i want to ask some question about exhaust system :

1. how to determine the diameter and length of header pipe,determine the size of muffler, determine the baffle (basic calculation) in 4 stroke 1 cylinder engine ( generally in motorcycle )?

2. what is back pressure? whatz the effect on performance? how to avoid back pressure?
 
3. i want to give valve on my motorcycle exhaust pipe (4 stroke 1 cylinder, 107 cc)like EXUP system in yamaha and Suzuki Exhaust Tuning (SET). is it good? how to determine the valve position and the mechanism of the valve, so it can open and close depend on engine rpm?

thank u for the attention:)