Then why not run 8 single exhaust systems
Headers are takeing advantage of tuneing, collectors are also needed for this tuneing, the rest of the exhaust system is primarily dealing with just flow. If you understand what is really going on with a tuned length header/collector you will understand why they are run together... the piston forces exhaust out in one big "lump" really fast, this "lump" goes flying past the end of the pipe of the next pipe in the firing order, as that mass of exhaust flies past the other pipe the inertia it has pulls a vaccume on the adjacent pipe, if that pipe is just about to dump the exhaust then the first lump will help pull it out of that pipe, it is the venturi effect. In a normal V8 there is an un even firing order from side to side so you get two lumps really close together, this is where the X pipe comes in, or the old fashioned less effective H pipe. It allows that big supper lump to bleed from one side to the other and allows scavenging from one side to the other, now the ultimate exhaust would be to join the two pipes into one at just the right time for the two big lumps to help pull each other out the entire length of the pipe in a single exhaust configuration. After a certain point in the system there is gonna be no significant tuneing effect and simply raw flow numbers are gonna make the most HP and a single pipe with the same cross section as a dual system will flow more, then it becomes a matter of packageing the system and most cars will have a hard time fitting a single 3.5 inch pipe with out dragging the ground or bending it too tight, this is the only reason dual exhaust became popular for high power machines. A dual system has more heat loss due to surface area than an equal cross section single and heat loss costs HP also.
Now there is gonna come a point where simply raw flow rules supreme even over scavenging but that is top fuel territory (not even nascar or F1) where 8 single exhaust pipes rule but even then they still tune with the primary lengths instead of just running the exhaust right out of the head.
Here is how scavenging works
http://www.youtube.com/watch?v=pzhM9e5ZpbASee how he lets his breath pull more air to fill the bag with one breath instead of the 6 or 8 individual breaths, that is like how one exhaust "breath" from one cylinder pulls on the next one to better empty the cylinder and if you can get the cylinder more empty then you can pull in more A/F on the intake stroke and make more power. The pipes by them self would never make as much power if they were not able to pull their neighbors exhaust and by running duals you are limiting there ability to help there neighbor.
