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::: Exhaust Guide :::

What is an exhaust?

Obvious question you might think but nevertheless a good place to start when discussing the available options, benefits and pitfalls of exhaust systems on any car not just VWs. An exhaust serves several purposes:

1 - Remove the by-products of combustion from the engine and vent them into the atmosphere away for the occupants of the car.

2 - Silence the noise of the engine.

3 - (On modern, post '92 cars only) clean up the emissions of the engine before venting them into the atmosphere.

Point 1 is simple and just requires the use of a length of pipework shaped to direct the gases in the right direction. The fulfilment of point 2, however, impairs the ability of the exhaust to carry out point 1 and point 3 harms it further



As performance tuning becomes more and more popular there is an ever-increasing range of options for the aftermarket buyer and this article will help you make an informed decision about the sort of exhaust that is most suitable for your requirements. As you can see, as the main functions of an exhaust are contradictory there is always going to be a trade off when selecting an exhaust and what suits one car/person may not suit another. The following factors should be considered when researching and buying an aftermarket exhaust:

  1. Cost
  2. Longevity
  3. Weight
  4. Performance
  5. Sound
  6. Ease and quality of fitment
  7. Quality of manufacture
  8. Compatibility with car

Obviously cost should be balanced against the other factors but absolute cost can vary enormously. A bog-standard mild steel replacement for a Mk2 GTi can cost around £100 (sub $200) if you shop around. A bespoke stainless item can cost 5 times that, but is likely to last five times as long…. Not that that matters if you only plan on keeping the car for a year or so.

Which bits are which?

An exhaust is made up of several sections that pipe spent combustion gasses from the engine to the rear (or sometimes side) of the car. The pipework incorporates a number of silencers (known as mufflers in the US) throughout its length to reduce noise. Starting at the engine end you have the exhaust manifold which on most production cars is a crude bit of cast iron. On a regular 4 cylinder VW engine this takes gas from each of the 4 cylinders via four separate "pipes" and terminates in a single or double outlet. This then joins to a "downpipe" which takes the gas down under the car and joins to the main exhaust system. On modern VWs you then have a section incorporating a catalytic converter, this tends to sit as close to the engine as possible as they only work when hot and the closer to the engine they are the quicker they get hot.

An important point to note is that they have an optimum operating temperature and moving them too close to the engine can result in them overheating! Typically, they are situated approximately under the front seats.

Older cars do not have catalytic converters and simply have 1 or 2 intermediate sections. The intermediate sections pipe the exhaust gasses to the back of the car where they pass through the rear silencer and out through the tailpipe. When buying an exhaust system you tend to only get the pipework from the downpipe back or from the 'cat' back. It is possible to purchase and replace individual sections however.

The manifold ought to last the life of the car and so only gets replaced for performance purposes, although cast manifolds can crack through heat and abuse. Note: Exposing the manifold to a rush of cold water (speeding through a flood) can cause cracking!

The catalytic converter may not last the life of the car but ought to last most of the life of the car and is typically the single most expensive part of the exhaust.

A similar principal applies to other formats of engine except the configuration is different. For example a V8 may have a pair of manifolds (one for each cylinder bank) and a pair of down pipes that may or may not meet to travel to the rear of the car as one or two separate bits of pipe.

Materials

There are basically 2 choices here, mild steel (often zinc coated) and stainless steel. Mild steel is much cheaper but obviously rusts so expect a mild steel exhaust to last not much more than 3 years or so. Stainless does not rust (well imperceptibly slowly anyway) and so lasts much longer and most stainless exhausts come with a lifetime guarantee. Sadly stainless exhausts are considerably more expensive than mild steel items but a good investment if you plan to keep the car a while.

Because stainless does not rust, such exhausts often have an attractive polished finish as opposed to the dull and subsequently rusty finish of a mild steel exhaust. This polished finish only lasts if you keep the system clean as stainless steel, like most other metals, still tarnishes. A sensible finish (as found on most Jetex systems) is a painted black finish as this helps protect the metal against the elements. In terms of tailpipe appearance it doesn't really matter what the exhaust is made from as the tailpipe finisher can be a different material all together (such as chrome) or simply plated. 

It is worth noting that stainless steel gets hotter quicker than mild steel and also expands noticeably when hot. This means that after a bit of use stainless exhaust joints work loose so when cold the exhaust clatters and rattles. It is only once warm that the metal expands and the joints tighten that everything quietens down. This is particularly common on VWs as the sections tend to be joined with compression clamps rather than bolted unions. The expansion of the metal also means that an exhaust that sits straight and neat when cold does not continue to do so when hot!

Once you have decided to go down the stainless route you should also be aware that the quality of stainless steel varies and some exhausts are merely plated with stainless steel. 304 and 316 grade are the best with 409 grade having comparatively less corrosion resistance. Some manufacturers are now offering mid priced heavy duty mild steel systems or light duty stainless systems.

Noise

One of the key reasons for upgrading to an aftermarket exhaust system is noise, both more of it and tuning the noise to create a sporty sound. More noise is achievable by using less silencing, either reducing the ultimate number of silencers or using sports silencers that flow more gas at the expense of silencing. Silencing tends to slow down the flow of air through an exhaust so more performance is often accompanied by more noise.

To enhance the noise your car makes you can get away with changing just the rear silencer and not the entire system. The type and volume of noise an exhaust makes varies from manufacturer to manufacturer. From experience Jetex and Milltek tend to be sporty but refined, Scorpion tends to be a bit loud and Magnex sits somewhere in the middle. Noise/sound is very much a matter for personal taste so ask around on forums and at owner meets.

It is worth noting that most track days impose noise restrictions on cars taking part so if you are likely to be tracking your car it is advisable not to go for the loudest system on the market. Having said that, 100dbA is a common limit and not even my old raucous 1.9Mk2 came close to this in a sound test. Consider also the sort of driving you do before choosing a noisy exhaust as a lot of aftermarket systems are unbearably boomy at motorway speeds, particularly if your car is short geared and therefore high revving.

Performance

Most standard exhausts are designed to be cheap quiet and inoffensive. Performance is a long way down the priority list. The performance of an engine is proportional to the amount of energy you can release (and harness) by burning fuel and therefore the more fuel you burn (assuming good combustion) the better the performance. To do this, not only do you have to get the fuel and air into the engine quickly but you have to get it out quickly.

It follows that you can improve an engine's performance by fitting a free flowing exhaust and performance claims for aftermarket exhausts vary from about 2 to 10 BHP. I would take these claims with a pinch of salt, changing the exhaust alone (and I mean the entire system, not just the back box) may yield 3 or 4 BHP but nothing particularly noticeable on most VWs. You will tend to get perkier performance higher in the rev range where it becomes important that the engine is breathing well. That is not to say don't bother but do be realistic, it is no good getting gases out of the engine if the intake side is not breathing properly. Performance exhausts work well with aftermarket air filters to improve the overall breathing of the engine and you will notice it rev far more freely after these two mods. They should be regarded as a good starting point for further mods and there is no point spending money on cams and heads etc if the basic breathing is not spot on.

Performance exhausts tend to flow better by having larger diameter pipes and less restrictive silencers. When forcing exhaust gases through a restrictive standard exhaust the engine uses BHP on the exhaust stroke to expel burnt gases. By reducing resistance to this expulsion of gas less power is wasted and therefore more becomes available at the flywheel. The amount of gas an exhaust can flow is proportional to the velocity of air multiplied by the average path area so simply fitting greater diameter pipes ought to yield gains. This is true up to a point but as the pipes get wider the velocity of air decreases and there comes a point where the pipe diameter is simply too big. This particularly affects low down torque which benefits from higher velocity air flow.

The larger pipes and typical stainless construction tends to mean that aftermarket exhausts can be MUCH HEAVIER than standard. Not only does this give fitting headaches but the extra weight negates the minor BHP improvement. If you seriously crave additional performance (not just pub talk BHP) then go for the lightest system you can find. The Milltek system on my G60 is literally half the weight of the Scorpion one that was on there when I brought the car and is considerably lighter than standard.

Utilising sports silencers also aids flow. Regular silencers tend to be based on a reverse flow design which passes the gasses backwards and forwards through a series of baffles within the silencer greatly reducing flow velocity albeit whilst eliminating a lot of noise. Sport silencers tend to be a straight through design which pass gas through the centre of the silencer in a straight line. The centre perforated tube of the silencer is encased in a sound absorbing material which reduces noise (although not as well as a regular silencer) with minimal flow impedance.

Some aftermarket systems even reduce the total number of silencers to further aid flow and in some cases replace them with resonators. A resonator is a chamber that contains a specific volume of air which is designed to cancel out certain frequencies of sound and is typically found in the centre section of aftermarket exhausts. Some tailpipe designs also incorporate integral resonators, which in certain instances are specifically tuned to enhance the noise of the exhaust.

Exhaust tuning is becoming a serious black art for many modern tuners as big gains can be obtained by tweaking the exact length of the primary pipes, secondary pipes and overall exhaust.

It is worth buying a system that is tried and tested and has been specifically designed for the car not just cobbled together from a load of universal silencers. Each time gas is expelled from an exhaust valve a pressure wave occurs in the exhaust manifold. These pressure waves often interfere with those from other cylinders creating turbulence or stalling the air flow in the manifold and thus impairing performance. By designing the manifold tube lengths such that these pressure waves do not interfere and in some cases create a vacuum to help draw gas out of other cylinders you can see large power gains. Big gains are therefore often seen by fitting stainless steel tubular exhaust manifolds (known as headers in the US) which instead of being cast are made from stainless steel pipework welded to a head mounting plate. Ford pinto engines and the Rover K series both respond well to this. Sadly the gains are limited on most VW engines and you are best sticking to a 'flowed' standard manifold. This involves replacing your standard manifold with one whose internals have been machined to remove casting imperfections as well as smoothed and widened to enhance gas flow. 4-branch manifolds (as the stainless tube items are known) are obviously a good solution for engine transplants where clearance is an issue as they can be shaped differently to standard. Unfortunately the stainless steel gets very hot and has a habit of damaging anything that sits near the manifold in the engine bay, particularly steering column boots.

The method used to create the required bends in the exhaust pipework also affects performance. Standard exhausts are cold crush bent which means that the cross section of the pipe deforms as the bend is made which will impair flow at this point. Good performance exhaust pipework is mandrel bent which involves softening the metal with heat to avoid distortion. The cross section of the pipe remains consistent at the bends and flow is not impaired. This is obviously a more expensive process and some systems just crush bend but compensate for the cross sectional distortion by using oversized pipes.

The final area for performance improvement is the catalytic converter which started being fitted to cars in the UK from about 1990 onwards (although were introduced in the US in 1975).

A catalytic converter converts three harmful components of a cars exhaust into harmless compounds. Hydrocarbons (unburned petrol) are turned into carbon dioxide and water, carbon monoxide (a by-product of burning petrol) is turned into carbon dioxide and nitrogen oxides (the heat in the engine forces nitrogen in the air to combine with oxygen) are turned back into nitrogen and oxygen. Unfortunately cats are highly restrictive and impair the overall performance of the exhaust system. For UK cars registered before August 1992 they are not mandatory and can therefore be removed. Cars registered after this date must have a cat in place in order to pass an MOT.

Having removed the cat it is replaced with a cat bypass pipe which is simply a piece of pipework that sits where the cat would previously have been and incorporates a threaded hole to mount the lambda probe in. I would recommend fitting a cat bypass pipe that incorporates a small resonator as bypass pipes without one tend to be very boomy. Ditching the cat on any car will yield a noticeable improvement in performance, particularly in conjunction with a full performance exhaust and decent induction kit/air filter and you should not need to pay more than £90 for a cat bypass pipe. For cars registered after August 1992 it is possible to fit a performance cat that simply flows better than a standard equivalent. I have no experience of these but have read independent reports that imply good gains can be had by fitting them in place of the standard item.

Fitting

Unless you have access to a pit or set of ramps, fitting an exhaust can be a tough job due to the limited access.

We will not go through the process step by step (see your workshop manual for this) but will mention a few useful pointers. The worst bit is removing the old system and this should not be tackled without access to a hacksaw, large hammer and an angle grinder. Because mild steel corrodes so easily, by the time an exhaust needs replacing all the unions have usually rusted together and the securing bolts are so corroded no spanner will get a purchase on them. The easiest thing to do is just to cut the exhaust into pieces with an angle grinder and grind the heads off any securing bolts. This means that the old exhaust becomes scrap but then it probably was anyway!

Read into this the fact that it is often difficult and time ineffective to replace individual sections of a mild steel exhaust, and usually if one section is rotten the remainder of the exhaust is probably on its way out as well. It goes without saying that care must be taken when working under a car with an angle grinder particularly around the fuel tank, fuel and brake lines. You should also take care if the car has been run recently before changing an exhaust as it is likely to be very hot! If you are removing an old stainless system then you may be able to separate the joins and save the exhaust as they tend not to corrode together.

Once the old exhaust is off the car you should offer the new system up and bolt it loosely in place. Apply regular exhaust assembly paste to any joints if fitting a mild steel exhaust but use clear silicone sealant on stainless exhausts. It may sound odd but the regular assembly paste does not stick to shiny stainless steel. Do not tighten everything up until the car is back on the ground and you are sure everything is straight. There is no point spending a premium on a nice looking exhaust only to have the tailpipe stick out at a wonky angle. It should be noted that twin pipes and letter box style tails are far harder to fit neatly as they must be mounted perfectly straight. A round tailpipe can be wonky as you like but it won't show, the same is true of an oval pipe, but to a lesser extent.

When you are happy that everything is lined up (pay particular attention to where the pipe loops over the rear axle) slowly and carefully tighten everything down taking care that the various exhaust sections do not twist as you tighten them up. Make sure that you align any exhaust clamps so that the nuts are easily accessible but not so they protrude unnecessarily below the car. Some tailpipe designs require the bumper aperture to be trimmed for fitment, such as DTM style pipes and some large twin pipe designs. Check this out before buying an exhaust as it may not be a great idea to chop the bumper around on a car such as Corrado or Rallye where bumpers are expensive and hard to come by.

It is always a good idea to renew the exhaust rubbers when fitting a new exhaust. Light exhausts should be fine with standard rubbers but heavier big bore systems may require heavy duty rubbers to stop the system from swinging around and bouncing up and down on the underside of the car or back axle. Powerflex (and others) do a range of heavy duty polyeurethane mounts (about £8 a pop) and C&R in Nottingham supply some very useful rubbers with a chain inside them (about £6 a pop) that do not stretch at all. Fitting them can be a pain with the chain rubbers but they are an ideal solution for a heavy exhaust. You will need to unbolt the hanger from the car, fit the rubber to the hanger and exhaust and then re-bolt the hanger to the car. A good proven solution for heavy exhausts is to use 2 standard VW rubbers either side of the centre section (just in front of the rear axle) and 2 chain rubbers on the rear silencer.

As a slight aside, you will want uprated rubbers if you ever plan on doing a track day at brands hatch! From experience we can tell you that the severity of compression at the bottom of paddock hill bend snaps standard exhaust rubbers with alarming regularity. We recall snapping one or two during each and every track session, usually on the first or second lap out of the pits so the exhaust rested and clattered on the back axle for the remainder of the session. The fact that it was a rather heavyweight system probably didn't help matters!

Common problems

The most common failure mode for an exhaust is simply brought on by old age, particularly if you still have a mild steel exhaust. Short journeys are a killer for exhausts as a lot of water is produced as a by-product of combustion, particularly on start up. When the exhaust is hot this is no problem as it simply evaporates but if the engine is stopped before the exhaust is hot then this water just sits in the exhaust causing rust to form from the inside out. The problem is made worse as the water combines with certain compounds in the exhaust gas to form acid which slowly eats through the exhaust.

After a while your refined VAG 4/5/6 cylinder engine will suddenly start sounding like six and a half litres of American muscle car engine. Sadly this is not an indication that you have just hit the jackpot in the free BHP lottery, but an indication that your exhaust is "blowing". This means that exhaust gas is escaping from somewhere other than the tailpipe. Follow the sound to track down the leak and if you are unsure, place your hand near (not ON!) the source of the sound and you should feel the exhaust gas escaping. The leak will either be from a rusty hole (Prone areas are seams on silencers and the welded joints where the pipes join the silencers) in which case the offending section of the exhaust needs replacing, or from the union between two sections. If it is the latter, separate the joint, apply some exhaust assembly paste/silicone sealant and reassemble to sort the problem out. Remember, however, that stainless exhausts often blow at the joints until they have warmed through and the metal has expanded fully so only worry if the leak still exists when the exhaust is hot.

Some rusty holes can be patched up with gun gum (a heavy duty exhaust repair paste) or even welded but this should only be regarded as a temporary and cheap fix.

Another common exhaust problem is the exhaust banging on the underside of the car. Do not be fooled, in most cases the exhaust is not banging on the underside of the car but on the back axle! Where the exhaust goes over the trailing arm clearance is limited and this causes most exhaust "clonking" problems on VWs. The most likely culprit is tired or split rubber exhaust mounts so check these first as tired mounts will allow the exhaust to swing around more than it should. Secondly, check the fitment of the exhaust, if the bend in the pipe does not line up with the trailing arm then clearance will be impaired and every time you hit a bump or put stuff in the boot the two will meet…. Noisily! It is worth noting that lowering the car makes the problem worse as it reduces static clearance by bringing the trailing arm closer to the exhaust, and fitting a larger bore exhaust does the same. In both instances it is advisable to use uprated mounts to minimise the movement of the exhaust. It is important that your rear bump stops are in tip top shape (we recommend using new OE VAG items, not poly ones) as these limit the amount of travel the back axle can do when you hit a bump. If the stops are tired then the first thing the back axle will hit on its way north is the exhaust.

Exhausts can often make odd rattling noises, often at very specific frequencies. The first thing to check in this case is the heat shields that protect the underside of the car from the heat of the exhaust. These often work loose and rattle around as a result. If these are in order or the rattle is a deep rattle rather than a tinny rattle then you have probably blown the baffles in one of the silencers. The rattle will therefore be coming from a specific silencer. The various internal parts of the silencer will have come loose and will be floating around freely in the outer casing. If this is the case, replacement is the only option and it should be done ASAP as it is possible for the loose parts to block the exit to the silencer which will cause the engine to stall or suddenly lose power (not great if you are in the process of pulling off a hair brained over taking manoeuvre).

Finally, it is possible, albeit not that common, to suffer a blowing exhaust manifold. This is easily diagnosed as the car will sound much like a tractor with the noise emanating from the area around the exhaust manifold. The problem could either be a blowing gasket between the head and manifold / manifold and down pipe or a cracked manifold. If it is the latter then the manifold is scrap as it is not possible to cost effectively repair cast iron. If it is the former then you simply remove the manifold check for flatness and replace the offending gasket.

Such a blow could ultimately be caused by the manifold warping so it is important to check for flatness on the mating faces before reassembly, and you may need to flatten the face of the manifold slightly before refitting. A word of warning though…. Removing the manifold is a nasty job. A lot of ancillaries often need to be removed first to gain access and in some cases the inlet manifold also needs removing. Having gained access the nuts will be rusted onto the manifold studs and it is all too easy to snap the old seized studs leading to expensive problems if they become stuck in the head (they will need to be drilled out and the holes re-tapped). For this reason we would only tackle such a job yourself if you are very time rich and money poor!

Which exhaust?

There are many different manufacturers producing a bewildering array of exhaust options so my best advice would be chat to other owners on-line and at meets to see what experience they have had with various systems. Pay attention to comments about quality, fitment and noise. Form your own opinion on appearance and take performance gain claims with a pinch of salt. If you are a Corrado owner check out the following thread:

http://www.the-corrado.net/forum/viewtopic.php?t=25077 

As well as buying off the shelf it is possible to get bespoke systems made up. This is useful if you have built a one off car or want a spec that no manufacturer produces. Hayward and Scott are pretty well renowned as are Powerflow. A word of caution regarding Powerflow, however and that is to remind you that they are simply a franchise and the quality of the work varies from outlet to outlet. We had an excellent Powerflow system made up for my 1.9Mk2 but then had a real nag of a cat bypass pipe made (by a different outlet) for my G60 which I subsequently replaced! Therefore pay heed to the location of the outlet people used if they subsequently recommend a Powerflow exhaust to you.

Finally, exhausts are very car specific, just because it looks similar to the one on your car does not mean it will fit. For example, the exhaust for a big bumper Mk2 is different to that for a small bumper car as the tailpipe is slightly longer to ensure it sticks out behind the larger bumper. This means be wary of buying second hand unless you are absolutely sure the exhaust is off your exact model of car. Phrases such as "yeah mate it should fit any Mk2 or Corrado" should have you scurrying away from the relevant e-bay listing. That's not to say don't buy second hand as you can often get a good bargain but do be wary and double check the application.

Disclaimer : Matey-Matey accept no responsibility for any of the information contained within this document or the accuracy thereof. It is intended as a helpful guide and is solely based on personal experience. The authors also wish to stress that the methods highlighted are centred around personal opinion and there may be other equally credible ways of performing this conversion.

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