Headers do more than just funnel spent gases out of the engine compartment less restrictively than manifolds; they draw spent gases from the combustion chamber by a process called scavenging. During the exhaust stroke of the engine, the exhaust gas is forced out of the combustion chamber through the open exhaust port by a combination of lower pressure outside the chamber and pressure from the rising piston. The exhaust will continue to be drawn out as long as the exhaust valve stays open. If the timing is off and the valve closes with exhaust left in the cylinder, you lose efficiency. Headers draw maximum exhaust from the cylinder during the duration of the exhaust stroke.
The inertia from the escaping exhaust gases moving through the header pipes creates energy pulses that form vacuums, pulling additional exhaust out of the combustion chamber so that the maximum amount of fresh fuel and air can be drawn in. The more fresh fuel and air you can pack into the cylinder, the more power the engine can make. The less energy the piston has to use pushing exhaust out of the cylinder, the more it can use for something fun, like moving you down the road.
The primary pipes are the main components of any style of header. The most important consideration when selecting headers is the diameter of the primaries. According to the many header company tech folks we talked to, one of the most common mistakes rodders make when buying headers is getting pipes that are too large in diameter.
The diameter of the primary pipes directly affects the speed (or flow velocity) of the exhaust mixture traveling through them. Simply put, gas moves faster through a small tube than a large one. Change the pipe diameter and you change that speed. You also change the rpm where the torque peak occurs. The lower resistance of a large-diameter header moves the torque peak into higher rpm; a smaller-diameter pipe moves the torque peak to a lower rpm. Bolt on a set of headers that are just too big and your torque peak will move so far up the rpm range, you'll never see it again--and you'll wonder why your new headers screwed up your motor.
The length of the primary pipes also affects torque. Where diameter affects the torque peak relative to rpm, the length affects the shape of the torque curve. Longer primaries provide more torque below the peak and reduce it past the peak. Shorter primaries provide more torque above the peak at the expense of below-peak torque. More torque in the low- to mid-rpm range is important to rodders who want that feeling of seat-of-your-pants performance on the street. Longer primaries also reduce the chance of escaping exhaust's being drawn back up another pipe. 8)
