The exhaust of your engine is considerably more complex than just some pipes that lead to the back of your car, and once you have a look at exactly what a header is and what it is capable of, you'll have a greater appreciation of this.
In our first article we looked at what the function of an exhaust system is and why keeping heat and limiting volume in your exhaust is important (this is not back pressure!). A header is a system that uses volume, heat and something called an exhaust pulse to the advantage of your engine and can free up some substantial torque that you can then use at your wheels instead of pushing noxious gases around.
Manifolds and Headers
First off you need to understand what the difference is between an exhaust manifold and a header. From even a casual visual inspection the differences should be clear. Image one shows an exhaust manifold from a late 60's Ford V8. Image two shows a modern header, also for a Ford V8.
Image One: Manifold
A quick glance at the header shows that it is multiple pipes... one pipe per cylinder, actually. Instead of collecting all the exhaust in one box and then forcing that into a pipe, a header gracefully allows each cylinder its very own pipe that is later fed into the exhaust pipe in a much more organic fashion. The first reason that headers work better is because with a manifold your cylinders are often fighting against each other just to push out the exhaust gases and with a header each is free to push only the gases that it has created.
Exhaust Pulses
OK, now here is where things start to get pretty slick. A properly tuned header not only frees up your engine by keeping it from fighting itself to get rid of its exhaust, but it can also literally make the exhaust easier to flow out of the pipe. It does this by making use of something called an exhaust pulse.
Image Two: Header
What moves it down the pipe is the pulse in front of it, and the pulse in front of that one. As each pulse travels down your exhaust system it develops higher pressure in front of it, and lower pressure behind it. Each pulse pulls the one behind it along. A well tuned exhaust system will almost show a true vacuum between pulses which makes two amazing things happen: Your exhaust is pulling itself down the pipe without assistance from your engine, and when the exhaust stroke happens it is pumping gas into a vacuum... the exhaust gas is literally being pulled from the cylinder.
Given that as much as a third of your engine's potential is being used to do nothing other than pump exhaust gases around, you can see how this could add a substantial amount of torque to your rear wheels.
Keep in mind that a properly tuned header and exhaust system isn't adding power, it is simply freeing up power that your engine already had.
Pulse disruption
For all this to work, the exhaust pulses have to be sheparded all the way through your exhaust system and out the pipes or it all falls apart. Once the pulses are disrupted you are back to pushing raw, uncooperative gas through your exhaust and you are right back to square one. What are some of the things that can cause the pulses to disrupt? Heat loss is a big issue, which is why you see heat tape on the exhaust of high performance motors. That isn't to protect other things from the exhaust heat, it is to keep the heat in the pipe, because as air cools, it gains density and becomes harder to push. A large volume increase will also cause a temperature drop and density increase and will quite quickly kill of any coordinated exhaust pulses. The common suspect in that case are exhaust pipes that are too large, or large areas where exhaust can collect such as hollowed out catalytic converters. Most types of mufflers are pulse killers, as well.
Tune it!
The really really really smart fellers that spend their entire life as a monk in Tibet studying exhaust systems with David Carradene can do something pretty amazing with a tuned header and exhaust system: They can actually change the torque output curve of your engine. Torque can be flattened out and stretched out over a larger range, caused to come in earlier or later, caused to spike... all according to what the engine is going to be used for. This is obviously an expensive proposition and is not the kind of thing that you are going to accomplish in the backyard with a blowtorch and some solder but be aware that it is possible and the differences can be impressive.
You might also consider reading the first article in this series HERE.
The third article in this series (crossovers) is HERE.
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