, a profitable 82% sales boost to gladden the hearts of the Essex multinational's "bean counters".

The 4WD car market begins with the £4,872 Fiat Panda and culminates in unlisted Audi Sport Quattro (£58,000 for eight UK imports) and Porsche 959 (£100,000) machines capable of 155 and 186 mph, with many more hornologation runs of the required 200-off now available on the Continent. Ford's presence has undoubtedly increased the public's awareness of all 4WD cars. For example, Audi has practically doubled its UK sales in 1986, climbing from 541 to 976 assorted quattro sales.

Audi led us back into the 4WD performance car and finds a substantially different percentage of quattro customers in each different model line: under 5% of Audi 80/90 customers take 4WD. whereas 25% of 200 saloon buyers optfor the £24,000 quattro 200.

Understanding Audi 4WD throughout its commercial range is made easier by the fact that all models have a North-South front engine layout (four or five cylinders) ahead of the front axle line. All the Ingoistadter quattros for public sale feature the unique and expensively machined hollow shaft arrangement within their five speed gearboxes, allowing power to be fed forward from the central differential without a bulky external transfer case.

Hopefully our illustration will make the Audi's unique system clear. It is worth emphasising that only Audi parent VW has ever tried using the Audi system (the Passat syncro estate, no/sold in UK). It was not a particularly successful transplant and VW now has its own 4WD solution, one made necessary by the use of a transverse engine for the now universally accepted front drive formula.

Whilst the Audi system is compact and light (an average weight gain of under 70 lbs) it has been criticised since its 1980 debut on two main counts. Firstly, a fixed power split underlines the nose-heavy Audi's understeer in quattro trim. Also, it has been attacked because the differential locks (centre and rear) must be manually engaged when the going gets rough.

Audi has yet to offer the public anything but a 50-50 power split in the quattro, but from August 1985 the factory competed their S1 Sport derivatives with an uneven power split facility. This was major step. Audi had always used three differentials (front, middle and rear) for the quattros on sale, but had competed with only front and rear differentials previously, using a solid interconnecting shaft.

Naturally, Audi Sport's enormously powerful competition quattros were pretty hard to handle on tight tarmac curves. minus a central differential to equalise front and rear axle speed disparities. Yet Audi Sport's initial solution was to cut 12.6 inches from the wheelbase (Sport quattro) and add the 20 valve engine to catch a more sophisticated opposition than they had initially faced.

August 1985's 1000 Lakes Rally showed us that Audi had decided they would have to have a central differential for competition, after all, with an uneven power split biased to the rear. However, they didn't just come up with a central differential and the American Torsen Gleason limited slip control and leave it at that. For a subsequent winning appearance in Sanremo they used both two and three differential transmission layouts according to the surfaces and average speeds faced!

For the 1985 RAC Rally one works Audi appeared with the two differential layout and the PDK (Porsche Double (K) clutch) live speed. The PDK has also been seen in Group C use, but Audi used its own Volkswagen at Kassel-manufactured six speed gearbox for rallying more often in the months up to Audi Sport's retirement from rallying in June of this year.

Confirming Austin Rover Motorsport's view that transmission technology was critical to gaining further special stage speed, Audi also used Ferguson Viscous Couplings in association with its power splitting experiments, Lancia and Peugeot experimenting in Group B with Viscous, Torsen or ZF plate types of differential controls as they tackled rallying's wide variety of climates and surfaces.

As the engineers experimented with competition layouts to beat the opposition, one could find a multi plate limited slip differential installed on a "soft" torque setting at the steering end of a competition car, with a Torsen Gleason aft and a Ferguson Viscous Coupling for apportionment of inter-axle slippage.

Ford was one of the few to stick with the use of all three differentials of the VC type in competition (Group B RS200) and the same company championed their use as showroom technology dazzlers within XR4x4 Sierra (two. one middle, one rear) besides single installations for front drive (Escort AS Turbo) and rear drive (Sierra RS Cosworth).

Thus, although Audi gained a commercial and competitive advantage by its 1980 quattro system it is totally untrue to say that everyone else in the now flourishing 4WD market (or in competition) has slavishly imitated the quattro.

In competition terms Peugeot swiftly outmoded the front engine quattros, the French adopting a transverse mid-engine. uneven power splits via Ferguson. patented components including a central Viscous Coupling and both front and rear limited slip action by ZF, at least initially. Power bias changes were achieved by three primary choices in epicyclic gear sets to alter power splits at service halts between 25-75% front-rear, the most common 33-67% all round choice and a near replica of Audi's natural bias, actually 45-55% at Peugeot.

Why? Because the consensus of competition 4WD engineering opinion is that only minimal power need be fed to the front wheels on dry tarmac, but that ice demands the equal power delivery for traction.

Translating this into a 4WD system that will flexibly suit the enormous variety of public road surfaces has seen the World's finest engineers come up with anything but the Audi quattro system's permanent 50-50 power split and vacuum-operated differential locks.

The simplest and sometimes least satisfactory 4WD systems come mostly from Italy, France and Japan. They are basically front driven cars of the current transverse engine school that carry an additional rear differential to provide rear drive too. They lack the refinement of a central differential and are normally operated as part time 4WD Disengagement may be by an over-run freewheel or dog clutches, usually activated by a lever alongside the main gearchange.

Typical of this 4WD class with front and rear differentials only are Fiat's Panda 4x4, Citroen's Mille Pistes version of the Visa, or the Japanese Subaru, Toyota Tercel or Honda Shuttle 4WD. Normally an owner will be discouraged in the owner's manual from running 4WD permanently (Citroen is the exception with disengagement only necessary to park).

If you leave the 4WD engaged and try to park most of the examples quoted, the cries of distress from wheels and creaking transmission will soon tell you why a central differential is desirable to civilise 4WD from Japan and Italy. Toyota arid Subaru have both exhibited superior 4WD systems based on triple differentials, and Lancia has displayed stunning transmission capability around its new Delta HF 4WD, of which more anon.

As we have seen, Audi is in a class of its own, so next we have the makers of front engined, rear drive saloons who have gone into 4WD. Good examples are Ford Sierra/Scorpio, BMW 3-series and Mercedes 4Matic on its newest model line, 200-300.

Basically their problem is to add front drive with as little clutter as possible, but all of them have needed sonic sort of central transfer box from which emerges a front propshaft to teed an extra forward differential and halfshafts. Added weight is way up on Audi, typically in the 110 lb range, and the trio quoted have the inelegant but necessary solution of feeding one driveshaft through the wet sump area of their various six cylinder rnotors.

The advantages that Ford, BMW and Mercedes share are that they have used those central differential cases to also house planetary gear sets and provide an unequal power split (typically one third (34%) front and two thirds (66%) rear). This means this trio can be driven on slippery surfaces with a restrained rendition of rear drive's oversteer. This avoids many of the adapted front drive 4x4's depressing understeer characteristics, a trait many drivers find harder to contain in slippery circumstances.

Mercedes add complex microprocessormanaged electronics to supervise sophisticated central and rear differential multi-plate locking devices, preferring to run the car in two (rear) wheel drive until slipperiness is sensed and the electronics engage front drive and the locking of differentials in split second stages. Finally there is a new breed of front drive adaptations that seem to set new standards. First is the simple Golf twin differential layout on sync to It depends upon a propshaft tail-mounted Viscous Coupling (further developed over a Ferguson base) to effectively act as a power apportioning device.

The Golf will normally run with front drive characteristics and a nominal 5% power idly turning the back differential. When it hits ice and the front wheels lack grip, the VC senses the difference in axle speeds and feeds power rapidly to the rear wheels. When the VC senses the front wheels are gripping again, fwd status quo is swiftly restored.

The Golf is a fairly simple 90 horsepower carburettor model that will be coming to Britain this Autumn, hopefully costing less than £10.000. A new breed of hatchback 4WD from Mazda (323 Turbo. 16-valve) will also make its mark from August, and Lancia look certain to bring in its Delta HF 4WD by January 1987.

I don't know enough about the Mazda to offer any constructive technical comments on the transmission, but the Lancia is extremely sophisticated. Built on the transverse 2-litre powerpack of the Thema Turbo and crammed into the five door Delta, Lancia's 4WD features a trio of differentials that are unusually laid out and controlled. Lancia's "centre" differential is within the gearbox and the front drive layout has been straddled on one side by a Viscous Coupling (D/S) and on the other by an epicyclic gear set. The eptcyclic is designed to preserve front drive characteristics with a 56% front and 44% rear power split.

At the back a Torsen ("Torque Sensing") differential apportions power transversely according to grip. The wheel and tyre with most traction gets most power. All applied via the interaction of worm gears for each inner wheel shaft and planetary gears, the latter with helical patterns pivoting on the outer casing and in mesh with two crown wheels. In cornering action the planetary gears are activated, acceleratmg one crown wheel and slowing the other in response to differing wheel to halfshaft input speeds.

It is a remarkable package, part of an astonishing small car Next month I hope to convey some impressions of how the Lancia and the differing commercial and competition 4WD systems perform in a wide range of circumstances, group by group. hopefully assisting those who ask "Which 4WD is the best for me?" -- J.W.