Adam's Brief
Fox Tuning Guide

My Fox on a dyno, Sept 2001.

Purpose

This page is meant to be a starting point for anyone wanting or even merely thinking about doing some upgrades to their VW Fox.  It is by no means complete, but it should give you a rough idea of what can (and in some cases should) be done.

As time permits, I will expand the information beyond this page to include instructions on how to perform many of these upgrades.  For now, however, they will only be outlined here.

Feel free to save this file to your computer (click "File" then "Save as..." on the top left part of your web browser) for future reference.  Do not, however, save this file with intent of plagirizing the information for your web site(s).

I am still working on the content and formatting of this page, but I have decided to put it up before I'm finished it, so that whatever information is here will (hopefully) help answer some commonly-asked questions.

Theory
An engine can be compared to an air pump.  To get more output from the pump, you have to be able to deliver more air to it, and also expel the larger volume of air that it'll pump.  Once this is done, other things can be done to the pump's internals to make it have a better output.

VW Fox

• (Restrictive) Exhaust Gasket:  The very first thing any Fox owner should do is make the exhaust flow better.  From the factory, the Fox's engine (as sold in North America) was restricted in the power it could make.  This was accomplished primarily by the gasket between the downpipe and catalytic converter (or resonator in 1987 and 1988 Canadian models).  Although the exhaust pipes are somewhere around 1.5" in diameter, the gasket cuts flow to less than 1.0", effectively "choking" the engine, thereby preventing it from making a lot of power.  To fix this, simply go to a VW dealership and order the gasket designed for an Audi 4000 4-cyl.  I have heard that the original restrictive Fox gasket is now superceeded by the non-restrictive Audi part.
• (Restrictive) Exhaust:  If you need to replace your exhaust system, do not get one from the dealership or a local muffler shop.  For approximately $150 USD you can have a tuned cat-back system from a company such as Techtonics ("cat-back" means that it replaces everything after the catalytic converter;  'everything from the cat back').  The Techtonics exhaust has 2.0" diameter pipes, all the bends are "mandrel bent" (meaning there is no restriction in diameter at the bends), and it is designed specifically for the VW Fox so there are no fitment issues.  For those wondering, the transverse muffler is replaced by a round resonator and the rear muffler is a DynoMax UltraFlo unit.  The exhaust system comes with all the hardware needed to install it, and it is in 4 pieces:  1) a flange to bolt to the catalytic coverter, 2) a section of pipe + the resonator + more pipe (all welded together), 3) the DynoMax muffler, 4) a piece of pipe to go from the muffler to out under the bumper.  The Techtonics setup follows the path of the stock Fox exhaust.  Other companies, such as Leistritz and Eurosport also make cat-back systems.
• Exhaust Manifold & Downpipe:  The exhaust manifold that comes stock on Foxes is a 4-to-1 design.  This means that the gases from each of the 4 cylinders are merged into one outlet, all within the exhaust manifold.  While this gets the exhaust out of the engine, it is by no means the most efficient way.  The best setup for street use is to use a "dual outlet" exhaust manifold and a corresponding downpipe;  the exhaust manifold combines the exhaust gases into 2 outlets - one from cylinders 1 and 2, and the other from numbers 3 and 4.  The two outlets are joined into one within the downpipe that leads to the rest of the exhaust.  Techtonics sells all you need for this upgrade;  they do so in two different ways:
• Buy their exhaust manifold and downpipe (I have heard the manifolds they sell are brand new Audi 4000 4-cyl manifolds)
• Find a 1979 or earlier VW Dasher / Audi Fox and buy the corresponding Techtonics downpipe.
• A third way might be to use an Audi 80 4-cyl exhaust manifold and downpipe.  I will be performing this upgrade on my Fox sometime during winter 2001/2001, but I will not be able to comment on fitment until I actually try it.  The exhaust manifold will bolt up to the Fox cyl head, but whether the Audi downpipe will fit is my concern.  The Fox and Techtonics downpipes go over the driveshaft, whereas the Audi piece looks like it goes under it.
At this point, you should upgrade to a high-flow catalytic converter as well.  Techtonics sells a 2.0" high-flow catalytic converter;  using the Techtonics catalytic converter I passed the emissions testing without any problems here in Ontario.
If you are wondering about headers, don't.  On a street car, especially one driven year-round, they will not have a long lifespan, and the dual-outlet exhaust manifold will flow almost as well as a header.  No one makes a header for Foxes anymore, so you most likely won't find one either.
• Intake:  The intake setup on Volkswagens, especially when compared to other cars, is actually very well designed.  If you own a 1987-1990 Fox, you should find a 1991-1993 Fox and grab the intake manifold and throttle body from it.  The Digifant Foxes (1991-1993) have a better-flowing intake manifold and a bigger throttle body.  By itself a bigger intake manifold and throttle body won't give you much in the way of gains, but when combined with the exhaust work mentioned above, you will gain a bit more power and better throttle response.  Furthermore, switching to a reusable K&N (or other brand) air filter will get you marginally better flow.  Do not expect 10% more or whatever the advertised gains are;  the biggest reason for upgrading to a K&N filter is because they're reusable - you only need to buy one!
• 1987-1990 - some people recommend drilling holes in the lower part of the airbox.  As long as the car isn't sucking hot air, this might gain you a few extra HP.  There is no way to mount a cone-shaped air filter on these Foxes.
• 1991-1993 - if you don't drive the car in below-freezing temperatures, you can install a cone-shaped air filter instead of the airbox.  Again, do not expect any significant increases in power.  The noise under full-throttle acceleration, however, is very cool!  Here is some information on air filter upgrades.
• Camshaft:  Once you have taken care of the airflow in and out of the engine, and providing everything is still running perfectly (i.e. no leaks, weird noises, etc.) it might be a good time to install a camshaft.  With a sport camshaft you have to give some serious thought to what RPMs you drive your car at, how driveable it should be around the city (especially in stop-and-go traffic) and whether emissions and fuel consumption are a concern.  The camshaft is what controls when the intake and exhaust valves open and close, the rate at which they open and close, and how long they stay open.  Techtonics' catalog has a great description of all this, so read up in there.  Essentially you can't have the best of everything - serious high-RPM horsepower is at the cost of lower low-end power and a rough idle.  For a cylinder head with hydraulic valve lifters (like the stock VW Fox one), I believe the best compromise is the Techtonics "Factory 268" camshaft;  this is allegedly the camshaft used in sport models in Brazil.  I used one on my Fox for a while - emissions were fine, the idle was a little bit lumpy, and the power above about 4500 RPM was very nice!  Some camshafts aren't legal for use in all States, so be sure to comply with local laws.
• An adjustable cam gear might be a good idea to install along with a sport camshaft.  An adjustable cam gear allows you to move the power band up and down, to suit your driving style.  Also, since Digifant Foxes are limited to 6000 RPM, bringing the powerband down a bit (about 2 degrees advanced is perfect) helps make use of the power the engine creates.
• Getting more fuel to the engine once a sport camshaft is installed is also a very good idea.
• 1987-1990 - I am still doing research on this.
• 1991-1993 - I am using a custom-modified fuel pressure regulator that puts out 4.0 bar of fuel pressure.  It is a direct replacement for the stock unit (even though it looks a bit different), and helps the engine get more fuel, especially at higher RPMs.  It is available from Motronix Motorsports (talk to Dave there, and tell him Adam with the VW Fox sent you).  I've also heard that Porsche 944 fuel pressure regulators put out more pressure than the stock Fox one, but I have not been able to determine exactly what pressure they put out.  It is a common myth that a Corrado G60 fuel pressure regulator puts out more pressure;  it has the exact same Bosch part number as a Golf piece, which is the same as a Fox piece, so there is no reason to upgrade to it.  Using Corrado G60 fuel injectors, on the other hand, is a good idea, as they flow more fuel than the stock Fox ones in the same amount of time.
• Please be aware that the camshafts for engines with hydraulic lifters and for those with solid lifters are different and are not interchangeable.  I am mentioning this, in case anyone plans on installing a cylinder head with solid lifters (why you would want to do this is outlined in the next section).
• Removing the rev limiter would also be adviseable at this point, so you can make use of all the power the sport camshaft will make.
• Cylinder head:  The steps above can fit into about $1000-1500 CAD for the parts.  Once you go beyond the above steps, the price starts to get quite costly;  the performance gains are still there, but they cost more to obtain.  Just like with the intake and exhaust modifications, the more air a cylinder head can flow, the more power it will produce.  This is accomplished with the use of bigger valves (stock on a Fox are 38mm / 33mm) - Golf GTIs and even later model base Golfs and Jettas came with what is commonly known as a "big valve head", meaning that they have 40mm / 33mm valves.  The difference from larger valves is only felt in the higher RPMs, when more flow in and out of the head is required.  Simply swapping on one of these big valve heads will gain you a bit more power up in the high RPM range.  If you have the money, it will be worth it to do some work to the head before you install it.  Here are some things that can be done (by a machine shop):
• Oversized valves - again, more flow = more power.  I have 40.5mm / 33.5mm valves installed.  I have heard that up to 42mm / 35mm valves will fit, although I do not know how this is possible.  (Have a look at here for a comparison of different valve sizes).  Furthermore, my valves are made of stainless steel, and they are called "competition" style, because the stem narrows near the head of the valve to promote better flow.  The benefits of stainless steel valves are:  stainless steel is considerably stronger than the stock sodium-filled valves, they dissipate heat better, they are substantially lighter, and the added strength of the stainless steel allows a thinner valve stem, which 1) reduces reciprocating weight, and, more importantly, 2) allows a thinner valve profile, permitting more air to pass around the valve stem and valve head.
• Port & Polish:  In addition to being able to physically flow more air, the cylinder head must do so efficiently.  A Port & Polish accomplishes this by smoothing the passages through which the air/fuel mixture flows.  The intake side is smoothed out of any bumps and casting imperfections, yet left slightly rough so the air/fuel mixture continues to tumble as it enters the combustion chamber, to ensure a proper and homogenous mixture of the two.  The exhaust side is smoothed out to as close to a mirror finish as possible, so there are absolutely no obstructions to the exhaust gases, and so they exit the cylinder head as smoothly and quickly as possible.  Ideally, this should be done on a device called a "flow bench".  A flow bench measures the speeds of the airflow through a cylinder head, and allows for precise measurements as to how the cylinder head should be ported and polished.
• cc'ing:  cc'ing refers to equalizing the volume of each combustion chamber, for engine balancing purposes.
• Decking / Shaving:  This is done be shaving the bottom surface of the cylinder head beyond the factory specifications.  By shaving the bottom of the head, the combustion chamber is made slightly smaller.  Since the stroke inside the block remains the same, the same volume of air/fuel gets compressed into the smaller combusion chamber, thereby increasing the compression ratio.  The higher the compression ratio, the more power the car makes.  This is at a cost of more wear and stress on the engine, as well as the need to run higher octane gasoline.  There is also a physical limit to the amount a cylinder head can be shaved - at a certain point, on the exhaust and intake strokes when the valves are fully open, the piston will hit the open valve.  This is only a concern with heavily shaved cylinder heads;  in fact, I don't know whether it's even possible on an 8v head.  A friend of mine has a 2.0 16v that runs somewhere above 12:1 compression and he ran into this problem getting the car to this stage.
• With a stock 8v head, and even a mildly shaved one (like mine - shaved 0.015"), the engine is a "non-interference" engine.  This means that in the unlikely case that the timing belt would break, a fully-open valve will not be hit by a piston at top dead centre.
• The compression ratio can also be raised by using high-compression pistons;  they are discussed below.
• Converting to solid valve lifters:  while hydraulic valve lifters provide maintenance-free vehicle operation and a quieter engine, they do have their limitations.  Because hydraulic lifters are kept at the proper height be oil pressure, a camshaft can only be so wild.  If a camshaft had too quick a lift, the oil keeping a hydraulic lifter at the proper height would be "squished out".  Solid valve lifters do not require oil pressure to keep them at the proper height - they use specially-designed discs that are interchangeable.  Periodic valve lash adjustments are necessary, to ensure the distance between the lifter and camshaft lobe has the proper spacing.  Because solid lifters are purely mechanical, a camshaft can be as wild as you want it to be, without any lifter problems.  But, unless you are going to get a really wild camshaft, you have no need to worry about this upgrade.
• Displacement: There's a saying that says "There's no replacement for displacement."  With everything else the same, an engine with more displacement is guaranteed to make more power.  There are several ways to add displacement to your car:
• Over-bore the stock 1.8L.  There are pistons of varying diameters available in the aftermarket.  If going this route, it will be in your best interest to install high-compression pistons.  Even without over-boring, they will raise the compression ratio from the stock 8.5:1 to 10:1.
• You can also swap the crankshaft and pistons from an Audi 80 2.0L into the stock Fox block, but this is quite costly.
• The easiest ways to add displacement are to install a stock 2.0L VW/Audi engine.  See below for details.
• Engine Swap:  While an engine swap isn't a tremendous amount of work, it is not a quick and simple job.  If you are sticking with a VW/Audi 4-cyl engine, though, it is quite straightforward - essentially 'out with the old, and in with the new'.  If you are planning an engine swap as a future upgrade, you should plan out the modifications you do before it so you either save money and do the engine swap, or only perform upgrades that work with the engine you will be swapping in.  If you are planning on staying with a 4-cylinder 8 valve engine, you can safely perform the intake, exhaust and camshaft upgrades before, because they will work with a 2.0 block.
• 2.0L 8v - this is by far the easiest engine swap to perform into a VW Fox.
• "Bubbleblock" from an Audi 80, code 3A.  The Audi block is identical in height to the stock Fox one and the engine mounts are identical.  To make it work, you have to you'll have to swap on your cylinder head, intake and exhaust manifolds.  The reason is that the Audi design has the fuel injectors in the intake manifold rather than in the head.  The pattern of cylinders, oil passages and coolant passages is identical between the two blocks, so you can swap any 8v cylinder head on.
• "Tallblock" from an A3 VW, code ABA.  The tallblock is called a "tallblock" because it is 16mm taller than the older VW/Audi blocks (including the stock Fox one).  The cylinders, oil and coolant passages are still the same as the Fox block, so there are absolutely no concerns there.  The downpipe also has enough play in it so that it fits fine with the 16mm difference in block height.  Among the only things the ABA has going against it, is that it has a longer stroke than the Fox and 3A, and therefore doesn't like to rev up as quickly, and that there are some possible oil filter fitment issues.  One person I have heard from said he had to relocate his oil filter, because the stock bracket on the ABA block didn't fit once the engine was installed in the Fox;  his solution was the run two oil lines and have the filter mounted remotely.
• 1.8L / 2.0L 16v
• The simplest answer is "too much work and money".  A 16v should mount without too many problems, and it will bolt up to the stock 8v transmission.  But the battery will be in the way of the intake, and the exhaust manifold and downpipe will have to be custom-made, because a) the stock 16v exhaust manifold won't work in a Fox, and b) the bolt pattern and spacing is different from the 8v one.  With the money you would spend on getting a custom header made, you could do some work on an 8v head to make it as good as a stock 16v one.
• 1.8T
• An intriguing possibility.  The block is essentially the same as any VW 4-cyl block, so it should bolt up into the car without any big problems.  The alternator, power steering pump and A/C compresssor are in different spots from their Fox couterparts (except for the PS pump, since Foxes never had them), so those might pose a small problem.  Audi A4s and VW Passats came with longitudinal 1.8Ts, so those would be your best choice. Audi A4s also had mechanical throttle bodies, so that would make the swap a bit simpler.
• Audi 5-cyl
• Impossible, without extending the front end.  WAY too much work.  Different transmission too.
• VR6
• Again, custom all the way.  Mounts, exhaust, transmission adapter, etc..  Plus you'll add some extra weight to the already-nose-heavy Fox, which will negatively affect handling.
• Audi V6
• This has been done.  A V6 twin turbo Quattro Fox wagon.  The cost was allegedly astoundingly-high, and shouldn't really be reproduced, unless you have a huge budget.  Info here and here.
• W8
• A very intriguing possibility, with a base 275 HP.  But this engine will probably be quite heavy, not to mention posssibly quite wide with the exhaust included, and it is unknown whether it'll even fit in a Fox engine bay.
• Aircooled 4-cyl / 6-cyl boxer
• Since the original BX cars in Brazil came with aircooled 4-cyl boxer engines, the possibility exists to swap in some type of 4-cyl Porsche engine.  I imagine a transmission would have to be brought in from Brazil.
• Subaru 4-cyl boxer
• These engines are swappable into VW Vanagons, so they might work in Foxes too...
• Transmissions:  The better the gearing of the transmission, the quicker the car will be.
• The ideal transmission to hunt down is the 3M/9Q, because it has the tightest gears and a nice low 5th for low highway RPMs.  The 2P is also geared very tightly, but the 5th is fairly high, resulting in high RPMs on the highway.  The PSA and 2N are geared identically and will also yield an improvement in acceleration over the stock 4-speed.  I have a chart with the gear ratios of various transmissions here.
• .

Theory
There are several reasons for lowering a car.  Most of them center around improving the handling characteristics and performance, but in addition to this, a lowered suspension alter the looks of the car.  When lowering a car, specifically one like our Foxes with a MacPherson strut suspension in front, it is important to remember that for the best handling, the control arms should be parallel to the ground.  If you lower the car beyond this, it will negatively affect handling.  When a car is lowered, its center of gravity gets lower, reducing the body roll during cornering and also weight transfer.  Lowering springs not only lower the car, but also stiffen up the suspension in order to compensate for less suspension travel, and also to help with cornering.  This is the reason why it is recommended to use actual lowering springs rather than cutting stock springs a coil or two.  Springs made for lowering are often made so they have a "progressive" spring rate, meaning that up to a point they are fairly 'loose', but beyond this point they are stiffer;  this really helps on a car that is still used as an every-day street car.  In the Fox case, however, since real lowering springs are scarce, one may have to resort to cutting a set of stock springs.  I do not know anything about cutting springs, so I cannot comment on the effectiveness of this solution.  In addition to stiffer/shorter springs one should also upgrade the shock absorbers.  A stock-spec shock will not last long with shorter springs, because it is not meant to operate in the range that a shorter spring needs shock absorbing.

VW Fox
Unfortunately all companies that ever made lowering springs for the Fox have discontinued them.  You might get lucky and find some old stock sitting around at a tuning shop somewhere, but chances are you won't.  Several Fox owners have done research regarding any possibly crossover of parts from other VAG products.  While almost none of this has been verified in practice, it works in theory:
Front:  Audi 4000 / VW Quantum non-Quattro/Syncro springs + Fox shock inserts.
Rear:  Corrado G60 springs + Fox shocks or A2 Golf springs & shocks. [I am still trying to verify this.]

If you are planning on getting custom springs wound, it is adviseable not to lower the car more than 1.25-1.50".  The Neuspeed springs drop the car approximately 1.0-1.25", and this brings the control arms paralell to the ground, the optimal position for handling.  Here's a picture of my car's control arms, with Neuspeed lowering springs.
Shock absorbers:  There aren't many options for Foxes.  If you have a big budget, go with Bilsteins.  For adjustability go with Tokico or Koni.  For the "best bang for the buck" go with Sachs Super Gas.  I'm using these with Neuspeed springs and am satisfied.  The rear end still sits down slightly under hard acceleration, but for price, I'm not complaining.

Theory
The larger the rim size, and the smaller the sidewall size (prividing you keep the overall height the same), the less lean the car will have in turns.  This is because there is less sidewall to flex.  However, rims above 15" in diameter might not fit properly, and will also yield a harsher ride, due to less flex in the sidewall.

Tire sizes explained:  175/70-13.  175 = 175mm tire width.  70 = the sidewall height is 70% of the tire's width.  13 = rim diameter in inches.
Rim sizes explained:  5.5Jx13H2 ET45.  5.5 = rim width in inches.  The J refers to the design of the rim's lip (I have never seen anything other than a J).  13 = rim diameter in inches.  (I have not been able to determine what H2 means.)  ET45 = offset in mm.  [The offset is how far the mounting surface (the part that touches the brake when mounted) is from the centre of the rim - an ET0 would have the mounting surface in the exact middle, ET 45 means that the mounting surface is 45mm towards the outside of the rim moving the wheel closer to the inside of the car, and ET-45 would mean that the mounting surface was 45mm towards the inside of the rim moving the wheel farther from the inside of the car.]  Rims also have a hub size, which is specific to certain cars.  The hub size is the round hole in the middle of the rim, which centers the wheel once it's mounted on the car.  While it's possible to mount a rim with a bigger hub, it is quite dangerous, as the wheel may not be centered and will cause some serious steering/handling problems.  The bolt pattern, which isn't often stamped on the rim, is the number and spacing of bolts that hold the wheel to the car.  The Fox's bolt pattern is 4x100, meaning there are 4 bolts and they are 100mm apart.

Fox
The stock tire and rim size on Foxes is either:
5Jx13H2 ET45 with 155/80-13  or
5.5Jx13H2 ET45 with 175/70-13 or
5.5Jx13H2 ET38 with 175/50-13  [this was the factory-installed aluminum rims]
All rims have the 4x100mm bolt pattern shared by all 4-bolt Volkswagens and even some Audis.  Be careful with Audis, as some of them have a 4x108mm bolt pattern, and are therefore incompatible with the Fox.

In order to keep your speedometer showing the proper speed and your odometer the proper mileage, it is adviseable to stay with the same overall wheel diameter.  The following is a list of tires that have the same overall diameter:
155/80-13
175/70-13
185/60-14
205/50-15

When choosing rims to upgrade to, the following should be taken into consideration:

• Fitment.  Obvious - make sure they have the same bolt pattern, hub size and offset.
• Looks.  This is a personal choice.
• Wheel weight.  The less the unsprung weight the car has, the quicker it will be.

Any Volkswagen 4-bolt wheel will bolt up on a Fox, up to about 15" in diameter.  Above this, the rim might rub on suspension components, or on the body.  And even with 14"s, if the offset is wrong, the tire will rub on the body, even if the car is not lowered.  I have this problem with my rims - 6.5Jx14H2 ET33 with 185/60-14 designed for A3 Volkswagens.  While they bolt up without any problems, the tires rub on the inner fenders in the rear when the car goes over a bigger bump on the road.  Hondas share an identical bolt pattern and hub size, so they are another source for wheels.  BMWs, Nissans and other cars also have 4x100 bolt patterns, but their hub sizes might be different, making the wheels unsafe to use on a Fox.

One other option that is available which will make the car quicker (at the cost of a lower maximum top speed) is to run smaller-diameter tires.  The smallest-diameter tires available for 13" wheels are 175/50-13 and 195/45-13.  These will yield a much quicker acceleraion, but will lower your top speed, and also make your speedometer and odometer inaccurate.  Using this size tire will also help improve handling a bit, since they lower the car's center of gravity and reduce the sidewall height.

Theory
Brakes are among the most important upgrade you can do.  No matter how fast or slow your car is, if you can't stop it in time, you will be in big trouble.  For everyday driving, the stock brakes are adequate (otherwise, the factory would have used something better).  Here are various styles of front disc brakes explained:
Solid rotors:  This is exactly what it sounds like - a solid disc of metal.  Foxes come stock with these.
Vented rotors:  Many street cars come with these - essentially two solid rotors connected by fins to help with cooling.
Cross-drilled rotors:  These are usually aftermarket - the rotors have holes drilled right through the contact surface, again to help with cooling.
Slotted rotors:  These are also aftermarket - the rotors have grooves cut into them to help with cooling, and to get rid of the dust that is produced during braking.
And combination of the last 3 is possible, i.e. vented, cross-drilled and slotted rotors do exist.
Unless you do a lot of hard stopping withing a very short time (such that the brakes heat up so much that by the second or third hard stop they are too hot to work properly), you do not really need to upgrade to anything more than vented rotors and appropriate calipers.
For the rear brakes, again, unless your car sees track time, where the brakes would see many hard stops within a short period of time, you do not to upgrade beyond the stock drums.  The majority of braking is done by the front brakes, so you do not need to upgrade the rears on a street car.

Fox
We Volkswagen owners are very lucky because VW and Audi share brakes among many different models, and most brakes are interchangeable between various models, making brake upgrades a simple "out with the old, in with the new" job.
Front
Volkswagen brakes come in 3 sizes:  9.4", 10.1" and 11.0".  Only 9.4" and 10.1" are easily swappable onto our Foxes.  With 9.4" brakes you can use any rim size above and including 13", whereas with 10.1" brakes you have to use at least a 14" rim (and some 14" rims might not clear the brakes, so you'll have to test-fit them).

• 9.4"

In order to upgrade beyond the stock solid rotors up front, and stay with a 9.4" brake, you will have to get the following parts:  caliper, caliper carrier, rotor and pads to match the new rotor.  These can come from any of the following cars:
• VW Cabriolet (Rabbit body style, 1985-1993)
• VW Jetta GLI
• Audi 4000 & Coupe (non-Quattro)
• and others
• 10.1"

In order to go to the next size up, 10.1", which should be enough for everybody, you will have to hunt down a 16v Scirocco.  Grab the caliper, caliper carrier, pads and rotor.  (Obviously, the rotors and pads may be in bad shape if coming from a junk yard, so you might need new ones).

With all Fox brake upgrades, you have to switch the sides - whatever came on the left side on the donor car goes on the right side of the Fox and vice versa.  Good brake pad manufacturers include:  Ferodo, Mintex, Axxis Metal Master and others.

Rear
The stock drum brakes are good enough for even highly enthusiastic street driving.  If you still feel that an upgrade is necessary, Fox wagons came with larger drums in the rear (200mm vs. 180mm diameter), and many Volkswagens came with rear discs.  If upgrading to rear discs, it might be a good idea to also upgrade the master cylinder so that they get enough pressure, and while you're doing that, it would be worth your while to install brake proportioning valves so that you can adjust the brake bias (the balance between whether fronts or rears get more braking power).

Theory
Well, the theory behind modifying your interior is for it to be different.  This can be for looks, for functionality, for more information, or whatever else you want.  There are virtually no aftermarket parts specifically for the Fox, so doing anything is custom-fitting work.  Don't get discouraged, though, it's a lot of fun, and chances are you will end up with a completely unique setup if you work on it yourself

Fox
Here's a list of what can be transferred from other Foxes.  GL models, Wolfsburg models and even other Volkswagens can be a source for parts to make your Fox's interior different and/or better.

• Dashboard:  1987-1989ish Foxes came with a dashboard that had the radio in the dashboard and a "cubby hole" underneath it to store stuff.  The dashboard was modified somewhere around 1989 to include a knee-bar to help protect your knees in case of a collision.  While I haven't actually test-fitted the two dashboards, I imagine they are intechangeable.  The old-style dashboards also had a glovebox infinitely larger (and more useful) and that newer ones.
• GL options:
• Ceiling light with swiveling map light - easy to install.  Be sure to grab the sun visors, as they're a different shape.
• Map pockets on doors - easy to install.  Just remove your door panels, poke the screws through the material/vinyl, and the holes are already there in the backboard, ready to attach the map pockets.
• Tachometer - very easy.  Get an entire instrument cluster with a tachometer from a Fox, swap your speedometer into this new cluster (to preserve your mileage), reinstall.  Full instructions can be found here.
• Interior pieces:  door panels are interchangeable between 2-door and wagon models.  Foxes came with several different cloth patterns;  if you like a certain style more, just grab everything from the junk yard.  Within the cloth patterns, are both brown and grey colour schemes.
• Steering wheels:  Some Wolfsburg Foxes came with the "4-button" steering wheel.  There was also a sport steering wheel available as an option for Foxes.  Steering wheels from A1, A2 and B1, B2 Volkswagens will bolt up without any problems.  No horn re-wiring is necessary.
• Seats:  the following cars can donate seats that are a direct swap into Foxes:  Audi 4000, Audi Coupe, Volkswagen Quantum.  All other Volkswagens and Audis have floor rails that are wider than the Fox's.  It is somehow possible to swap the seat bases, but I haven't done this and have no idea how to proceed with it.  For the rear seat, I am unsure of what will directly swap over;  I did test fit the rear bench and backrest from my ex-1984 4-door Jetta GLI and it fit and had the same mounting points.
• Additional instrumentation:  such as the 3 VDO gauges found in Audis and GTIs are fairly easy to install.  For the old-style dashboards you can mount them in the cubby hole, or get a pillar mount.  I have 3 Audi VDO gauges (voltmeter, oil pressure, oil temperature) installed with a newer dashboard (picture) - one gauge is just taped on using its stock bracket to the radio console, and the other two are in a pillar mount on the left A-pillar (I believe the pillar is meant for an Eagle Talon - just take your A-pillar trim piece to a car tuning shop, and match up a gauge holder to your pillar mount).  The voltmeter just wires into a +12V and a ground, the oil temperature sensor should be installed in the oil filter flange, and the oil pressure sensor (depending on its style) can go either in the oil filter flange or in the back of the cylinder head (albeit with an angled adapter, so that it doesn't hit the battery).
• .

Theory
Again, this is highly personal - modifying a car to suit one's tastes.  This can be for looks, for better lighting, etc..
PLEASE don't be discouraged by the lack of parts in this section - improvise, modify, adapt and even make your own parts!

Fox

• Body kits, spoilers, etc.
• The following were available through Volkswagen dealerships as Autobahn accessories:  front airdam (mounts below the lower part of the stock bumper, using the bolts that are visible), side skirts (mount using 4 drilled holes on the inside of the fender wells), trunk spoiler (mounts using 4 drilled holes), rear window louvre, trunk luggage rack.
• Known-to-work front spoilers:
• Stock A3 VW front spoiler:.
• Mid-1980s Chevrolet S-10 pickup front spoiler:.
• Erebuni used to make a body kit, but it's discontinued.
• Kamei makes a universal-fit rear spoiler that is very similar in design to the Autobahn one.  The link is to an online store that carries them;  I have no affiliation with the store, but since I couldn't find a link to it on Kamei's web site, I used this one instead.
• No one in North America makes badgeless grilles, clear/smoked turn signals or taillights, etc..  They are readily available in Brazil.  I am working on getting a source for them.  As soon as I know something it will be posted up on my web site and on any and all Fox forums online.
• Lighting
• Light bulbs
• Be careful if trying to use the "high output" bulbs, as they are much hotter when on, and can actually melt the plastic housing of the stock headlights.  Also stay away from rainbow-coloured headlights, as they shine worse than stock bulbs!
• 1987-1990
• H4 headlight lenses are made by companies such as Bosch and directly replace the stock sealed beams.  H4 headlights give a much better light pattern on the road.
• With some work, you can also switch over to a different grille/light setup.  Click here for some ideas.
• The newer-style taillights fit on older-style Fox sedans with some minor modifications.
• The newer headlights and grille are a lot of work to swap over, because the hood, fenders and rad support are all different.  Unlike A-platform Volkswagens, the rad support isn't merely bolted on.
• 1991-1993
• No H4 upgrade is available locally, however, some Foxes made it from Brazil with Brazilian-spec H4 lenses.  As mentioned above, I will work on getting these types of parts from Brazil.
• Fog lights, etc.
• As with any car, you can get fog lights, driving lights and others and install them on your car.  Be sure to mount them and use them properly (i.e. use fog lights only when you need to see better, use driving lights only when you would use high beams, etc.)  From my experience it is more adviseable to go with a larger fog lamp that uses an H1 bulb, rather than with the purpley tiny ones.
• Other:
• Parts like front end covers are available (I have one made by CoverCraft), full car covers are available (I have a universal-fit one also by CoverCraft), and I'm sure there are some other parts out there too.

The information found on this page is a collection of what I have learned from many people.
Big thanks go to Jonathan, Joshua, Karl, Heather, Kenny, Louis, VWoT, the two online VW Fox clubs, Dave @ Motronix, and anyone I may have forgotten!