As you may already know, exhaust gas is a byproduct of combustion. These gases can be very harmful and need to be routed away from the cab of the vehicle, which is exactly what the exhaust system does. After the exhaust ports in the cylinder head, the headers are the first point of contact for these exhaust gases. How well your headers flow can have a massive impact on thermal efficiency and overall power output.
As stated above, the main purpose of exhaust headers is to take exhaust gases from the cylinder head and route them to the rest of the exhaust system. Typically, OEM exhaust manifolds are made of heavy cast iron and don’t have the greatest design. For the most part, OEM parts like the headers are built as cheaply as possible to keep the overall cost of manufacturing the vehicle low.
If you want to get the most performance possible, the OEM headers can be swapped out for tubular headers with a larger diameter and a better design. When an exhaust pulse comes through the header it creates a low-pressure area behind it. With a good design, this low-pressure area can help pull the exhaust pulses behind it. This is known as scavenging, and a well-designed header will greatly improve scavenging efficiency which leads to much greater flow.
The shape, length, and route of each header pipe has a massive impact on total flow and scavenging. In many applications, longer header pipes lead to better performance. Most exhaust headers are built as “equal-length” headers, meaning each header tube has the same overall length. In some applications, most notably Subaru EJ engines, the header tubes aren’t equal length which produces a unique exhaust note.