Audi B6/B7 S4: BBK V8 engine

Introduction

Audi’s BBK was a 4.2-litre, 40 valve V8 petrol engine that was first introduced in the Audi B6 S4. Key features of the BBK engine included its:

• Cast aluminium alloy block with 90-degree ‘V’ angle;
• Die-forged steel crankshaft with five main bearings;
• Cast aluminium alloy cylinder head;
• Chain-driven double overhead camshafts;
• Five valves per cylinder actuated by roller rocker cam followers;
• Variable intake camshaft timing;
• Variable intake manifold;
• Electronic throttle control;
• A compression ratio of 11.0:1; and,
• Maximum engine speed of 7200 rpm.

Although based on the AQH/AVP/AYS/BCS engines in the Audi D2 S8, the length of the BBK engine was reduced by 52 mm to 464 mm so that it could fit in the engine bay of the Audi B6 S4. This reduction in length was achieved by transferring the chain drive to the clutch side of the engine and using chain-driven camshafts (rather than the toothed belt and chain arrangement of the AHC/AKH engine).

Block

The BBK engine had a cast aluminium alloy block with 84.5 mm bores and a 93.0 mm stroke for a capacity of 4172 cc. The die-forged steel crankshaft was supported by five main bearings and, attached to the crankshaft, the BBK V8 engine had forged connecting rods.

It is understood that the BBK engine had cast aluminium alloy pistons with specially-designed valve recesses.

Cylinder head

The BBK engine had cast aluminium alloy cylinder head that was mounted on a triple-layer metal gasket. The double overhead camshafts were chain-driven and used roller cam followers – with hydraulic adjustment – to actuate the five valves per cylinder (three intake valves and two exhaust valves). Furthermore, the BBK engine had variable intake camshaft timing.

Roller rockers

To minimise inertia forces and facilitate the 7200 rpm maximum engine speed, the roller rockers were made of die-cast aluminium. Compared to the mechanical bucket tappets of the 32 valve V8 engine, the roller rockers reduced frictional losses in the valvegear and halved the oil delivery rate in the cylinder heads. The rockers were supported by a spindle shaft which was also used to supply oil to the bearings and the hydraulic valve lifters. Valve actuation, however, differed between intake and exhaust valves:

• The two exhaust valves per cylinder were actuated by a twin roller rocker that, in turn, was actuated by a single cam by means of a roller located between the rocker arms; and,
• The three intake valves per cylinder were actuated by a triple roller rocker that was actuated by a double cam by means of two rollers between the rocker arms. For uniform compression between the two cams and rollers of the roller rockers, the spindle shaft of the inlet roller rocker had a convex shape to prevent the roller rockers from tilting.

For the BBK engine, coolant flow direction was the same as Audi’s concurrent V6 engines. As such, coolant flowed from the cylinder heads, merged in the rear coolant pipe and then flowed to the cooler.

Intake manifold

The BBK engine had a fresh air intake system with two branches to reduce pressure losses, with the branches joined upstream of the throttle valve. While the 3.7- and 4.2-litre 40 valve V8 engines had a three-stage variable intake manifold (VIM) made from die-cast magnesium alloy, the BBK engine for the Audi S4 had a two-stage variable intake manifold. The VIM was controlled by the engine management system and, at low engine speeds, a long intake runner was employed to increase pulsation effects and torque output. At higher engine speeds, however, the short intake runner was used to reduce intake resistance and increase power output.

Injection and ignition

The BBK engine had electronically-controlled, common-rail multi-point sequential fuel injection via eight injectors that were positioned in the intake manifold. The injection and ignition process was controlled by the Bosch Motronic ME 7.1 engine management system which used a hot film air mass meter to measure intake air volume, two sensors for cylinder selective knock control and two heated lambda probes for adaptive lambda control.

The BBK engine had a compression ratio of 11.0:1 and 1-5-4-8-6-3-7-2 firing order.

Exhaust

The BBK engine had an air-gap insulated exhaust manifold, with the exhaust pipes of the individual cylinders were assembled in a cloverleaf configuration for each cylinder bank (i.e. a 4-into-1 arrangement).

Secondary air injection system

The BBK engine had a secondary air injection system so that the catalytic converters reached their operating temperature faster. In this process, the fuel mixture was enriched to produce a higher percentage of unburnt hydrocarbons in the exhaust gas. The secondary air injection – located downstream of the exhaust valves – then enriched the exhaust gases with oxygen, causing oxidation (i.e. after-burning) of the hydrocarbons and carbon monoxide.

Audi S4: BBK V8 timing chain rattle

For the Audi S4’s 4.2-litre BBK V8 engine, the cam adjusters, tensioners and guides are susceptible to wear. As per the table below, this can cause camshaft over-advance codes to be issued, a rattling noise on start-up, engines misfires, a lack of performance and the vehicle to enter ‘limp home’ mode.

By removing the rear timing covers, the cam adjusters can be replaced without removing the engine from the engine bay. Replacement of the tensioners and guides, however, requires the engine to be removed from the engine bay since the chain drive is located at the rear of the engine.

Please note that while not all BBK engines have experienced these problems, failures have occurred at mileages from as little as 60,000 kilometres. To extend the life of the tensioner guides, frequent oil changes with high quality oil are recommended. For a discussion of this problem, please see Audizine: B6/B7 S4 Timing Chain FAQ.