How it Works: The Exhaust System

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We all know the exhaust system is the means of carrying exhaust gases, the products of combustion, to the rear of your car. Their initial job, before pollution and sound were added into the equation, was to discharge the gasses into the atmosphere rather than the car – particularly given one of the constituent gases created by the internal combustion engine is carbon monoxide - poisonous even in small quantities.

The reason for carrying them to the rear was so that they can be dispersed into the turbulent air stream at that point. In some cars, even this was insufficient to prevent some of the gas getting back into the car, making it necessary to eject the gas into the even more turbulent air to the rear of the rear wheel. Station wagons with opening rear windows were particularly vulnerable in this respect.

The Life of the Exhaust System

The life of the exhaust system is generally dependent on the quality of the materials used and the operating conditions. The biggest problem is with corrosion from the inside, as one of the biggest products of combustion is water vapour, which combines with other products to form an acidic solution which attacks the steel. In a car which only does short trips, the vapour condenses in the pipe and muffler, and more rapid corrosion occurs than in a car doing longer trips which gets the system hot enough to keep the vapour in suspension until it reaches the atmosphere.

Heavier materials will stave off the evil day as will systems coated with various materials such as aluminium, zinc, tin or lead. A development that started to become more popular in the late 1970s was the use of stainless steel in the system - with the promise that it would last indefinitely, but at a significant cost penalty, as well as being more difficult to work.

Damage to the Exhaust System

A lot of systems don't last long enough to corrode - they are bashed by stones, driveways, kerbs and the like, as on many cars, the exhaust is the lowest point. Ask a bona-fide country driver, one that uses unsealed roads regularly, and they will tell you it is not uncommon to find the bottom of the muffler to be worn right through. Another cause of damage is loose clips, broken mountings, or even failed engine mountings or torque stays. The former two can let the pipes twist into a position where they can be struck by other moving parts of the car, such as axle or propshaft, and the latter two can cause excessive bending or movement of the pipe which can lead to failure by bending or fatigue.

The Major Parts of the Exhaust System

Generally there are four major parts, the 'front pipe', consisting of the length of pipe between the engine manifold and the muffler; the muffler itself, which consists of a round or oval box divided into several chambers; the catalytic converter and the 'tail pipe which is the length of pipe from the muffler to the rear of the car. There are of course exceptions to this layout, for example where two mufflers are used, or when for reasons of assembly the front pipe or tail pipe is in two parts. We should also consider such things as clips, clamps and mountings as a part of the system.

Front pipes first. Generally the front pipe is the longest lasting part of the exhaust system because it gets hot quickly and does not suffer from corrosion. However, it can suffer fatigue failure, as it is close to the source of engine vibration, particularly if the pipe is not installed correctly, or if clips or stay brackets which secure the pipe to some part of the power unit, usually the bell-housing, are left off or not fitted correctly. In cars with an east-west engine configuration, it is most important that the steady-clip is installed correctly, otherwise the normal engine movement will cause early fatigue at the joint between the pipe and the manifold.

Mufflers come in many shapes and sizes. Usually the size is determined by the available space left when all the other mechanical items have been fitted in, thus we often see cars with two mufflers because the space at any one point was not large enough for one muffler of sufficient size. Internally the muffler box usually consists of a number of compartments through which gas must pass before reaching the tail pipe and freedom. The basic idea is to control the expansion of the gasses as well as remove as much energy from them as possible.

Exhaust Tuning

Usually the compartments are tuned in respect of their volume in order to remove certain objectionable noise frequencies. For this reason it will often be found that fitting the muffler from one make or model of car onto another will result in objectionable noises at some point in the operating range. For the same reason, the fitting of so-called 'universal' type mufflers is not likely to be as satisfactory as the original equipment type part. E-Bay buyers beware. The tail pipe is generally the most vulnerable from a corrosion point of view because it runs the coolest, and so collects the most condensation.

Additionally the usually tortuous shape will provide a low point at which the corrosive mixture will concentrate. Tail pipes need to be correctly aligned to prevent fouling against items such as rear axle housing, shock absorber, chassis longitudinal member, fuel tank, rear apron panel, and the bumper bar. Fitting it correctly can be like the proverbial Chinese puzzle. Sometimes it is necessary to remove some other components in order to fit the pipe. After it is fitted, it is essential that it maintains its correct position, otherwise it will rattle or be worn through. Mounting clamps and clamps to the muffler should be kept tight.

Exhaust Mountings

In order to isolate the exhaust system from the rest of the car, some form of flexible mounting is used to support the weight of the system. The front end of the system is usually carried by the engine, either at the manifold, or at the support clip mentioned previously. In the region of the muffler will usually be found some sort of rubber mounting of varying complexity, but usually providing lateral, vertical and anti-rotation stability. Another mounting will be found near the rear of the tail pipe This will also be of a type which will provide lateral as well as vertical support.

It will be seen therefore that if any changes are contemplated to the mountings, a number of factors need to be borne in mind. A lot of thought will have to be put into the design and location of the original mountings, so that a departure will be likely to cause such annoying things as body drumming or "booms", or transmission of engine vibration to the passenger compartment.

How To Fit an Exhaust System on an Older Car

If you have decided that you need to replace some part or all of the exhaust system of your car, the first thing to do is to examine the whole system carefully to determine just what you need. For instance, as well as the pipes, do you need new mounting rubbers, clips, clamps, flange gaskets, manifold clamp nuts and bolts, support brackets, or whatever? Perusal of the Workshop Manual, and the Illustrated Parts List in the dealer's parts department may reveal parts missing from your car, or parts needed which may not be obvious.

It's always a good idea to replace clamps in any case, as the old ones are be distorted and may not give a good seal. Another thing to watch is that some manufacturers market their service parts exhaust system as individual parts, whereas the original system might have been in two or even one part, the components being welded together. If this is the case with your older car, additional clamps will be required. Another point to watch with this set-up is that if you are replacing a muffler which was originally welded on with a clamped-on type, you should be careful where you cut the pipe, as otherwise you may end up with the muffler too far back, or with insufficient engagement between the parts.

Fitting a Replacement Exhaust System

First of all the usual warnings:
  • Don't go under a car supported on its jack alone.
  • Don't use bricks or rocks or drums to support a car, use proper stands.
  • Before attempting to undo any of the nuts and bolts around the system, it will pay to treat all the threads and pipe joints with a penetrating oil, or one of the spray on preparations such as WD-40. Allow this to soak in for a while before starting to undo them.
With the car jacked up to gain access underneath, note the clearances to the various components under the car, then remove the components you intend replacing. To get the tail-pipe out, it may be necessary to jack the car in such a way that the rear axle can be lowered so that it is hanging on the springs. This can be done by placing the support stands under the chassis, or another suitable part of the un-derbody. Fit the new parts, but at this stage don't tighten any of the clamps or clips. Make sure that the clearances are adequate everywhere.

The jacking arrangements should now be changed so that the axle is back in its normal position with the springs taking the weight with the stands under the axle. The mountings should be all under the same strain, and there should be at least an inch (25 mm) of clearance between the pipe and the floor or other parts. Now tighten the system fixings up, starting from the rear in the absence of more specific advice. With everything tightened up, start the engine and check for leaks at the joints. If any are found they can sometimes be cured by rotating the s clamps carefully, or by moving the clamps slightly fore or aft. If the U-boit type clamp is used, sometimes a leak can be cured by positioning the U-bolt closer to the end of the slot.There are a number of joint sealing compounds on the market, but their use should not be necessary if the pipe joints are a good fit.

Exhaust Modification

The first thing you should do before modifiying your exhaust is to ring your insurance company. Make sure it is noted on the policy - because if the worst was to eventuate, and you had a modified exhaust , this could be cause for your insurer to reject your claim. Further, modifications to the standard exhaust system can be illegal in some States, so before hacking your system about, or spending your hard earned cash on exotic mufflers, or pipes, check that you are not going to break the law, and so provide additional revenue to the State.

Usually a so-called "sports" system will be noisier than standard, so you have to be careful. If you live in the country or drive a lot on unsealed roads, a useful modification can be a skid plate on the bottom of the muffler, welded on and extending forward to be either welded to the front pipe or attached by a U-bolt clamp. It does not need to be very heavy, say 1/8in. (3 mm), but can be a real help in preventing damage to the bottom and front.
Porsche Cayenne Exhaust System
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