think of a garden hose <straight pipe> this setup tends to favor extremely smooth easy to use low end power. Beginner bikes, chainsaws and weedwhackers although no pipes are closer to straight pipe characteristics than expansion chambers.
think of a garden hose that was trying to pass a football through it,bulge in the middle with slightly tapered cones. This shape has more of a "hit" to the power delivery but wide and controllable.older non powervalved bikes and big dirt bikes ,cr500 etc.
think of a garden hose trying to pass a basket ball, large bulge with steep cones . this is a high revving hard hitting top end of power,think screaming 80's in the expert class.
To build an expansion chamber correctly you will need to measure just about everything in your engine,go thru a long and tedious math problem to know what kind of cones and where they go to help the dimensions of the engine. And then hope the engine can dissipate all the extra heat it is trying to harness so you don't melt the piston. To guess on making your own pipe your chances are better that you are going to have less power than you already have in most of your rpm range if not all the range or melt the piston than you have getting lucky .