The 2,144 cc ‘WR’ five-cylinder cast iron engine block of the original Group 4 cars was largely standard, but its ancillaries were optimised through 1980 to a new specification. The first requirement was reduced weight, the second was improved throttle response and finally the confection needed to be able to endure operating in extremes.

On all models from 1980-86, the engine was angled over by 27.5 degrees sloping left-to-right when viewed head-on (exactly as the road cars were). This arrangement helped to keep the front end as sleek as possible when the large inline engine was placed on top of the front wheels and drivetrain.

All of the rally engines were built by the specialists at Lehmann, who started by swapping the Bosch Jetronic fuel injection for Pierburg’s old-style mechanical system’s more direct approach to feeding the cylinders. Their main concern was power, and to the greater extent that meant the turbo, so the five-cylinder’s compression ratio was dropped from 7:1 to 6:3:1 in order to allow for higher levels of boost.

The standard KKK K26 turbocharger was fitted to the earliest cars but, at the 1981 Swedish Rally, smaller turbines were fitted to try and reduce the ponderous throttle lag of the original system. On the road cars the intercooler was mounted next to the turbo behind the right-hand pair of headlights but on the rally cars it was moved to a central position lower down – and after the oil cooler was put on the boot lid, the available space was filled with a larger intercooler.

Run as a dry sump engine, the oil tank was placed in the boot and fabricated, along with the fuel tank, by Matter. As with the road car, the reverse flow cast aluminium cylinder head featured hemispherical combustion chambers and two valves mounted vertically per cylinder. A new head was developed in time for the 1981 Sanremo Rallye to improve cooling.

From the 1981 Rally Portugal onwards, the standard single overhead camshaft was replaced by a new design with six bearings forged to optimise the spread of power for sharper acceleration. This first round of turbo modifications meant that the amount of available boost rose to a variable 1.5-1.8 bar.

In total, these enhancements took the standard ‘WR’ Quattro output of 200 brake horsepower and 206 Ib.ft of torque, and beefed it up reliably to 300 bhp above 5,500 rpm and a maximum 304 Ib.ft of torque, which was delivered at 3,250 rpm. By late 1981, output had risen to 320 bhp.

In order to future-proof the engine it was necessary to think ahead to when Audi Sport would start its almost endless battle to remove weight from the Quattro. To this end, a second homologation was made for an alloy version of the ‘WR’ block with GG cast iron cylinder liners, shaving 22kg (48lb) from the weight. This engine would be debuted on the 1982 Tour de Corse and continue into the Group B era inside the Audi Quattro A1 and A2.

The alloy engine development was led by Dr Fritz Indra, who had formerly produced such exotica for BMW’s competition and high-performance division. In total, the engine design team numbered four people, as the ever-forthright Dr Indra declared that more people would just get in the way.

The A2 featured the sleeved-down cylinders to give options of 2,110 cc or 2,135 cc in order to get the Quattro beneath the 3-litre engine capacity line laid out in the regulations, which in turn allowed the car to run at a total weight of less than 1,000kg (2,205lb). The smaller engine capacity was countered by fitting a more potent turbo, the KKK K27, with either a small 12-blade turbine or larger 14-blade turbine depending upon the characteristics of the event.

This was almost as far as development would reach on the original 10-valve engine, which in 1983-84 saw 360 bhp as the standard output. From 1983 onwards, attention in the engine shop shifted towards the new 20-valve ‘KW’ engine that would eventually be fitted in the short wheelbase cars. Some elements of the new engine were competition tested first on the 10-valve engines during the 1984 season, most notably a new ‘Umluft’ wastegate, which gave an additional 500 rpm in the optimum range and reduced throttle lag.

These were areas where the technology within the Audi Quattro began to integrate usable parts from Porsche’s dominant Group C sports car, the 956 and later 962 variant. While relationships between Ferdinand Piech and the Porsche family were seldom cordial, the two brands that they represented were able to work collaboratively to tremendous effect.

Such support was highly valuable when trying to solve the riddles of the Audi Sport quattro. Its 20-valve cylinder head and twin overhead camshafts immediately kicked available power up to more than 400 bhp. Another benefit of the new cylinder head was that the turbo was relocated away from the fuel lines, removing one of the major causes of conflagration in the early rally

Until the smaller car’s weight gain could be reversed, the performance from the 20-valve would be blunted. This was most clearly seen under acceleration, because the peak torque of the engine was 1,000 rpm higher up the rev counter than it was on the 10-valve, which meant a lot of the available grunt was squandered.

This is where the addition of Walter Rohrl’s forensic abilities as a test driver came into their own – not to mention his close ties to the Porsche werk. Endless variations of wheels and transmission were tried, but the lion’s share of the work fell to Bosch in getting the ECU to optimise the power and torque for the singular demands that the Sport quattro made.

Not until the start of the 1985 season were the fruits of this development programme truly revealed, when Rohrl showcased the much more usable Sport quattro, taking the lead of the Monte Carlo Rally from Peugeot and remaining in contention for victory until the very last stages. Of course, a fair degree of the performance was down to Rohrl’s love affair with the Monte, but Hannu Mikkola was not far behind.

The arrival of the Audi Sport quattro S1 then gave the team licence to throw caution to the wind. The full force of the 500+ bhp S1 remains astounding to behold, provoking sights such as Stig Blomqvist popping a wheelie as he left the start line on one stage. In Finland. On gravel.

The ultimate specification would be that used by Walter Rohrl to set a new record time at Pikes Peak. With only 20km (12.5 miles) to cover, this unique engine was claimed to have been putting out more than 600 bhp by Audi -but was confirmed years later by Rohrl to have been 750 bhp.

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