The most complex device on your car is one of the most widely updated items on any machine. Any shaded mechanic with a tool and a 6-pack can do exhaust jobs, but when you’re working with off-the-shelf parts, the finesse and engineering of developing a great exhaust system is overlooked on most.
When discussing headers in specific, it’s hard to have more information from someone about what and why past “more flow is faster, bro.” So, I’m going to address that and give you all you need to know about why they’re upgraded, and help remove some of the myths around them, as well as the concepts in general, to make an effective exhaust system.
Exhaust Manifold vs Exhaust Header
This is a common issue that comes up whenever headers show up. Why is it that stock parts are typically referred to as manifolds, but aftermarket variants are termed as headers? Although they do the same thing, the distinction is that the exhaust manifold is a sturdy cast iron part for all cylinders, while the exhaust header is constructed up of independent steel tubes for each exhaust outlet, welded to meet with a collector or multiple collectors to carry the exhaust gasses down to a single pipe, ready to pass through the remainder of the exhaust system.
What’s scavenging, and how do the equivalent size headers assist?
Essentially, scavenging can be spoken of as the inverse of backpressure. With an effective exhaust flow, the outgoing gasses can produce a vacuum behind the outgoing pulse. When the valves expand between the cylinder strokes, there is a happy medium where the intakes and exhaust valves are at least partly open at the same time. At this small period, the vacuum would aggressively pull clean air through the intake valves, leading in an effect that is not different to what you will get from moderate forced induction
More air in the cylinder implies additional power, extra power means more happiness.
2 things happen when the exhaust valves are opened; the first is a high-pressure shock surge generated by the quick expansion of hot exhaust gasses as they escape the valve, moving at approximately the speed of sound (about 767 miles an hour). After that is the real blast of exhaust gases. This is best left unchecked by leaving the exhaust headers via the collector in the remainder of the exhaust network.
Once this shockwave hits the end of the tube and adjusts the ambient pressure, a low-pressure pulse returns the tube to the cylinders to pull the air into the cylinder.
Where equally sized headers come into the equation is to aid here, it is by guaranteeing that each pipe from the cylinder head to the collector is of the same size, that the exhaust bursts have the same length to travel to get there. That means, there is always an empty path for the exhaust to release as much back pressure as possible for the next burst to come back down that line, meaning that the optimal requirements for scavenging are as similar as possible.