This is a collection of online articles I have found with reference to the Opel/Astra 4x4. I can't guarantee it's accuracy.

Leading upto the Opel Astra/Kadett 4x4 Project.

In 1983 Norwegian Martin "Mister Rallycross" Schanche wanted a 4WD Ford Escort Mk3 to become European Rallycross Champion again, after claiming the title already in 1978, 1979 and 1981. From 1982 the sport of Rallycross was re-opened (!) for four-wheel drive cars and Austrian Franz Wurz (father of Alexander Wurz) claimed the 1982 title with an ex-Mikkola factory Audi quattro while the 1983 title went to Swede Olle Arnesson with a "homebrew" Audi quattro. In the by then for Hewland working British engineer Mike Endean that man Schanche found somebody capable to realise his own idea, a 4WD that could be changed by hand while competing from 28:72 (front:rear) drive stepless to 50:50 by a hydraulic system. The resulting 4WD was called Xtrac (not X-Trac nor X-trac as often seen) system. The car was ready for the British Rallycross Grand Prix at Brands Hatch in December 1983, where Martin used it with an 1860cc 560bhp Zakspeed turbo engine. Mike Endean was there as well as Erich Zakowski (Mister Zakspeed) and Peter Ashcroft of Ford to witness the Norwegian. In 1984 Schanche won "his" title back and all other cars were looking like wheelbarrows against the XR3 that we used to call "Thor’s Hammer". (The Swedish mag 'Teknikens Värld' conducted a car test in 1984 to find out that the XR3 did the 0 to 100km/h [or 0 to about 61mph] sprint in 2.5 seconds.) BTW, that first Xtrac XR3 was later used for Rallycross by the Britons John Smith and Barry Squibb and is nowadays owned by Mike Endean himself. It is rebuilt to Brands Hatch 1983 specs and was used by Endean, who lives as pensioner on one of the Channel Islands, for many a year in hill-climbs.

Schanche and Endean in 1984 were trying to sell their brainchild to Ford (working on the stillborn Escort RS1700T RWD by then) or other manufacturers. While being at the Zakspeed workshops one day Schanche was visited by Rauno Aaltonen who did a test drive with the Xtrac Escort on a car park. Aaltonen was very impressed and told Opel about Schanche’s beast. Opel’s Karl-Heinz Goldstein encouraged Endean soon to work for the German factory team, while Schanche found himself somewhat out-booted. However, Endean left Hewland to set up his own company Xtrac and started to work with Opel on their Group S project, the Opel Kadett 4x4 (not a Group B car and not a Vauxhall Astra 4S or whatever). When the car was ready the drivetrain was the same as in the Schanche XR3 and when it was presented to the press the bonnet was safely locked. Why? Because there was a FORD engine underneath it, a 1860cc Zakspeed turbo mill, and it was all but in the interest of the Opel factory team that the World or even their own GM bosses should know anything about that fact.

However, when the Group S was aborted by the FIA Opel had two 4x4 Kadetts ready, equipped them with normally aspirated 2.4 litre Opel engines of the Ascona 400 and used them for Paris–Dakar, where they went into utter chaos, as Opel was not able to carry enough shock absorbers to Africa to replace the broken ones. Both these cars later went to Endean’s buddy-buddy Briton John Welch who used them in the British and European Rallycross Championships. One of these cars was later bought by Swede Tommy Kristoffersson, as an Xtrac system donor for his own Rallycross Audi Coupé S2. If the other one is now property of Vauxhall in the UK – I do not know.

Source Unknown: From http://forums.motorsport.com/forums/showthread.php?t=112669&page=3

Opel's GroupB Manta's Paved the way for the 4S.

Talking rally car and groupB developments at Opel in that era, it is interesting that indeed a 4x4 version of the Manta 400 existed. This prototype carried the reg plate GG-CM 537, which bears an interesting similarity to the reg plate of the car used for the Manta 400's WRC debut (both registered at same time). However the 4x4 conversion was carried out by British company Ferguson for Opel. Why this 4x4 prototype was never a route followed further is hard to guess. Indeed it is only my guess, but since it had the same relative low torque 2.4 atmospheric engine, I doubt the Ferguson 4x4 equipped Manta 400 was actually faster than the RWD one.

As it seems Opel realised already before its launch that the Manta was not a big enough step forward, so not long after its launch Opel started working on a proper group B supercar, called Opel Kadett 4S. Opel showed some fantasy with the car as - apart from 4x4 - it had a mid engine that was located behind the front axle rather than in front of the rear axle. Early versions had a 500BHP Zakspeed Turbo engine, but then the discussion of replacing group B by group S for the end of 1987 came and Opel replaced the Zakspeed engine by its known Manta 400 engine, making this an ideal group S car. However the accidents in 1986 meant group B was stopped and group S never happened, such the Kadett 4S was a stillborn project. The groupB Kadett 4S actually even started one rally, the British national Audi Sport Rally, where Andrew Wood finished 4th behind 2 Ford RS200 and a Metro 6R4. This event was in October 1986, 2 months before the ban of groupB, which very much proves how unlucky the timing of the project was.

The car was developed by Opel alongside the Manta B 400 model which consisted of the same changes. Irmscher and Cosworth were hired as partners for the project, Cosworth to deliver a 16 valve double cam crossflow head for the engine, and Irmscher for the exterior and interior styling. Cosworth delivered the heads to Opel and Opel soon discovered a major mistake. The plan was to use the 2,0 litre engine block but this did not produce enough power. Time was running out and Opel badly needed to do something. Opel took the 2,0E block and gave it an overbore, installed larger pistons, other pistonrods, and installed the crankshaft of their 2,3 litre diesel CIH style engine. Results was a 2,4 litre engine. The 2,4 litre engine gave way to some massive power outputs using the 16 valve head. The street versions of the 400 therefore came with 144 bhp (107 kW; 146 PS) engines, using the Bosch fuel injection of the Manta GSi and GT/E series. However in race trim they were delivered putting out some 230 bhp (172 kW; 233 PS), which could be improved further to a staggering 340 bhp (254 kW; 345 PS) + still using normally aspirated engine components.

The Opel Ascona i400 project, and later Manta i400, was put into production in 1978 in collaboration with Cosworth. Opel had delivered cylinderheads and parts from their 2-litre 16-valve project engines and Cosworth took up the development and improvement. It is therfore not accidential that the i400 cylinderheads has 20 degrees valveangles as well as light tappets. The development of this engine was acutally started already in 1972, and Cosworth revised the pistons, conrods and crankshaft and ended up with a displacement of 2400 ccm and a poweroutput of 240 Hp. The crankshaft has 8 counterweigths and a stroke of 85mm, which makes it almost identical to some of the crankshafts from the Omega A C24NE engine. In standardspec with injection, that Opel produced for the streetversions of the Ascona/Manta i400, the engine provided 147 Hp. In Phase 1 the effect was raised to 240 Hp, Phase 2 at 261 Hp, Phase 3 at 280 Hp and Phase 4 at 300 Ho. Today effects of more than 345 Hp can be achieved with a fully optimized 2.5 litre i400 engine.

Source http://www.rallye-info.com/carspecs.asp?car=219

GroupS was dropped before it started.

After a second fatal incident during the 1986 rally World Championship regulators pulled the plug once and for all on Group B cars, in a stroke sweeping aside the hive of exciting and technically interesting Group S cars that were gestating behind the scenes.
Before this happened though, some of the secret manufacturer supercar projects were well underway and a skeleton regulation structure was already in place. ‘After many meetings with the manufacturers we arrived at the conclusion that we've got to stick to around 300bhp. Ten cars will be the minimum, but it's not impossible that the manufacturers will build 20 or 30, all strictly identical, which must be homologated and cannot change during that year.
‘There will be no evolution during that year, but it will be no problem for the following year - just produce 10 cars at the beginning of each year. That puts each manufacturer on the same footing at the beginning of the year.' That was FISA technical commission president Gabriel Cadringher speaking in the mid-'80s of the planned Group S regulations. These were never definitive, but Cadringher's ideas were to mandate side, front and vertical impact tests, obligatory steel rollcages and a minimum weight of 1000kg. In its attempt to regulate a maximum 300bhp, FISA's Group S rules were expected to allow 2.4-litre normally aspirated engines and 1.2-litre forced induction units. Group S cars would have been allowed to run in 1987 WRC rallies, but not for points, with the intention of contesting the complete 1988 World Rally Championship.
But just one month later FISA decided to stop the rally supercars short. Group B cars were outlawed from the beginning of the 1987 season, and FISA's proposed revision of the supercar rules was stillborn, leaving Group A the top level of contemporary rallying.

In response, all non-Lancia works drivers signed a four-point statement that was issued to FISA. It read as follows: ‘The drivers totally support FISA's efforts to control both spectator and driver safety, but we feel that the current proposals will not achieve the desired results and would request that consideration be given to the following points:

1 The drivers are very concerned about the use of turbochargers from both the safety point of view and that of control of power in rallying.
2 The drivers are very concerned about the use of plastic and inflammable materials in current rally cars.
3 The drivers agree with FISA's objective of a maximum power in rally cars of 300bhp but wish to point out that current Group B cars have developed many safety features by way of suspension, steering, brakes etc, which will not be available in Group A cars.
4 We recommend that the use of slick tyres be completely banned in rallies.
Given all the previous points, we believe that a total change to Group A for World Championship rallying is not a solution to all the problems that currently exist. Many manufacturers without suitable Group A cars will be forced out of the sport and, as a result, we would respectfully request that FISA gives consideration to a Group S formula with the following major characteristics:
A Normally aspirated engines, maximum power 300bhp.
B No plastics or inflammable materials.
C Limitation on aerodynamic devices.
D A crash test for all rally cars.
E Minimum production qualification that will allow as many manufacturers to contest World Championship rallies as possible.
‘With the introduction of the measures listed above, the drivers believe that safety in rallies will be greatly improved without destroying the stability of our sport.'
Unsurprisingly, the drivers didn't get their way, but many agreed with them and, given the lead time in production, prototype Group S cars were already cropping up in likely places.

Fiat's Experimental Composite Vehicle (ECV) was Lancia's answer. Its S4 Group B car had a tubular steel frame, which was replaced in the ECV by a load bearing carbon fibre and Kevlar tub. In the show car even the wheel rims were carbon fabric with aluminium honeycomb centres. There was now a composite propshaft, and the ECV retained the S4's Hewland gearbox, together with its linked turbocharger/supercharger induction arrangement.

(The last part re- supercharger is inacurate btw.. the ECV engine had 2 sequential turbos.)

As for Peugeot, PTS chassis engineer Jean-Claude Vaucard explained their position at the time of the ban: ‘There was a lot of scope left in the 205T16, particularly with new technology for the transmission. We were at the beginning of four-wheel drive, and if we were to stay at that level [in 1986] it would have been archaic.'
When Group S was mooted, the Peugeot team reasoned that because the front wheels prescribe greater radii than the rears in a corner, the locking characteristics of the viscous couplings used at the time were the reason for the understeer generated on corner entry.
Consequently, adjustable mechanical slip limiters were being developed for the centre and axle diffs of PTS' Group S transmission - until the ban came - but the car's concept was not in dispute: ‘We were always going to have a rear-mounted, mid-engine layout,' explained Vaucard, ‘but the exact definition of the concept depended on the final engine regulation. With the forced induction equivalence ratio that FISA had decided for Group S we were going to use a 1200cc turbo. But we didn't think that was a good solution because it was expensive, runs at very high speeds and will break often.'
Like Lancia, PTS had also started to build the basic bones of its Group S car and, amongst other technological advances, twin dampers at each wheel were proposed, but there was scepticism over active suspension: ‘If you can change the car's level very quickly it can be an advantage from one stage to another, but when you change the level of the car you change the camber too, and therefore any advantages inherent in the tyres are finished. What you gain by having an optimised ride height you lose out by at least three times by not having good camber.'
Testing a 205T16 with an automatic clutch showed it was 0.5 seconds quicker from rest to 400m than a conventional car, but Vaucard discounted any possibility of using a twin-clutch gearbox. This was considered too heavy and not beefy enough to during this period, John Wheeler, also considered having a front engine behind the front axle the way to go for good weight distribution in a rally car that is to be used off road. But in the end with the company's RS200 rally car this was outweighed by the disadvantages of excessive heat in the footwell, packaging of the steering and exhaust, catering for left and right-hand drive and front driveshaft lengths. In the end, the team achieved a similar effect with the engine mounted at the rear and the transmission as far forward as possible.
It's likely then that a Ford Group S concept would have needed little variation from the basic RS200 philosophy to be competitive, though Wheeler concedes ‘it does look as if Audi were further down the line to Group S, but Lancia and ourselves were in rather different positions - we both had cars which were a step further toward Group S.'

Most teams designing a Group S car favoured the rear-mid-engined format. Toyota was one, with its transverse in-line four or longitudinal V6 (or four cylinder) sitting behind the occupants and driving all four wheels. Lotus was rumoured to have been involved in this Group S development, and in 1985 a prototype was tested in Eskdalemuir forest by team boss Ove Anderson and driver Bjôrn Waldegärd. It also tested at the Bagshot military proving ground in England, before disappearing off the radar.

Then, out of the blue, the four-cylinder prototype appeared in public at the Goodwood Festival of Speed in June 2006. Both the four-wheel drive Toyotas were designed before it had been decided to restrict forced induction Group S engines to 1.2-litre, so the turbocharged in-line twin cam four of 2.1-litres seen in the Goodwood car had a maximum output of... 750bhp.

The transverse car then seen at Goodwood the following year was the actual one that was tested in 1985. It is clad, as is its sister, in lightweight carbon/Kevlar bodywork resembling that of a Mk1 MR2, covering a tubular ‘birdcage' chassis/roll over frame. The weight of the transverse car totalled about... 750kg.
The then President of TTE, Ove Andersson, did most of the early testing of this car before the programme was cancelled. A very experienced and successful rally driver in his own right, Andersson has driven the gamut of rally cars, including rear-engined Alpine A110 Berlinettes and the Lancia Stratos - a short wheelbase, rear transverse engined, two-wheel drive car, yet he stresses that it is an understatement to say that driving the Toyota Group S car was unpredictable: ‘You never knew what it was going to do. With such a short wheelbase and such power in such a light car it could swap ends at any time, and without any warning.'
While discussing the various Group S designs that were around in 1986, and the potential performances and stage speeds of these extreme machines, Andersson, the experienced rallyman turned racer, reflected on the ‘86 FISA ban: ‘I think we were lucky, you know!'

Source Unknown: From http://www.motorsportforums.com/forums/showthread.php?t=112669&page=4&highlight=opel+kadett

Opel Kadett 400


Opel Kadett 400 driven by Tony Pond/Richard Leeke on the Nissan Rally in Cape Town, April 1984

History of the Kadett 400

The Opel Kadett was a rear-wheel drive version of the GT/E that underwent testing and rallying in South Africa where events are open to non-homologated cars.
The first car, one of three that was built in Germany before being taken to pieces and air freighted to South Africa in March 1984, was part of a project that was schrouded in secrecy.

The car itself utilised the normally aspirated 2400 cc engine, gearbox, suspension, axle and transmission from the Manta 400, but differed in looks in that it did not have a boot.
Weighing in at just over 1000 kgs, it was also a little lighter than the Manta despite still having steel doors and panels. Its aerodynamic devices included a tongue that protruded from the front spoiler and a lip that was attached to the top of the hatch.

At the time of its first appearance in South Africa, it was speculated that the car could be a basis for a future four-wheel drive challenger, although the engine might change.

In Tony Pond's hands, the Kadett 400 showed its handling potential in the Nissan Rally, although the car seemed slightly top-heavy and the car lifted its front wheels rather easily.
Pond was able to maintain second place behind van der Merwe's Audi Quattro and even set three fastest times out of the nine stages he completed, but retired on the tenth stage after the oil pump drive failed.