Header Design, Fabrication and Terminology 101

Casey Heerman, Assistant Technical Director of Burns Stainless LLC (a popular American Exhaust Design Consultant and Parts Supplier) recently posted this article in the K20A.org forum. Excellent read..

Header Material- headers can be made from a variety of different materials. 304 ss 321 ss Mild Steel Titanium and Inconel 625

304 SS - Is your run of the mill material and will work for most NA applications the most common thickness is .065 wall or 16g, to save some weight you can go to 18g .049 or even 20g .035 for the experienced fabricator

321 ss - Is found on most turbocharged applications in a 16g .065 wall thickness. The use of 321 is advantageous as it will maintain it's strength properties at elevated temperatures. 321 is also good for use in 20g .035 Na headers where better strength properties are desirable.

Mild Steel - Is your standard run of the mill tubing very easy to fabricate with, and very inexpensive. It does rust and it will eventually decay into nothing as far as I'm aware from a headercentric stand point it offer no favorable properties over mild steel, except for the slightly lower coefficient of thermal expansion

Inconel 625 - IS the most wonderful material to use in a header but heck we don't all make what bill gates does. 625 has excellent strength properties, fatigue resistance, it doesn’t grow as much as stainless and it insulated better then stainless. It is essentially a Super stainless alloy. It's very expensive and can be used in wall thicknesses of .028 22g to fabricate NA headers, which can provide a great weight savings.

Titanium - I have heard mixed opinions on the use of Ti for header construction, some say it wont last some say it will regardless it's extremely hard to bend and even harder to find in the sizes needed so we won't delve into this too much. However if used it can offer a wonderful weight savings.

Head Flange - This is the base of the header normally offered in two different materials 304SS and Mild Steel. Both materials have good properties, the mild steel being quite a bit better for the beginner builder as it does not warp near as much when you weld it. When using a 304 SS flange it is going to warp quite a bit, it will require surfacing, a 20" face grinder is great for this. Flanges should be the same shape as the exhaust port and a certain amount larger to allow for the tubing to be fit to the port a rule of thumb is large enough to fit .065 tubes.

Primary tubes - These XX number of tubes will channel exhaust gas to the merge Collector. They should have both a length and a size measurement along with notation of steps.

Example: Primaries: 28" X 1.75 Od : This would indicate a primary length of 28" OAL starting at the face of the cylinder head and ending at the very end of the tube.

The primary tubes length and size is very important, for different length and sizes you can actually change the power curve pretty significantly if you have an engine that responds well to exhaust changes.

On a Tri-Y or 4-2-1 header they still have primary tubes, the primaries on a tri-y start at the face of the head and end at the end of the tube that enters the first set of collectors. Immediately after the primary tubes end the secondary tubes begin.

Secondary Tubes - These start Immediately after the primary tubes end, they Include a portion of the collector. This is another area of adjustability that can offer good header tuning. When the header is constructed properly you will be able to see a change in the engine by adjusting the size and length of the secondary's.

Merge Collectors - Merge Collectors are MOST COMMONLY REFFERED TO BE THERE INLET SIZE. The inlet size is the Od size of the tube that the collector must slip over. a 3" Collector to me means 3" primary tubes.

Parts of the collector
Inlet - As stated above, in our world this will 99% of the time be a slip fit. If your going to call the collector refer to it by this size

Collector Choke (minor diameter) - This is the size the can make or break a good header This doesn’t need to be 3" unless you have something super weird going on with your engine. I have not design a Honda header yet that even needs close to a 3" Collector choke. I’m not saying it wont or cant happen, I’m just very skeptical of it

Final Outlet - The final outlet can be configured a variety of ways. Most common for road going cars that need mufflers is a transition from the choke to the correct tailpipe size and a ring (ring is a straight section of tubing where you can slip fit weld a flange V-band etc.) . Another configuration that we often use and popularized if not Pioneered is the megaphone with reverse cone we have used this in many applications with great success and use it as often as possible. When using a large diameter final outlet like a 3" on a K series engine, if bogging is epxerienced at low rpm you need to consider retuning the engine, this not a fault of header, large exhaust or open exhaust

Header Configurations
4-1: a 4-1 Header is typically going to operate in a farily narrow rpm band of say 4000 RPM, people that ask a 4:1 to do more then this are asking way too much of the header. However the rpm range can be lengthened by the implemetion of a megaphone and reverse cone and will work well when sized correctly

4-2-1 or Tri-Y: A Tri-Y will typically operate in a very broad rPm range. When designed correctly they will dramatically increase the mid range without loss of peak HP

And here's a recent example of the excellent work done by Burns Stainless. Exhaust shown is a 4-1-2 long primary design for a K24 road race engine. Material used to fabricate the header is 321 stainless. One day I will have such an exhaust on the SP. Price of header? Don't ask..