Here is a rundown of how I made my chamber for my RD400 based Drag racer.Now I'm no expert so if you use my dimensions, to build yourself a pipe, and it doesn't work.Well don't blame me.
So this pipe is made for a Drag bike that will make its peak power at 11000rpm and it has had the exhaust outlet milled out too 41mm from the std 38mm.
This explains why the header pipe starts at 41mm instead of 38mm and why the belly is 125mm
This pipe is not even close for using on the street but the methods used in making it still apply.

These dimensions why developed by using  BIMOTION. Then refined by running hundreds of simulations on MOTA.
This pipe has not been run on the Drag strip yet so it still all theory at this stage.

Here are some MOTA dat files...use at your own risk.
RD400E standard
RD400E Drag motor