The thread Jamie mentions contains (in my opinion) mostly blah-di-blah.
The term back-pressure relates to the notion of exhaust gases being reflected when they encounter a restriction or obstacle (such as a bend or muffler) in the exhaust pipes. The reflections create a a pressure wave traveling back towards the exhaust valve (hence the term back-pressure), increasing the resistance for the gases coming from there and slowing the flow of gas in the process.
Below are some links to texts that go deeper on exhaust-related topics.
Hope this helps,
Ruedi
Exhaust Header Design Ideas:
http://tom.marshall.tripod.com/exhaust.html
Calculate Exhaust Header Length:
http://www.nightrider.com/biketech/c...austlength.htm
Comparison of two Exhaust Manifold Pressure Estimation Methods:
http://www.fs.isy.liu.se/Publication...CSSE_01_PA.pdf
Multiobjective Design Optimization of Merging Configuration for an Exhaust Manifold of a Car Engine:
http://reynolds.ifs.tohoku.ac.jp/edg...ons/ppsn02.pdf
Exhaust Manifold Design with Tapered Pipes Using Divided Range MOGA:
http://www.ifs.tohoku.ac.jp/edge/library/rifs2003.pdf