The Audi C5 RS6 4.2L V8 BCY powerplant is one of the most solid and powerful production engines ever produced.
This still holds strong today, with some examples above 200,000 miles and performing at original power levels. The internal combustion engine is an all-aluminium alloy 4.2-Liter Twin-Turbocharged (Bi-Turbo), double overhead camshaft (variable valve timing), 5-valve per cylinder version of Audi's 4.2-Litre 90° V8 engine.
It was developed and manufactured in Germany and the UK by Quattro GmbH and Cosworth Technology (now known as MAHLE Powertrain). With the addition of the two turbochargers (one per cylinder bank), the power output of the V8 engine was increased to 450 brake horsepower, and generated 428 pound-foot of torque. The combination of the 4.2-litre V8, variable inlet valve timing, and twin turbochargers gives the RS6 an exceptionally wide power band, with peak torque available practically all the time at 2,000-5,600 RPM.
Based on the existing 4.2 V8 BCY from the Audi C5 S6, this engine was tuned with the assistance of VW Group subsidiary Cosworth Technology (now MAHLE Powertrain), and featured two parallel turbochargers, known as 'biturbo', with two side-mounted intercoolers (SMICs). Enlarged and modified intake and exhaust ports on the new five valve cylinder heads, together with new induction and dual branch exhaust systems, a re-calibrated Motronic engine management system, revised cooling system, and decorative carbon fibre engine covers complete the upgrade.
Further detail of the engine includes fully sequential electronic multi-point fuel injection with intake manifold-sited fuel injectors, mapped direct ignition system with solid-state high voltage distribution using eight individual single-spark ignition coils, NGK longlife spark plugs, and three knock sensors. Engine management uses a Bosch Motronic ME 7.1.1 engine control unit, which controls all functions of the engine operation; including fuel delivery, ignition system, valve timing, emissions control, and torque reduction control—the latter operating in conjunction with the the Bosch ESP 5.7 Electronic Stability Program, as part of the "Anti Slip Regulation" (ASR) traction control system.
Transmission – 3 Shift Mode (Drive, Sport, Tiptronic) 5 Speed Automatic
The RS6 has an electronically controlled five-speed ZF brand, 5HP24A model, Tiptronic automatic transmission with lock-up torque converter in all forward gears. It has "hill detection" capability which applies braking until forward movement after stopping on an upslope. The transmission control unit (TCU) incorporates "fuzzy logic" with its "Dynamic Shift Program" (DSP), with the shift pattern adapting to suit individual driving styles. The DSP will adapt to the users shifting style in Tiptronic, and apply that to the Drive setting. It will also adapt to the drivers use of heavy or light throttle, and related shift points (typically the more throttle the later the shift occurs). The Tiptronic manual control can be achieved with either the gear selector shift lever, or the steering wheel mounted paddles. The option to have two manual shifting locations is very helpful depending on style of driving or conditions. The TCU will automatically shift up at redline (6,500 RPM), and down at around 1,000 RPMs when in Tip mode. You can also manually shift using the steering wheel mounted paddles when in either Drive or Sport mode, and the computer will then revert back to Drive or Sport automatic shifting if no other manual paddle shift is made within 30 seconds.
This transmission is however known to have weaknesses, especially when addtional power from a modified ECU/engine is applied. There are a few companies in the U.S. that rebuild and upgrade the stock transmission for between $3-6k. Labor to replace the transmission is around $2-3k. With associated parts and fluids used during a transmission replacement, parts and labor can easily total around $10,000; not for those faint of heart or wallet. The stock torque converter is also very prone to failure due to a weak internal part from the factory, but has been replaced on almost every car at this point.