Header Construction 101

There are many different shops that construct custom headers for Honda, Acuras and other makes.  Many are less expensive and cheaper versions of the top performing units.  At SMS Products (SMSP) the quality of construction is as important as how the header performs.  You can’t race and win at the track with a header that is cracked.

When possible, SMSP designs their headers with the minimal amount of bends in order to provide the best flowing tubes. A header will add heat to an engine compartment and that is why it’s critical to package the primaries and secondaries (if a 4-2-1 header) in a way to reduce the addition of heat in the engine compartment.

All header tubing is measured by the outside diameter (OD) and wall thickness (gage).  Manufacturers use any of the following tubing gages.

14g      .083”

16g      .065”

18g      .049”

20g      .035”

Thinner walled tubing is used for 2 reasons, to reduce weight and increase the ID of a given size of tubing for more flow capability.  Reliability of the welds with the thinner wall tubing can be retained if the welding process is controlled.  Butt welds located in high stress areas can take a lot of abuse.  So at SMSP we will always back purge a butt weld in a header tube no matter if the material is mild steel or stainless steel.  While this adds to the cost of the header it also adds to the life of the header.

However, before one can perform a butt weld the joint needs to be prepared.  Most times a section of tubing gets butt welded to a bend that was cut somewhere along its arc.  While fabricators use mandrel bent tubing for headers there is still a small reduction in diameter throughout a bend.  Depending on the tubing and bends orientation the fabricator might have a slightly larger ID going into a smaller ID and this will promote some turbulence, nothing drastic but still not the best situation.  Therefore, SMSP feels the best way to prep tubing for a butt weld is to ensure the ODs match up (assuming the fabricator is using the same gage tubing on either side of the butt weld).  Here’s an example of the proper way to prep a joint for butt welding.

Some companies don’t feel that a back purge is required on a mild steel butt weld but we disagree.  This becomes even more important when there are multiple welds in sections of tubing that contain many bends.  When the header is run and heats up the tubing will expand.  The tubes will grow and this growth imparts additional stresses on the tubing and welds.

Back purging at SMSP is done using 100% Argon gas that is flowed through the ID of the tubing during welding.  This Argon purge displaces the oxygen and prevents any oxides from forming in the weld.  These impurities in the weld are sometimes referred to as sugar.  The weld is bumpy and somewhat jagged on the inside.  This weld will most likely fail in time.  The thicker walled tubing with no purge will last longer but it is more susceptible to cracking than the same thickness of tubing that was back purged.  So while trying to save a little time and money by not back purging, the customer is getting something that will probably fail over time.

Here is a SMSP ceramic coated mild steel header whose butt welds were back purge that had a little incident.

Even with this damage no welds failed.

This is a cutaway of some 304 stainless tubing that was butt welded without using a back purge.

Notice how rough the inside of the weld is.  This is the case even when the outside looks great.  Always remember the exhaust gases don’t pass around the outside of the tubing so they don’t care how good the outside of a header looks like.  The inside is the most important.

Here’s a picture of the same tubing but with the Argon purge.

The result is a much smoother weld that will last much longer than a non back purged weld. 

Here’s a picture of the 2 welds side by side.