Header design, like many aspects of engine design, is the subject of many misconceptions or over-simplifications. The most fundamental (and non-intuitive) thing to understand about headers is that they are not, primarily, intended to enhance exhaust flow. They are primarily intended to enhance
intake flow.
The only article I've ever seen in the mainstream auto mags that actually explains this is the one by David Vizard for PHR:
http://www.popularhotrodding.com/eng...h/viewall.html
The one quibble I'd have with David's article is that he uses the term "velocity" in describing why excessively large tube diameters are bad. This makes people think that what is desired is high velocity in the exhaust flow itself - which is not what is wanted.
To give a simplified picture, there are two things moving through a header pipe: pressure waves, and exhaust gas. Think of a river flowing, with waves on the surface from a passing boat. The wave motion is mostly independent of the flow of the underlying water.
Same thing in a header - we have pressure waves moving rapidly through exhaust gas, which is also flowing out, but at a much slower rate. The pressure waves are what we use to perform the magical scavenging/intake flow trick, so we want them to be as intense as possible, which means we want to confine them into a tight space. But at the same time, we don't want the engine to have to do a lot of extra work to have to push the exhaust gas flow through the pipe. Balancing these two goals is the essential tradeoff when sizing primary pipes.