Purpose

The throttle body is the part of the engine that lets air in.  It is composed of 2 flaps (often called "butterflies"), which open and close accordingly with how far the gas pedal is pressed.  A larger throttle body will allow more air into the engine, thereby improving the performance - both throttle response and horsepower.  This upgrade should only be done after you've upgraded the exhaust system.  The Fox's stock exhaust is so restrictive that the increased airflow from a larger throttle body will make the engine run worse when compared with the stock one.

If you own a CIS/CIS-E Fox (1987-1990), then there are upgrades that you can perform.  If you own a Digifant I/II Fox, you already have the largest stock throttle body that VW put on it's 4-cylinder cars, so there's no point in looking for a larger one since they don't exist.

In my 1992 Fox the primary butterfly is 35 mm, and the secondary 52 mm.

Also, it is highly adviseable that you have bettered your exhaust system prior to performing this upgrade.

Easy:  find a 1991-1993 VW Fox in a junk yard and get the intake manifold and throttle body.  They are a direct bolt-on for 1987-1990 Foxes, and include a blank spot to mount the cold-start enrichment injector, aka the "5th" injector.  The only thing you will have to do, is cap one vacuum fitting which is unused by CIS and CIS-E.
Difficult:  read below.  This should only be attempted if you have exhausted every possible resource and cannot find a 1991-1993 VW Fox to loot parts from.

This is a rather extensive modification, so don't try to perform it yourself if you don't have the experience with porting metal.  I do not suggesting going this route unless you absolutely cannot find a Digifant Fox intake manifold and throttle body.

Also, do not be fooled by the adapter plates that some tuners sell.  While the throttle body itself may be larger, if the air still gets into the intake manifold through the same sized opening, you'll gain nothing.

This larger throttle body is found on ALL A2 Golfs (i.e. 1985-1991), as well as most Digifant cars (including the 1991-1993 Fox).  Don't be fooled, you can't use the same intake manifold as the Digifant cars (except the Fox one) though - they don't have a place for the cold start enrichment valve!

The Golf throttle body is nearly the same as the one Neuspeed sells (same size, slightly different design) and the large Audi throttle body as well.

The following are directions for fitting a large throttle body from an A2 Golf to a CIS-E Fox.
Please keep in mind these instructions are here for your convenience, and if you don't have the proper skills and tools, don't undertake this upgrade.

After obtaining another CIS-E Fox intake manifold, have your local machinist have a go at the manifold. The following needs to be done to it:  Using the gasket for an A2 golf throttle body / intake manifold mating, draw the inside lines on the manifold. You'll notice that (usually) the lines go out further than the manifold.  Well that's where the machinist comes in. Have him (her) weld extra aluminum around the end of the manifold where you ran out of metal. Now you're ready to port it!  Redraw your inside port lines on the mating surface of the manifold (where the throttle body goes) and start going. This takes quite a deal of time, so be patient.  One other way is to cut out the end where you have to port most BEFORE getting the welding done (using a torch of some kind), THEN welding on just enough so that you don't have to grind away at it for days.  Make sure to add enough aluminum to it so you can mount the throttle body too (i.e. add enough so you can drill and thread holes ).

After you have ported out enough that the throttle body and intake manifold match each other, mark the position of the holes on the welded side (the other side should already line up).  Drill 2 holes for your screws, and have them threaded (either at the machinists, or do it yourself)  Now you can polish the top of the manifold (inside) if you like, then mount the throttle body.  Be sure that when you port it, you create a good shape on the interior so that the air flowing in doesn't get turbulated (i.e. funnel it back gradually for better flow quality).

After this, you might as well get all the ports at least gasket matched. This is farily easy to do on your own if you have a dremel-style tool.  Take the gasket used for mating the intake manifold to the head, and draw the inside of each hole on each corresponding port. Using a bit for aluminum (soft metals) you can now port out the first inch or two of the runners.  Polishing is also another thing you can do afterwads.  Make sure you don't get any metal bits into the ports, and blow out the intake manifold when you're done!

You now have a larger throttle body with a matching intake manifold for your Fox.  Following the instructions in your Bentley manual, remove the existing throttle body / intake manifold together (don't forget to mark all those vacuum lines!).

Here's a tricky part.  Look at your old throttle body, and look at the new one.  Notice that the throttle cable attaches differently.  Well you have to fix that.  Not hard.  You have to remove the top portion of the big one, remove the next piece (remember how the springs go), then do the same from the other.  Use the second piece from the small one (trhe piece that holds the throttle cable) and the top from the big one. I think these should be correct, you may want to make sure there is only one piece above the cable holder.  Now, reverse removal of old unit, with new one.  Follow all stock torque values, and enjoy!

Big thanks to Josh, a fellow VW enthusiast for this information!!

If you have any questions, you can e-mail me, but because I haven't performed the upgrade myself, I cannot relate anything from experience; you can also contact Josh with questions about this.