manifold (fig. 4-47)
connects all the engine cylinders to the exhaust system. It is usually made of cast iron.
If the exhaust manifold is properly formed, it can create a scavenging action that will
cause all of the cylinders to help each other get rid of exhaust gases. Back pressure (the
force that the pistons must exert to push out the exhaust gases) can be reduced by making
the manifold with smooth walls and without any sharp bends. All these factors are taken
into consideration when the exhaust manifold is designed, and the best possible manifold
is manufactured to fit into the confines of the engine compartment.