what pipe for .......

This is a very good question because it's one of my "pet peeves" when somebody asks what pipe gives the most horsepower for a 400EX for instance. On a stock machine, you can build 10 different pipes and not really see more than 1 HP between all of them but as you build a motor the right pipe can make all the difference. The reason being, head pipe diameter can adjust your torque up and down, head pipe length will adjust where your power comes in at and the muffler system or megaphone on the end of that tuned length will also make a difference. Each rider has his own style of riding - one may ride the machine like a 125 always wide open and in a high rpm mode and another may ride like an open class rider short shifting and using the torque of the motor. The pipe can be adjusted to your riding style, the cam opening and closing and the size of the motor by changing head pipe size, length and whether or not you have a megaphone. Then you also have the question of whether you are running a spark arrestor with discs or screens. The discs tend to flow more; the screens usually kill the power pretty bad on a performance motor. With the disc-type exhaust systems (SuperTrapp, White Brothers, FMF, etc.), you can adjust back pressure for your riding style also. More discs giving less back pressure and more top end usually; less discs creating more back pressure giving more low end and throttle response. My point is, one pipe may work for one person and not for another. If you are building an engine you need to talk to your engine builder, discuss what type of riding you are doing and your riding style. I hope this answered your question without getting technical. I like to keep things simple