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

Hello I'm a CAD noob but have made a 100 or so pipes for bike/superkarts. I would be running miles from that design. Aftermarket pipes for dirt bikes rarely match the OEM pipe in the over rev area (and sometimes in the run up to peak power and peak as well). True they might look better :-) If you're going to do all that work you might as well make the end result worth it, and that comes down to the design, which is another subject all together.

To answer your question using the canvas to get the angle of the bend would be where I would start then using the loft to from one diameter to the next but for me unless you have a black belt in CAD a sheetmetal program is going to do that instantly as well as give you a printed pdf and/or a DXF