[Technick]

Basically when you heat something up it expands, when it cools down it contracts, with the exception of water. So the further down the exhaust pipe the exhaust goes the slower the exhaust goes, that is why most stock exhaust systems have the pipe diameter reduced the farther it gets from the engine, to keep the velocity up , which helps the scavenging effect of the exhaust. Why do you think that turbo's are as close to the exhaust manifold as possible? If it was back at the tailpipe it would produce little if any boost! That is another reason why turbo's have a lag issue. It takes "time" for the expanding hot exhaust gasses to get the impeller moving.The hotter the exhaust gases are the faster the turbo impeller spins! Basic physics!
Now close to the cylinder head, where the exhaust is the hottest, and fastest, you can get away with a "stepped" header which also helps scavenge the exhaust. By creating a change in header diameter (slightly larger, usually by 1/16") one can cause a "low pressure" area at that point that will help pull the exhaust down the header pipe. This does not work at the tail pipe or elsewere as the velosity and heat are not enough to sustain the effect.