Introduction
I have to start this off with a warning. There are a LOT of calculations and theories when it comes to headers. From the way it has been discussed on various forums lately it is obvious how little they are understood. The links below will be great resources, so read this to get an overview I guess, then tackle the real IN DEPTH theory.
What are Headers?
Your stock exhaust system starts right at the head, with a manifold. The manifold has a very short runner from the exhaust port to a tube that connects all 3 ports on the head together. Headers on the other hand will have a single tube (primary) run off each exhaust port and then merge into a collector. It is the diameter and length of these tubes that make the intake pulses perfect for certain RPM ranges.
Here is a picture of a stock manifold on a 3.4 DOHC (similar to the pushrod motor)
Here is what a header looks like. This is a pair actually, and made by Steve Hamm. The rear header on this set is much better designed than the front.
You can see the collector on the above picture as well. The collector is a very important piece. It is what the primaries are welded to in order to end the primary length, and to keep from having 6 pipes running the length of the car. The design and length are what you should watch for. A smooth tapper to the final exhaust diameter is very much desired. A sharp transition will cause turbulence and problems with the flow as each cylinders pulse reaches the end of the primary..
Another key term is the flange. The header flange is what the primary tubes are welded to, and then the header flange itself is bolted to the head.
Why Headers Work
This is where it gets complicated. In order to understand why it works, you have to understand what is going on in the motor. When the piston is being forced down from combustion, it creates a high pressure area in the cylinder. When the exhaust valve opens, you want a low pressure in the exhaust manifold/header so that the high pressure in the cylinder will force (push) the exhaust out. As the piston is rising back up, it again is causing a high pressure area, but you have the intake valve opening up before TDC as well. This overlap needs the intake to have high pressure, the cylinder to have high pressure, and the exhaust system to have low pressure.
Because the valves open and close, it causes a pulse of exhaust to come out the port. These pulses are high pressure waves of gas, but also carry a sound wave that travels the same pipes at a faster rate. These pulses not only go out the exhaust pipes, but also bounce off the pipe walls and turns. This might be where "backpressure" became a keyword to people.
It is the effect of this pulse that travels backwards that we are concerned with. In a manifold, you dont have much room at all, so the wave is coming back almost immediately. When it comes back as the valve is opening, you have a high pressure area in the cylinder AND the exhaust manifold/tubing. The gas isn't going to start flowing out of the cylinder until it has a greater pressure, aka the piston rising back up. This means more residual exhaust gas is going to be in the cylinder after the exhaust closes and the piston is on the induction stroke. Not good.
The reason headers work, or SHOULD work is that they are properly designed. This is not the case most of the time however. You can't just add longer tubes to the exhaust and call it a day. There are calculations involving cylinder bore diamter, stroke, valve bowl diameter, etc. You have to match up the length of the pipe to the peak torque RPM as well. This also means every tube on the header is to be the same diameter and length. If they are not, you will throw all the calculations out the window and condemn yourself to glorified manifolds. They may work better than manifolds but are far from perfect.
The formula for pipe length is this (use this as a starting point as its not written in stone)
Length of primary pipe (in feet) P = (ASD^2)/1,400 d^2
A = exhaust valve opening period in degrees of crankshaft rotation
S = stroke length in inches
D = cylinder bore in inches
d = exhaust valve port diameter in inches
The diameter of the pipe is another very important item. Too large and you kill the velocity, and with it your low end torque. A few HP gain at high RPM is not worth low end torque losses for a daily driver. The same book with the length calculations has info that is contradictiary to what Ed, owner of HeadersByEd says to do. Of course, I was reading in the 3.4 DOHC context which is most likely a different animal altogether. For diameter, the book says to use the diameter of the valve throat (not the valve itself, but the area below the valve, as a guide. You of course have to match the flange to the exhaust port but the pipe diameter afterwards can taper to the right diameter.
I need to add more information to this page, as I do many other pages. If you have anything to add please post a comment. I do want to add right now that when going to maximum performance from a set of headers, you have to be aware of your computer and O2 feedback setup. Long tube headers rule, but they are difficult/impossible to tune with a single O2 sensor. A heated O2 is a must but even then it may be difficult to tune on a street driven car. With this in mind, it may be in your best interests to go with a shorter tube design which will still give you excellent results but not as much as one made for race only.
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