Exhaust System: Description and Operation

Exhaust System

COMPONENT LOCATION









OVERVIEW
The 3.2L SI6 exhaust system is fabricated from stainless steel and is supplied as two separate assemblies; a front section incorporating two catalytic converters and a rear section incorporating centre and rear silencers.

EXHAUST MANIFOLDS
The exhaust manifold comprises two separate manifold assemblies. One manifold is used for cylinders 1 to 3 and the second manifold is used for cylinders 4 to 6. The manifolds are sealed to the cylinder head with a gasket and secured with 14 bolts.
Each manifold comprises three fabricated branches which merge into an integral catalytic converter. A threaded boss is positioned where the three branches merge and provides for the fitment of a pre-catalyst HO2S. The catalytic converter outlets have offset flanges which mate with corresponding flanges on the front section exhaust system.
A bracket on each outlet flange allows for the attachment of an exhaust manifold heat shield.

FRONT SECTION
The front section has two flanged pipes which mate with the offset flanges of the exhaust manifolds. The flanges are sealed with gaskets and secured with bolts. The flanges are connected via curved pipes to decouplers which provide a flexible joint in the system. The outlet from each decoupler comprises a curved pipe with a threaded boss which provides for the fitment of a post-catalyst HO2S.
Each pipe is connected into a smaller catalytic converter which in turn is connected to a double flange which mates with the rear section of the exhaust system.
Two mounting rubbers are located at the rear of the flexible coupling and are attached to a bracket mounted on the front subframe.

REAR SECTION
The rear section has a double flange which mates with the double flange of the front section. The double flanges of each section are sealed with a gasket and secured with studs, washers and nuts. Two short pipes from the double flange connect into the centre muffler.
The centre muffler is a fabricated unit with a capacity of 7.6 liters. The muffler comprises a pair of perforated baffle tubes contained within the muffler body. The two baffle tubes merge into one single tube which provides the outlet from the muffler. A rubber mounting is attached to the rear of the muffler and attaches to a hanger bracket located on the underside of the vehicle floorpan.
A single pipe from the rear of the centre muffler is routed along the underside of the vehicle body to the rear muffler. The pipe enters the rear muffler through a connection in the forward face of the muffler.
The rear muffler is a fabricated unit with a capacity of 29.3 liters. The rear muffler is positioned transversely at the rear of the vehicle, forward of the rear bumper. The muffler comprises baffle plates and perforated baffle tubes. Each end of the muffler body has an outlet which direct exhaust gasses to exit at the rear of the vehicle. The muffler is supported by a single rubber mounting on the Left Hand (LH) side and by two rubber mountings on the Right Hand (RH) side.

CATALYTIC CONVERTERS
The catalytic converters are fitted in the front section of the exhaust system, after the HO2S.
The HO2S monitors the exhaust gasses leaving the engine. The engine management system uses this information to provide accurately metered quantities of fuel to the combustion chambers to ensure the most efficient use of fuel and to minimize the exhaust emissions.
In the catalytic converters, 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 platinum, which is the active constituent for converting harmful emissions into inert by-products. The platinum adds oxygen to the carbon monoxide and the hydrocarbons in the exhaust gases, to convert them into carbon dioxide and water respectively.