Building High Performance Headers | Getting the hot exhaust out of the cylinder is a primary process of a combustion engine.
If you can't get the exhaust out, you can't get fresh fuel and air in. In order to help the engine breathe, headers or exhaust manifolds are high on the upgrade list as these are the first connection to the exhaust ports of an engine.
The terms headers and exhaust manifold are often interchanged, but can be categorized differently. The stock exhaust on most street car engines is made from thick cast iron and is usually referred to as an "Exhaust Manifold". These manifolds collect multiple exhaust ports of an engine and merge the into and single outlet within a fairly short distance. The thermal properties of cast iron make it an ideal material for this purpose and it is durable.
"Headers" are usually made of tube steel and have much longer runners for the individual ports before merging together. Depending on the type of engine, the individual runners may be designed to be all the same length, or varying lengths.
The type of engine you have will greatly determine the type of exhaust you need. A naturally aspirated motor will be very sensitive to the design of the exhaust. Changing the length of the runners or the way they merge together can sway the power output by 30hp or more.
Turbo motors aren't quite as sensitive to the runner design, but can see huge differences due to materials used. A tube steel turbo header may make more top end power, but a cast iron manifold may spool faster due to the heat retention keeping the gasses hot.
When it comes to building a custom turbo exhaust, there are 3 materials we can choose from. Stainless Steel Tubing, Schedule 10 Stainless Steel Pipe, or Cast Iron or Stainless. The cast options are limited to whats available to purchase. The processes and equipment involved in making a high quality cast part limit the process to dedicated manufacturers.
If a cast manifold is not made for your engine, you probably can't have a cast manifold. The most common type of header is made form Stainless Steel tubing, make sure that you know what type of stainless before you begin. 
Stainless tubing is durable and can handle high exhaust temperatures, especially when using 304 or 321 SS tubing. The thinner tubing offers a considerable weight savings over cast manifolds, but at the cost of durability. It's not uncommon for a tube header to develop stress fractures over time.
The ultimate material for custom street headers is Schedule 10 pipe, especially when heat retention keeps the turbo spooled. This is made up of cast sections of pipe that are meant to be welded together.
You get the thermal benefits of a cast manifold but the design freedom of tubing. The wall thickness of schedule pipe is about 4 times as thick as tubing. This makes it much more durable and able to handle extreme exhaust temperatures and resist cracking, choose wisely.
