Before I Start here is a disclaimer - I am not a professional CAD or Mechanical Engineer to know much about this. Also, All information I have is from the internet and have NO practical knowledge. Here we go then. I am currently working on building a High Performance Free Flow Exhaust for my Bike (Yamaha FZ16). Attached is the CAD design I made for the Exhaust (I am not a professional CAD designer but do it as a Hobby - by the way this is my first CAD Drawing). I was looking in to Details about how to make a FFE. I got a formula to develop a FFE. Here is the link to Document which gives details on how to calculate the Diameter/Length of the pipe. http://www.bgideas.demon.co.uk/tmanual/Tm_Ch4.pdf Using the above formula I get the Diameter, Length of the header pipe, secondary cone etc. When I calculate the formula I got Diameter of the Primary Pipe around 30mm. I took 35mm as I will have bend just after the Gases Come out of the ...