The ADU engine of the Avant RS2 represents the pinnacle of the classic Audi 20-valve turbocharged five-cylinder engine development. It was created through the systematic evolution of the ABY engine previously used in the Audi S2 and was developed in close cooperation between Audi and Porsche. The objective was to significantly increase the performance of the proven 2.2-litre turbocharged inline-five while maintaining full everyday usability and long-term durability.
The engine was based on a robust cast-iron inline five-cylinder block with a displacement of 2,226 cm³, a bore of 81.0 mm, and a stroke of 86.4 mm. The aluminum cylinder head featured dual overhead camshafts and four valves per cylinder, for a total of twenty valves. This four-valve technology, first introduced in the Sport quattro and subsequently refined in the 3B and ABY engines, provided excellent gas-flow characteristics and highly efficient cylinder filling, even at elevated engine speeds.
Compared with the ABY, the ADU received a larger KKK K24-7200 exhaust-gas turbocharger, capable of delivering a greater airflow rate and offering significantly higher performance potential. This was complemented by a larger and more efficient intercooler that substantially reduced the temperature of the compressed intake air. The lower intake-air temperature increased charge density, improved knock resistance, and enabled higher sustained power output.
The fuel system was also extensively revised. Larger fuel injectors supplied the additional fuel required to support the increased performance. The Bosch Motronic engine management system received specially developed calibration maps for fuel injection, ignition timing, and boost-pressure control. Combustion was operated very close to the optimal performance limit, while cylinder-selective knock control ensured engine reliability. Under full-load conditions, the engine employed targeted mixture enrichment to provide additional cooling for the combustion chambers, pistons, and turbocharger.
Furthermore, both the intake and exhaust systems were optimized from a flow-engineering perspective. The exhaust system, developed with Porsche's involvement, featured larger pipe diameters and significantly reduced exhaust back pressure. This enabled the turbocharger to operate more efficiently and deliver greater airflow, particularly at higher engine speeds. At the same time, pressure losses in the intake tract were minimized, further improving cylinder filling.
The Bosch Motronic coordinated all engine functions through a sophisticated management system. It continuously monitored boost pressure, intake-air temperature, coolant temperature, lambda values, and knock activity. One of its most notable features was the overboost function, which temporarily increased boost pressure under specific operating conditions in order to deliver maximum torque. In addition, the software incorporated extensive protection strategies that reduced boost pressure or retarded ignition timing whenever critical operating conditions were detected.
The result of this comprehensive development program was an output of 315 hp (232 kW) at 6,500 rpm and a maximum torque of 410 Nm between 3,000 and 4,000 rpm. With a specific output of approximately 143 hp per litre, the ADU ranked among the most powerful production engines in the world when it was introduced in 1994.