Almost every exhaust system uses some form of a merge collector. The key role for this merge collector and the most obvious one is to combine the exhaust pipes of the different cylinder into one tailpipe, which could flow to the turbo or the tailpipe.
The NASCAR Sprint Cup regulations for example make the routing of the exhaust system and its assembly quite a challenge, especially where a reduced weight of the exhaust system is needed while maintaining reliability.
When designing a sheet metal part which is folded you need to understand what happens when a flat sheet of metal is fabricated or folded. To calculate the actual pattern dimensions you can use the bend allowance or bend deduction method.
Next to the design of an exhaust system and its gas dynamics, the induction system of the engine is equally as important. To ensure that the flow into the runners is following a smooth path, the upper bellmouth must be designed carefully.