Purpose

A 'header' is a replacement for the exhaust manifold and downpipe.  It is designed to give optimal flow from each cylinder, for optimal high-RPM performance.

I can't quote any laws or theories, but I can explain how and why it works.  (Please correct me if I'm wrong.)  With the stock exhaust manifold, the exhaust gases from each of the 4 cylinders are channeled to a single pipe on their way to the exhaust.  This causes some turbulence within the exhaust manifold and possible along the downpipe as well.  Because all 4 cylinders don't fire at the exact same time, there are varying amounts of pressure within the exhaust manifold.  What a header is made of, are 4 pipes (1 from each cylinder), each of equal length.  They eventually meet up, first joining into 2, and then the 2 into 1, from where they connect to the single exhaust pipe.  With the pipe from each cylinder being the same length, the exhaust gases don't interfere (as much) with one another during the engine's operation.  This reduces the turbulence in the exhaust, yielding some more power.

Headers, it should be noted, are designed for high-output engines.  I think they aren't legal everywhere for street use, and are also not as durable as an exhaust manifold and downpipe.  I hear it is a good idea to have them ceramic-coated and wrapped in special header-wrap (sorry, I don't know the proper term), if they'll be used on a street car year-round.

Some dyno testing that Techtonics did revealed that you get essentially the same power out of a header as you do out of the dual outlet manifold + dual downpipe combination.  Unless you're running a race car that will only see track use, a header isn't worth your time or money.

The only header currently made, that will fit the VW Fox, is made by Pacesetter and is designed for a 1.6L VW Dasher engine.  It will bolt up without problems to a VW Fox engine (just like a VW Dasher exhaust manifold), but I don't know how it's design will affect the bigger 1.8L VW Fox engine.

Here's the link to where you can order it at Impact Parts.
 

Model	Year	Description	Part #	Price
VW Dasher	1974-1979	Header 1.6	70-1097	$90.97 USD

I don't know much else aside from what is contained on this page.  Here is someone's account of what the Pacesetter header is like:

Well, I had the chance to try out the header I bought for a Dasher. I was changing the head on my daily driver car, and decided it was a good time to put the header in. Just a bit of advice, DON'T buy it. Not unless you want to modify it. It does not fit - It will bolt up nicely to the head, but does not clear the inner CV joint. The pipes need to be about an inch longer. Also, I had to grind down the flange with a Belt/Disc sander, because it is welded a bit sloppy, and would not have sealed to the head. Of course I did this before I tried the fitment, so now I have a perfectly smooth flange, which will not fit in the car. I can either try to add some lengh, or try to send it back, or have a $100.00 large paper weight.

It looks like there aren't any headers available for our Foxes.

I know Autotech used to make a Fox header, but they don't anymore.  I met someone at a VW Show in September 1999 who recently found an Autotech header at a local distributor, but it was old stock.