Building Your Own Headers

Building Your Own Headers
By O'Darrell Poole (c)

Over the years I have had many people ask me about building their own headers. Specifically what size and length of the primary tubes as well as the size and length of our collectors. In this article we will discuss the 4 most important design aspects of headers as well as explain some important points to remember. As stated there are 4 important aspects in proper headers design. They are in order of importance:

1) Primary Pipe Size:
The size of the primary pipe is the number #1 mistake we see in self designed as well as aftermarket headers. The size of the primary pipe should be the same diameter as the cylinder head valve throat, not the valve but the throat. Exhaust system effectiveness depends largely on maintaining gas velocities from the time the exhaust valve opens to the time it is deposited to the atmosphere. If for example your primary pipe size is to large (often the case because many believe bigger is better) and your exhaust valve throat diameter is one tubing size smaller than your primary tube size you have changed your gas velocity by roughly 20%. This change is gas velocity will affect the scavenging properties of your header because the pressure pulses (negative pressure) used to scavenge the cylinders will now come later in the exhaust stroke or even after the exhaust valve is closed.

2) Primary Pipe Length:
Determining the proper primary tubing length is extremely important to the timing of the pressure pulses used in scavenging the cylinders and ensuring fresh charge. If your primary pipe length is to short the pressure pulses may reverse so quickly that you may actually be forcing exhaust residue back into the cylinders. If your primary pipe length is to long then the pressure pulses will have no effect on scavenging because that negative pressure that we desire at TDC will happen well after TDC.

3) Collector Diameter:
Many believe and commit the same error here as they do with primary tube size. That is that bigger is better. The fact is that the collector has a bigger job that just merging the 4 or 3 pipes into 1 although this is one of its functions it also provides secondary scavenging characteristics for our headers. When the exhaust valve opens and sends spent exhaust gas down our primary tube and merges into our collector the pipe size difference between the primary and the collector causes a pressure wave reflection sending a negative pressure back up our primary pipe and assisting scavenging the pressure wave continues on past the primary pipe and into the collector when it emerges from the collector again it sends another wave reflection back up the primary pipe behind the first reflection and thus prolonging the scavenging effects and ensuring clean fresh charge is digested into our cylinders. Improper diameter will slow down exhaust gas velocities and thus delay the start of this secondary scavenging wave.

4) Collector Length:
Proper collector length is important in timing the secondary scavenging characteristics of our header. If the first 3 factors are correct the incorrect collector length (short) will cause our secondary reflection to be timed to close to the primary reflection and may allow for pressure to rise above atmospheric over TDC. If the length is to long then our secondary reflection may be too late in the stroke to be effective in assisting scavenging.

These are just a few of the important characteristic of effective exhaust system design a tuning. Stay tuned for more info on intake and exhaust systems in my future articles. We welcome your questions!!


O'Darrell Poole
Autocraft Dynamic Design Inc.
autocraftdynamicdesign@live.ca