Exhaust System: Description and Operation
Exhaust System4.4L V8 COMPONENT LOCATION - REST OF WORLD (ROW) FROM 2006MY ONWARDS AND NORTH AMERICAN SPECIFICATION (NAS) UP TO 2007MY
NOTE: X = Service repair cut point
4.4L V8 COMPONENT LOCATION - NAS FROM 2007MY
NOTE: X = Service repair cut point
EXHAUST SYSTEM
The 4.4L V8 stainless steel exhaust system is supplied from the factory as a one-piece assembly and is connected to the engine with two exhaust manifolds. It is attached to the underside of the body with six rubber mountings.
The system consists of a two downpipes with flanges which attach to the exhaust manifolds. The downpipes are welded into the catalytic converters. Two intermediate pipes are welded to the opposite end of the catalytic converters and connect into welded joints with the two front cylindrical silencers. Two connecting pipes are welded between each front silencer and the rear silencer assembly. Internally the rear silencer assembly is divided into two independent silencers each with its own tailpipe. The tailpipes on the naturally aspirated derivative are curved downwards and not visible from the rear of the vehicle.
Although supplied from the factory as a one-piece, welded assembly, in service sections of the system can be replaced individually. Service joints are identified by indentations on the pipes, to show where the exhaust pipes can be cut to accommodate the replacement service sections.
After the replacement sections are positioned, the system integrity is restored by sealing the service joints with one piece repair sleeves.
NAS Vehicles - From 2007MY stub pipes are used to connect the exhaust system to the new exhaust manifolds. The stub pipes contain a threaded boss which provides for the fitment of the pre-catalyst HO2S (heated oxygen sensor). The threaded boss for the post-catalyst HO2S (heated oxygen sensor) is relocated to a position mid-way between the starter catalyst and the main catalyst.
EXHAUST MANIFOLDS
ROW Vehicles - The fabricated stainless steel twin skin exhaust manifolds are unique for each cylinder bank. The bank B manifold has a connection for the EGR (exhaust gas recirculation) pipe. Spacers on the securing bolts allow the manifolds to expand and retract with changes of temperature while maintaining the clamping loads. Heat shields are integrated into the exhaust manifold gaskets.
NAS Vehicles - From 2007MY new manifolds are fitted to comply with NAS LEVII emission regulations. The use of these manifolds require the use of stub pipes to connect between the manifolds and the exhaust system down pipes. The fabricated stainless steel twin skin exhaust manifolds are unique for each cylinder bank. Spacers on the securing bolts allow the manifolds to expand and retract with changes of temperature while maintaining the clamping loads. Heat shields are integrated into the exhaust manifold gaskets. The bank B manifold has a connection for the EGR (exhaust gas recirculation) pipe.
CATALYTIC CONVERTERS
The engine management system provides accurately metered quantities of fuel to the combustion chambers to ensure the most efficient use of fuel and to minimize the exhaust emissions. Two threaded bosses are located each side of the catalytic converters to house the pre and post catalyst oxygen sensors. The engine management system monitors the sensors and uses the information to further improve the fuelling and exhaust emissions.
To further reduce the carbon monoxide and hydrocarbons content of the exhaust gases, two catalytic converters are integrated into the front down pipe from each exhaust manifold. In the catalytic converter the exhaust gases are passed through honeycombed ceramic elements coated with a special surface treatment called 'washcoat'. The washcoat increases the surface area of the ceramic elements by a factor of approximately 7000. On top of the washcoat is a coating containing metals, which are the active constituent for converting harmful emissions into inert by-products. The metals add oxygen to the carbon monoxide and the hydrocarbons in the exhaust gases, to convert them into carbon dioxide and water respectively.
Two catalytic converters are used in each cylinder bank down pipe. A starter and main catalyst are located below the downpipe flange and are monitored by two HO2S (heated oxygen sensor).