confused on how to work out 2 stroke expansion chamber

I am trying to basically reverse engineer aa 2 stroke pipe for a dirt bike, but I'm confused on how you would get the exact dimensions into fusion. ill attach some photos of some of the other people's cad pipes ive seen

That's a challenging one for sure, but definitely doable.

I think it would be best to try the loft tool, and create sketches at each stage where the measurements are known. You might need a 3D sketch of the centreline to help positioning each section along that line somehow, and you could potentially use that as a guide for the loft as well.

Alternatively, it's going to be a matter of using the sweep tool and guide rails.

You might also be able to use those drawings as canvases to help with reference for the Curves