WHAT A TOOL.

 This is all about FreeCAD and my lack of expertise with it. I managed to make the four into one adaptor and that appears to be the easy bit. I have tried all sorts of cunning tricks to make the header pipes wander about exactly where I would like them to meander. I don't want to use curves or a spline which are basically different words for the same thing as it is really hard to control their dimensions and even harder to create them physically. 

To manufacture curves is a bit of a bugger. You have two choices, you can pie cut tube and weld together which done well looks very posh or you can cut out the shape as two matching flat pieces of metal weld the seams together and pump them to shape with oil. I used to know a bloke that made superb two-stroke exhausts using hydraulic forming and a lot of expertise.

I am slowly getting to grips with FreeCAD and do find any 3D stuff entertaining. I thought the Draft Workbench was a 2D facility and a clunky one at that. I then noticed as you sketched with it co-ordinates appeared and you can enter the line or wire endpoints, curve the corners and it also gives one the total wire length. More importantly it has a box where one can type in the dimension for the third axis. I think I'm halfway towards making it do what I want.

This is a simple one. Now all I have to do is remember what I clicked. There are many clicks and they have to be in the right order. This is the first bit of pipe that worked. I have indicated the wire which is the Draft bit. 

This is the complete header to tailpipe joint. 

I expect to get a few weeks of fun with this and that is before I do the gas flow simulation which will probably be totally irrelevant as exhaust gasses come in pulses on an internal combustion engine and I doubt the algorithm can cope with those or if it can can I.