Exhaust headers help to improve an engine's performance by expelling gases from the cylinders after combustion. They make the removal of exhaust from the motor more efficient, which cuts down on the amount of work the engine has to do and improves its performance.
In a standard engine, the gases are transferred directly to the exhaust manifold, where they travel down the exhaust pipe and out of the vehicle. The problem that this set-up causes is that the gases from one cylinder can build up pressure in the manifold. The next cylinder will have to work harder to get rid of its own gases, which will waste unnecessary power.
The headers allow each cylinder its own exhaust pipe, and these all join together to form a larger collector pipe. The individual pipes are all the same length so the gases reach the collector evenly and separately in order to reduce the amount of back pressure on the manifold. When less energy is devoted to pushing out exhaust gases, the engine will run more efficiently.
Custom exhaust systems, such as headers, are usually not specific to a car's engine or design. The only things that definitely need to be taken into consideration are the proper sizing of the pipes and choosing the right gasket type and size.