The exhaust manifold is the component of an exhaust system which connects the exhaust ports on the engine block to the downpipe. Some problems you may have with a manifold are cracks, warping, broken studs and bad gaskets. Unfortunately most of these problems require a replacement of the manifold.
Exhaust manifolds are generally simple cast iron or stainless steel units which collect engine exhaust from multiple cylinders and deliver it to the exhaust pipe. For many engines, after market high performance exhaust headers are available. These consist of individual exhaust headpipes for each cylinder, which then usually converge into one tube called a collector. Headers that do not have collectors are called zoomie headers, and are used exclusively on race cars.
The most common types of aftermarket headers are made of either ceramic, or stainless steel. Ceramic headers are lighter in weight than stainless steel, however, under extreme temperatures they can crack - something stainless steel is not prone to.
The goal of performance exhaust headers is mainly to decrease flow resistance (back pressure), and to increase the volumetric efficiency of an engine, resulting in a gain in power output. The processes occurring can be explained by the gas laws.
There are certain vehicles that are prone to developing manifold leaks. Also, due to many different types and configurations of motors in modern vehicles, the degree of difficulty in replacing an exhaust manifold can vary greatly. Unfortunately, this also means the cost of such a job can vary greatly too. Some vehicles exhaust manifolds many even have a catalytic converter integrated into it. This setup can significantly increase the cost also.