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Exhaust manifolds bolt to the cylinder head, and allow all the cylinders on the head to merge into 1 pipe. Most manifolds are engine and year specific. Most manifolds are cast iron, some of the LS series manifolds are stainless steel. Contrary to what most people believe, cast iron holds up excellent to heat, which is why most manifolds are made from cast iron.

So, if cast iron manifolds are so great, then why do they crack? Exhaust manifolds crack for several reasons. Exhaust components are usually overlooked as a part that needs inspection, so maintenance and lack of inspection is the main reason. The largest contributor is exhaust leaks from gaskets. When you have an exhaust gasket that leaks, air enters that crack from the pulsation of the exhaust gases. That cooler air causes the hot manifold to go through a heat cycle more rapidly, and due to properties of the metal, it will crack. Worn exhaust mounts, missing exhaust mounts, broken support brackets also play a role in cracking the manifolds from the excessive vibration. Your climate also affects the manifolds. Vehicles that are in colder states cause the manifolds to cool alot quicker than normal when the engine is shut off, and cast iron does not like to be cooled rapidly.