Purpose

The purpose of a performance exhaust (sometimes called "sports exhaust", "higher-flowing exhaust", "cat-back exhaust", etc.) is to make the engine run better.

As with most of my other advice, I cannot quote actual laws of physics or gasoline engine properties, but I can tell you the ideas behind it.

When you buy a new car, the whole car is tuned to give you the best balance between comfort, fuel economy, performance, longevity, reliability, etc..  While this is a good idea, because the car will last a long time, will be good on fuel, will be relatively quick, comfortable, and have a good audio system, most cars are not tuned for performance when you buy them from the factory.  Some may argue that true sports cars like Ferraris and others are, but you only need look at people who own them and modify them.  While they are insanely quick (especially for our speed-limited roads in North America), there are those with big budgets who tune them up to 1000 HP and more.

Let me get back on the VW Fox topic.  VW of America, when importing the Fox from Brazil, decided for all of us that we would get an inexpensive car that was tuned down in the performance department.  (Not to mention lacking power mirrors, engine options, sports seats and many other options available in Brazil.  The exhaust system that came on the Fox was really badly designed as far as performance goes.  The diameter was about 1.0" - 1.25" and that was very reduced at each bend and especially around the rear "axle" where it got flattened.  Furthermore, a restriction ring was inserted into the exhaust system just ahead of the catalytic converter (or resonator on earlier Foxes).  This "donut" restricted to about 1.0" or less!!  Definitely not what you want for performance.  If you simply remove this "donut" your Fox will have engine output much closer to a Golf or Jetta of the same year.  (Click here for info on how to remove it.)
 

In my opinion the best option to go for is to buy a pre-made "cat-back" exhaust system that is specifically designed for the Fox.  ("cat-back" means that the system replaces everything on your car from the catalytic converter to the rear of the car.)  This means that someone at some point actually took the time to measure out the dimensions, bends and size of the exhaust to be best-tuned for the Fox.

Here are some features that a good exhaust system should have:

• Mandrel bending
• When a pipe is "Mandrel bent" this means that there is no change in diameter at the bends.  A conventional pipe bender will put 'creases' into the metal in order for it to bend around a turn.  A special machine for Mandrel bending does not put these creases in - the diameter stays constant throughout.  I have read that with the same diameter pipe, a bend that isn't Mandrel bent loses up to 20% of the flow!
• If you're getting a system made by a local muffler shop, be sure to inquire whether they have a machine that can Mandrel bend pipes.  The machinery is very expensive and few shops have it.  A Mandrel bent exhaust system also costs more, but it is well worth it.
• 2.0" or 2.25" diameter
• All of the exhaust systems I've seen that are designed for the Fox are 2.0" in diameter.  You shouldn't go above 2.25" unless you've increased the displacement of your engine.  The larger the diameter gets over 2.0", the more low end power and torque you'll lose.
• Stainless vs. Aluminized
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There are only a handful of companies who actually made aftermarket performance exhaust systems for the Fox, and only a few of those still do.  Here are the ones that I am aware of (please let me know if you know of any more):
• Techtonics tuning - http://www.tttuning.com/
• 2.0", Mandrel-bent, and it includes everything you need to hook it up (gaskets, bolts, hangers, instructions).
• I have one of these on my car and have absolutely no complaints.  The quality is superb, it fit without any problems, and included everything I needed.
• Eurosport - http://www.eurosportacc.com/
• From their web site: "We use an increased diameter of mandrel bent aluminized steel tubing in our designs to ensure maximum flow and horsepower.  Our systems will give you a 7-12% horsepower gain along with a sporty, track tone."  (Direct link to Fox exhaust page.)
• No mention of exact diameter.  Clamps, hangers, instructions and 2 year limited warranty included.
• Thanks to Chris Chan for info on Eurosport's exhausts.
• Leistritz - http://www.leistritz.com/sportsound/index.html
• Now discontinued, but they still have some stock left (January 2000).
• A fellow Fox owner tells me it fits all Foxes (not only 1987-1990 like their web site says).  He also says that the bends weren't perfect and required some tweaks to make it fit; the work required was minimal and only around the heat shields to prevent minor rattling.
• Their web site claims that both torque and horsepower and significantly improved in the 2500-5000 RPM range, peaking precisely at 4057 RPM).  There is no mention of diameter.
• Autotech - http://www.autotech.com/
• They used to make them, but their web site no longer lists them.  You may be able to find some old stock in a local shop.