Know your Exhaust

Text - Matthew Mulvale

Today’s exhaust systems have to perform substantially more tasks than a few years ago. They traditional functions such as silencing noise, preventing gas leakage into the passenger compartment and contributing to maximum engine power with minimum fuel consumption. But they increasingly incorporate new technology, including from 1992 onwards, standard fitment of a catalytic converters in order to meet stricter regulations on emission levels. Catalytic converters represent a responsible investment in a cleaner, healthier environment. If your exhaust loses its normal healthy sound then it is time for a check up. Act quickly and if a replacement is necessary, insist on a good quality exhaust system.

DESIGN. Why do exhaust systems differ so much in shape? Their function is always the same, yet no two car exhaust systems are exactly the same shape because the construction of each vehicle differs. The exhaust system has to be accommodated within the space available under the vehicle floor, operate under extremely high temperatures, move with the engine, keep clear of the brake and fuel pipes AND still maintain ground clearance.

VIBRATION A car engine is attached to flexible mountings to reduce engine vibrations. The same is true for the exhaust system that is bolted to the engine manifold and must be free to move with the engine. Lengthways expansion of up to ten percent is not uncommon so the exhaust must not only withstand engine vibrations but also continue working efficiently despite abrupt changes in ambient temperatures – e.g. cold water from the road surface quenching a hot exhaust.

DOWN PIPE. This carries gases from the exhaust manifold. Here temperatures can reach over 1000 degrees C when a vehicle is climbing or accelerating hard. If noise levels were measured just 50cm from the outlet of the down pipe the reading would be about 130 decibels, compared with a reading in the high 90s when measured 50cm from the tailpipe, rather like the difference between standing next to a pneumatic drill and listening to loud pop music on a radio. Down pipes don't often fail, but if they do the reason is usually fatigue fractures rather than corrosion, because the high temperature operation and/or stainless steel material combine to resist corrosion very effectively.

CATALYTIC CONVERTER - the 3 way catalytic converter fitted to many modern exhaust systems converts harmful pollutants such as nitrogen oxides, carbon monoxides and hydrocarbons into harmless emissions of nitrogen, carbon dioxide and water vapour.  The catalyst is a precious metal coating applied to a wash-coated ceramic or metallic substrate. On a ceramic substrate of 1 litre volume, the multi-channel structure, combined with the high surface area wash coat, would result in an area three times the size of a football field. The catalytic converters used in exhaust systems are designed to provide extended service life. Correct operation of the converter relies on proper engine tuning, use of lead free fuel and maintenance of the overall exhaust system condition.
Potential problems - poisoning of the catalyst by use of leaded fuel, melting of ceramic because of engine malfunction, breaking of ceramic through physical contact with a road obstacle – e.g. high kerb. 

FRONT SILENCER BOX In a typical modern 4 cylinder, 4 stroke engine, between idle speed and maximum rpm combustion takes place between 1,500 and 12,000 times per minute. The resulting shock waves must be 'treated' by the front silencer box to reduce noise levels and ensure correct gas flow through the system. Potential problems - external damage by contact with obstacles in/on the road leading to internal failure.

INTERMEDIATE PIPE. Usually the most complex part of an exhaust in terms of pipe bends. It links both front and rear silencer boxes, bypasses the rear suspension leaving enough clearance for full suspension movement over rough road surfaces and, at motorway speeds, can be subjected to gas temperatures of around 500 degrees C. Potential problems - fatigue fractures at severe bends, or general corrosion or a combination of both.

REAR SILENCER BOX. The rear silencer performs a function generally similar to the front silencer but treating a different component of the exhaust noise. For example using fibrous absorption materials to attenuate high frequency noise. Potential problems - hitting an obstacle with the tail pipe when reversing. Corrosion, made worse by stop-start driving over short distances.

EXHAUST SYSTEM LIFE. Corrosion works from the outside in, and from the inside out! External corrosion is worsened by bad weather conditions, particularly when salt is spread on the road to melt ice and snow. Internally acids produced during the combustion process pass through the system as gas doing little harm - BUT on short journeys, exhaust system temperatures are lower, allowing the gas to condense on the inside walls forming a very acidic liquid which attacks mufflers and pipes alike.

So there you have it! A brief tour around your vehicle exhaust system. A part that is often overlooked until it starts making some ungodly noise, but a part that is extremely important in the running of your car. It makes sound financial sense to keep your exhaust in first class condition, have it checked regularly, especially after the winter spell. Remember if your exhaust is noisy, it doesn't have to mean a new exhaust. It could be a loose bracket or blown manifold gasket. Always insist on viewing the problem when taking it in to one of these 'fast-fit' places and, if it has blown, there are special 'tapes' and 'putties' available to 'patch it up' until you can afford to replace it. If you do neglect your exhaust, your fuel consumption and performance, not to mention your wallet will suffer!

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