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Exhaust Manifolds
The point of most aftermarket engine parts is to increase engine efficiency by creating as much mechanical force as possible from the energy stored within the fuel in the fuel tank. The engine has to breathe freely. Aftermarket intakes help the engine inhale plenty of fresh, cold air. Once that air is mixed with fuel and exploded in the engine, it has to go out the exhaust system. Smooth exhaling is vitally important to your engine.
A typical stock exhaust system is made up of the exhaust manifold, catalytic converter, back box, and various lengths of piping to connect them all. When it comes to performance exhausts, big pipes with as few restrictions as possible are required in order to avoid backpressure. Think of blowing hard through a drinking straw versus through a 2" pipe - big difference! Backpressure reduces the power of your engine because instead of directing energy at the crankshaft, your engine has to push the exhaust out under pressure. The greater the pressure, the greater the power loss. Some backpressure is inevitable, but there's a lot you can do to a stock exhaust system to minimize it such as replacing the exhaust manifold with an aftermarket manifold.
Manifolds are purely performance-based upgrades. Not only are their pipes bigger than stock, but each cylinder gets its own specially tuned pipe. The lengths, curves, and inside diameters of minifold pipes are specially designed to follow the rythym of your engine's exhaust valves so that every time a cylinder expels air, an empty pipe awaits. So, there's very little backpressure for the pistons to work against.
There are basically two main types of exhaust manifold available: stainless steel and ceramic-coated. Stainless steel manifolds provide a smooth surface for the quickest possible airflow. Ceramic-coated maniolds are usually stainless steel or cast-iron manifolds that have been coated with a heat-resistant coating that keeps your engine bay a bit cooler. You'll also notice that most manifolds have a design in which the four exhaust pipes join into two bigger pipes, and then the two join into one final pipe that connects to the rest of your exhaust system. This "4-2-1" design is most effective on higher revs, the "4-1" manifolds deliver a bit more power at lower RPM's. You can expect to see a gain of about 3 to 4 horsepower right out of the box.
