Exhaust headers differ from exhaust manifolds. A header provides a dedicated tube for each cylinder head exhaust port, while a manifold collects and routes all exhaust ports to a single outlet, which is in close proximity to the cylinder head. Individual pipes that are dedicated to a cylinder are referred to as primary pipes. On header designs that feature the bank of primary tubes grouped together into a common outlet, this common outlet is referred to as the collector. It’s where all of the cylinder head’s primary tubes are “collected” together as a group. (Chapter 7 discusses exhaust system–related math to aid in determining the appropriate header primary-tube diameter and length for a given application.)
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Although an exhaust manifold does allow exhaust gases and pressure to exit the cylinder head, transferring to exhaust piping, an exhaust manifold is generally considered a compromise in terms of performance. A manifold provides an initial path for exhaust exit. The exhaust pressure differs from cylinder to cylinder. An exhaust header provides an individual primary tube for each cylinder, which offers a more efficient and more evenly balanced exhaust route for each cylinder, with far less backpressure.
In a perfect world, the exhaust header tubes for each cylinder head have equal lengths, allowing each cylinder’s exhaust flow and pressure to be somewhat equal. In reality, depending on the specific engine and vehicle, equal-length headers may or may not be practical, simply in terms of fit and available space. Regardless, the use of headers (rather than exhaust manifolds) should, in most cases, provide a performance benefit and allow the engine to breathe more freely with less restriction. In addition, a tubular header provides a weight reduction as compared to most cast-iron exhaust manifolds.
Selecting the appropriate exhaust header for a given vehicle involves a number of considerations. First of all, establish your priorities, which differ depending upon the individual application. While upgrading from cast OEM exhaust manifolds to tubular headers generally provides a degree of performance improvement in any application, some enthusiasts are more concerned with maximizing engine power and others may be more concerned with ease of installation.
If performance is key, you need to select headers based upon the engine’s breathing requirements in terms of matching tube diameter and length to maximize low-end torque or high-end horsepower. Long-tube headers are ideal for enhancing torque at lower engine speeds, but depending on the vehicle, long-tube header installation may prove to be more difficult. In addition, you may run into ground clearance issues with a specific brand and model of header.
Ease of Installation
If your biggest concern is ease of installation, headers with shorter tubes and/or headers with unequal-length primary tubes may provide a better fit. For a street-driven vehicle, ease of installation and headers that provide better clearance both underhood and at the belly of the car is a better choice, even if that means not being able to absolutely maximize the engine’s performance potential. This might involve choosing headers with slightly smaller tube diameters, and sharper bends, and as a consequence it’s more restrictive. If, for example, your particular engine ideally wants a header that features 1.75-inch primary tubes, but a header with 1.5-inch primary tubes fits more easily without the need to modify inner fenders, power steering lines, wire harnesses, etc., you may be better off living with this slight compromise. When a race car is built from scratch, you have the luxury of planning and designing for an ideal exhaust system. With a production vehicle, unless you’re willing to make necessary modifications, the reality is that you may have to make a few compromises in terms of sacrificing a few potential foot-pounds of torque or a bit of horsepower.
Consider your budget as well. If you’re not planning to run the vehicle in competition, do you really need to buy the highest-dollar tuned-length pro racing headers? The answer is no. Also consider header material from a cost standpoint. All things being equal, mild steel uncoated headers offer the least expensive initial cost. Ceramic-coated stainless steel headers are likely the most expensive option. Think about how your headers will be used. Mild steel eventually rusts, either from the outside in or from the inside out. Adding a ceramic coating or painting with high-heat paint can extend steel header life.
A ceramic coating protects the exterior surfaces, but moisture inside the tubes can eventually take a toll. If the vehicle will be driven year-round in varying climates, the best choice is stainless steel. Even if they cost more initially, stainless steel headers pay for themselves in terms of longer life. If you plan to drive the vehicle on a limited basis, for example during warm weather months only, ceramic coated-steel headers likely provide a satisfactory lifespan. Most of today’s header makers offer their products already ceramic coated. If you have mild steel headers custom fabricated, or if you’ve purchased a set of new or used steel headers that are bare or painted, my advice is to ship them to a qualified coating shop (such as Jet Hot) for blasting and ceramic coating. I’ve had a variety of headers, new and old, treated to these coatings, and I have yet to see one that has experienced any corrosion issues. However, I have seen problems with headers coated by less-than-skilled shops.
As a case in point, I once had a system coated by two different shops. I sent the headers to Jet Hot and the pipes to another shop that claimed to use the same process. Once everything arrived, pieces from both shops looked identical. After being stored in the same storage area in my shop for about two months, I was ready to install the system.
At that point, the Jet Hot pieces were in the same condition as new, while the other items showed serious signs of surface rust. I ended up shipping the flawed pieces to Jet Hot for stripping and re-coating, after which I had no problems. The system was installed onto a 1933 Ford three-window street rod that I had built for a customer. I saw the vehicle four years later at a car show, and the entire exhaust system looked the same as the day I installed it. My point? Don’t choose a coating shop without checking out its reputation first.
Header installation can be quite labor-intensive for the hobbyist or anyone who doesn’t install headers on a regular basis. Some are easy and some are a nightmare. The reason I’ve devoted so much attention to the subject of materials and coatings is to spare you from the need to remove and refinish or replace your first set of headers. It’s better to do the job once and be done with it.
Apart from fully custom-designed setups, common header styles include full-length (often called long-tube), shorty (often called block-hugger), lakester, and tri-Y. Long-tube headers are usually designed with (in the case of an 8-cylinder engine) four primary tubes that merge into a single collector. Long-tube headers are generally designed to favor low-end and mid-range torque.
Shorty headers, as the name implies, feature relatively short primary tubes. Shorties are popular for many hard-to-fit applications with regard to available underhood space and often help ease installation. Short primary tubes tend to favor the upper RPM range, but as a compromise for a street vehicle where space is an issue they remain popular. Lakester styles are so-named because of their use in early dry lake racing. This style features short primary tubes that immediately enter a long straight megaphone (tapered) style secondary pipe. Lakesters usually feature a cutout plate, allowing you to run through a pipe/muffler system or open, bypassing the rest of the system. This style remains popular for nostalgia/old-school hot rod buffs.
Tri-Y headers are regular headers that have two of the primary pipes merging into adjacent primary pipes (creating a Y intersection). The bank of four primary pipes is relatively short, connecting to a pair of larger-diameter primary pipes, ending with the two larger primary pipes merging into a common collector. The idea behind a Tri-Y design is to create a port separation by pairing the cylinders that are firing 180 degrees apart. Merging the primaries and transitioning into a larger-diameter pipe helps to increase exhaust scavenging. Pulling exhaust gases out more quickly promotes increased air intake, speeding up the combustion process, and theoretically producing more power.
Exhaust headers are made using either mild steel, specific grades of stainless steel (usually 304 stainless), or in rare cases, more costly materials, such as Inconel or titanium. Sometimes a “mystery mix” of materials, with a very diminished level of alloys, is used.
Steel tube headers normally feature 16-gauge or thicker mild steel tubing, while stainless primary tubes can be made with as light as .040-inch wall thickness. However, typical wall thicknesses are .049 inch/18 gauge and .065 inch/16 gauge. Flanges on the primary tubes that mount to the cylinder heads are traditionally made of either flat-stock steel or stainless steel, ranging in thickness from about 1/4 inch to a more-common 3/8 inch.
Headers that are designed specifically for turbocharger applications usually have a heavier wall thickness to better handle the high exhaust gas temperature (EGT) generated during turbo operation, in order to provide added durability under extreme temperature. As an example, Stainless Works makes OEM-replacement and custom stainless steel exhaust systems that use 16-gauge 304L stainless steel for its turbo applications. The use of 18-gauge stainless 304 is often found in the construction of stainless headers because it’s about 15 percent less expensive and about 24 percent lighter (with cost a consideration for most consumers and weight a consideration for racers). The heavier-wall 16-gauge stainless material is a popular choice for turbocharged applications where the exhaust tubes encounter higher temperature extremes and stresses.
Performance aftermarket exhaust system manufacturers most commonly use two grades of stainless steel: 304 and 321. Grade 304 stainless steel is high quality stainless, highly resistant to corrosion (both from exhaust deposits and from road conditions), and non-magnetic. Grade 304 stainless steel is used as an industry standard for high-quality exhaust components. Grade 321 stainless steel is often referred to as aircraft grade. It offers even higher resistance to heat, fatigue, and cracking at extreme heat levels. This grade is a good choice for extreme heat levels, such as those encountered in turbo exhaust systems. In essence, a high-quality stainless steel exhaust component, such as headers, pipe, muffler, etc., should be made of either Grade 304 or Grade 321 stainless steel.
Inconel, available in grades 625 and 718, is a high-strength nickelchromium-iron-molybdenum alloy, which is often referred to as a super stainless steel material. The high level of nickel makes this material useful for extreme applications, including Formula One, NASCAR Cup, submarine propulsion motors, naval military exhaust systems, etc. Inconel’s benefits primarily include its high-rupture strength and ability to withstand extreme temperatures for prolonged periods.
Compared to steel, titanium offers a higher strength-to-weight ratio, and its melting point is higher than other materials. However, the average enthusiast doesn’t encounter high-enough temperatures to justify its use. The primary reason to run titanium exhaust tubing is to save weight, period. If you want to spend the extra dough to lighten the vehicle, have at it. By the way, while Chevy touts the late-model Corvette as having a titanium exhaust system; it only refers to the rear muffler/tip section, not the entire system. The carmaker used titanium in the rear to shave off a few pounds.
Inexpensive stainless steel grades are commonly used by many OEMs that contain a “mystery mix” of materials, with a very diminished level of alloys. That’s why you often see an OEM stainless steel exhaust system that is rusty. In many cases, a carmaker uses just enough alloy to justify calling the material stainless, when in fact the metal formula may contain a pot metal mix of magnetic particles (ferrous) to reduce manufacturing cost. Although this hybrid stainless may last longer than a mild steel component, referring to this level of cheap metal mix as stainless is a joke that’s not funny. In addition to the long-term appearance and durability factors, cheap OEM “stainless” components also do not provide the same level of heat retention as a high-grade stainless steel. Essentially, OEMs use low-grade stainless material to reduce cost and help make the system last just long enough to survive the vehicle warranty period.
Owners of newly purchased vehicles that feature a stainless exhaust system are often frustrated and angered when they notice surface rust after only a few months. If you want real stainless steel, you must rely on the performance aftermarket, not the carmaker. When it comes to the exhaust system, carmakers are more concerned about shaving cost from their production runs, where quality aftermarket stainless steel header manufacturers know that enthusiasts are willing to upgrade to a high-quality product.
Thermal properties are always a consideration for exhaust headers. The heat generated by the combustion process should ideally be captured and retained inside the exhaust system rather than being allowed to radiate from the pipes. Containing the heat inside the pipes leads to increased power. Steel tubing tends to absorb heat, making the tube surface hotter as heat is released. This is detrimental to power and causes exhaust heat to increase underhood temperatures.
High-quality 304 or 321 stainless steel is better at retaining heat, promoting a more efficient combustion control (more power), and reducing underhood radiated heat. Of course, steel headers may be treated to reduce radiant heat loss by either installing thermal-wrap fabric or by the application of ceramic coatings. In short, if you’re using steel headers, it’s wise to have them coated (or buy them already coated) or to wrap the tubes with specialized heat-wrap. If you’re running stainless steel headers, the heat passes through more quickly and the surface temperature of the tubes is lower. If you want to go even further, stainless headers can be ceramic coated to achieve an even higher level of thermal management.
Tubular headers are typically made from either mild steel or stainless steel. Stainless type 304 or 321 is the most common. Materials and the need/advantages of specific coatings must be compatible. Mild steel absorbs heat much more so than stainless steel. Therefore, this steel is less efficient; it allows heat to absorb into the tube and radiate away from the tube. As a result, this raises underhood temperatures, and this allows heat loss, where ideally you want the heat captured inside the tube to be evacuated out of the tubes for superior exhaust travel and better performance. High-quality stainless steel tends to do a much better job at capturing exhaust heat with less radiation (lowering underhood temperature in the process). With that said, it’s more important to apply a thermal barrier coating to steel headers (typically a ceramic-based coating). The other factor at play involves preventing tube corrosion. Steel can rust, resulting in degraded appearance and a shortened lifespan.
Stainless steel, as mentioned earlier in this chapter, is available in various levels of alloy content. An OEM stainless steel exhaust header or manifold may actually consist of a mystery-mix of various metals, which may be prone to surface rust and excessive heat radiation. Whether you’re using steel or an OEM “stainless” component, they are both candidates for a protective/ thermal barrier coating. Even though a high-quality 304 or 321 stainless steel already offers appearance, longevity, and thermal benefits without requiring a thermal barrier coating, the addition of this type of surface coating further facilitates the header’s heat-capture and thermal efficiency.
Spraying high-heat header paint on steel headers is an old-school technique, and it’s certainly better than nothing. While it provides a level of corrosion protection on the outer surface, the tubes can still rust on the inside walls. I don’t think that even the best heat paint applied from a spray can adequately reduce heat absorption/radiation. In my opinion heat paint offers only temporary protection and appearance longevity. If you’re running steel headers, they should be ceramic coated. You can obtain this coating by either purchasing headers that are already coated by the manufacturer, or you can send bare-steel headers to a specialty coating shop.
It’s easy to have your exhaust headers ceramic coated. Whether new or used, bare, painted, or already coated, a quality shop can prep and apply heat barrier ceramic coatings to your headers. If the headers are used and have a few rust issues, the coater can strip them to bare metal and then decide if repairs are necessary before coating. Typically, coating shops don’t like to perform repairs, such as welding pinholes, etc. In cases of used headers that have damage, the coater likely will discuss this with you, likely advising that he returns the headers to you to be repaired prior to coating. If the headers have no damage, the coater strips to bare metal and prepares the surface. This may involve a light media blast, depending on the condition of the surfaces. Then the coating is applied. The application process of the coating and the formulas used are generally tightly held secrets.
Specialty coating companies can apply coatings that address both heat management and appearance. Sources include Jet Hot, Swain Tech Coatings, and Polydyn (also known as Polymer Dynamics). Jet Hot has many products. In addition to anti-friction coatings for various engine and driveline applications, they offer two levels of ceramic header coatings that are appropriate for street and race use. The Extreme 1300 series withstands up to 1,300 degrees F and comes in a high-luster (almost-gloss) finish, which is very smooth and very easy to clean. The Extreme 2000, which is designed for up to around 2,000 degrees F, is ideal for turbo applications. The finish on this version provides a somewhat rough finish.
In all, Jet Hot offers around 35 colors from which to choose. These coatings are ideal for street use, in terms of corrosion protection, appearance, and ease of maintenance. Swain Tech Coatings offers many specialty coatings for all aspects of engine, driveline, and brake systems. These include thermal barriers, anti-friction coatings, oil drainback coatings, etc. Its exhaust coatings focus on heat management for engine efficiency and reduction of radiated heat, and as a result corrosion protection is an additional benefit. Swain’s TBC coatings are specifically designed for extreme insulating and not intended for show car appearance. These are formulated for competition applications, but work for other applications as well. The finish is white, with a slightly rough texture, applied at a thickness of about .002 inch.
Polydyn produces a variety of specialty performance coatings for many engine, driveline, and brake components. Its ceramic header/exhaust system coatings are polymer composites with insulating ceramics, designed to reduce heat transfer and protect against damaging heat saturation. The exhaust coating is typically a bronze color with a semi-matte finish. According to Polydyn, when applied to the interior walls of a chromed pipe or chromed headers, the thermal insulating properties eliminate discoloration and bluing. I have used all of these firms for various header and pipe coatings, and I’ve been thrilled with each result. Time frames vary depending on their workloads, but the typical turnaround time has been around 1 to 2 weeks (sometimes much less). Cerakote is another service that specializes in exhaust coatings. They offer a wide range of colors and finishes. Flowtech is a performance header and exhaust system brand, which is part of the Holley product family.
As with the selection of carburetors, fuel injectors, tires, wheels, etc., bigger is not always better. When you consider header primary tube diameter, many variables come into play, including engine displacement, camshaft profile, and the intended vehicle use. Does the driver demand more bottom-end torque or power at higher engine speed, etc.? In a way, you can relate exhaust tube diameter to liquid plumbing pipe diameter. At the same input pressure, a smaller-diameter tube creates higher water pressure, while a larger tube results in lower water pressure output. In overall terms, smaller-diameter primary tubes promote low-end torque, but if tube diameter is too small, backpressure can increase to a detrimental level (loss of power and burnt exhaust valves). Largerdiameter primaries lend themselves to top-end power, but if too large, insufficient backpressure results in a loss of power, especially at bottom-end and mid-range operation. As far as primary tube length is concerned, longer tubes provide increased torque, while shorter tubes may aid in quicker exhaust gas scavenging.
Rather than buying the largestdiameter exhaust headers available for your engine, you need to match the tube diameter to the needs of the engine. In other words, you need to tailor the engine’s powerband within the target operating RPM band. If you want peak torque at, say, 4,000 rpm for a street car, the header tube diameter needs to accommodate this. If you have a racing engine where peak torque needs to be in the 6,000-rpm range and peak horsepower is in the 8,000-rpm range, you need to match the exhaust system to work best at those engine speeds. A street engine demands more torque at bottom and mid-range, while a race engine (depending on the application) can sacrifice bottom-end torque in favor of generating maximum horsepower. Header design is all about exhaust gas velocity.
Shorty headers provide easy installation because they reduce fitment issues and have fewer installation challenges. While not appropriate for a race engine for which you need to extract maximum power, shorties (even without equal-length primary tubes) still provide superior performance to a stock exhaust manifold. Unless you’re building a dedicated race car, it’s not worth obsessing over extracting every ounce of power. Step-tube headers have changes in primary tube diameter from the cylinder head to the collector. These changes in tube diameter along the exhaust path can affect the speed of gas flow. For a naturally aspirated engine, a step header features smaller-to-larger tube diameter. This means that tube diameter at the entrance (from the cylinder head flange) is smaller, with tube diameter progressively changing to larger diameter(s) until terminating at the collector. This aids in pulling exhaust gas out of the cylinder head and speeding up the exhaust pulse (improved scavenging). For a turbocharged application, a step header features primary tubes that transition from larger to smaller diameters, creating a squeeze velocity that helps to spool the turbo quicker.
Primary Tube Profile
In a perfect world, the first 2 to 3 inches of the primary tubes should be relatively straight. This extends the cylinder head exhaust ports, effectively making them longer and having greater volume. Unless you’re dealing with a race car application where this is possible (with no obstructions that allow a longer, straighter shot of the primary tubes form the cylinder heads), you may be forced to begin bands closer to the cylinder head exhaust ports in order to make the headers fit the confines of the vehicles.
Primary tube bends are all about making headers fit the engine and also fit with the engine in the vehicle. Bend radius is a limiting factor. It’s not practical to bend a pipe in a radius that’s tighter than the tube OD; it’s just too tight. This forces the fabricator to extend a tube farther before a bend. However, there’s a way around this. An example is Ultimate Headers’ approach when an immediate tight turn is required. They investment-cast high-grade stainless steel in a tight elbow, then TIG weld the cast elbow to the stainless steel tube (all of Ultimate’s headers are 100-percent stainless steel). This allows the tube to begin its bend much closer to the header flange without losing cross-section area. Tight bends such as this limit the effort of extracting maximum power, but because of fitment issues, tighter-than-desired bends are sometimes a necessary evil.
Flanges are welded to the primary tubes on a jig, which simulates the cylinder head. The flange is bolted to the jig to hold it flat. The jig holds the individual flanges in the proper spacing, such as when the primaries have multiple flanges. One flange per tube is usual or the front and rear tubes have their own flanges and the center two primary tubes share a common flange, such as with Gen I small-block Chevy. The jig secures the primary tubes to the cylinder head flange and holds all tubes in the proper locations for specific vehicle applications. Header manufacturers make patterns for every make, model, and year vehicle, in addition to variants for specific engines and engine accessories, such as A/C, power steering, etc. This represents an enormously time-consuming and costly investment. Prices could be many times higher.
Heat should be evenly distributed when the flanges are welded. If the welding is performed properly, heat is applied in a sequential pattern that spreads the heat evenly. Instead of fully welding each tube in order, the flange is released from the pattern jig and there should not be too much residual stress that causes a one-piece flange to warp or causes misalignment of individual flanges. Mild steel welding may involve either MIG or TIG welds. Stainless steel assemblies are almost always TIG welded.
Mandrel bending is necessary to prevent kinks and wrinkles when bending header tubing or any tubing for that matter. While various forms of tube bending are possible, the two most common include ram style and mandrel style. Hydroforming may also be used.
Ram bending is often called crush bending. It uses a female die to match the tube diameter with the angle of bend radius required. The tube is simply hydraulically drawn over the die. In the process, the outside of the bend stretches to make the outer wall thinner. At the same time, the inside of the bend compresses, featuring a crushed area that creates an unround condition within the bend radius. While this may be acceptable for passenger cars’ exhaust systems, it’s not acceptable for a performance system because tube restrictions are to be avoided.
Mandrel bending also uses an outer bending die, but additionally features a mandrel that is inserted into the tube. The mandrel consists of multiple “ball” rings that are connected via a flexible rod. The mandrel balls are free to move in an arc that follows the path of the die. This provides internal support for the tube, preventing it from collapsing or kinking, keeping the inside of the tube round. Mandrel balls are selected not only based on the tubing inside diameters, but also in terms of thickness and spacing. In fact, the narrower balls with closer spacing provide more internal tube support. This allows the manufacturer to create the optimum mandrel for specific materials and degree-of-bend radius.
Mandrel balls are also available in different materials and hardness qualities. For harder tubing material, softer balls are used; for softer tube materials, harder balls are needed. When used with steel or aluminum tubing, a harder chrome-plated steel mandrel ball is used. For stainless steel tubing, a softer aluminum-bronze mandrel ball, called AB18, is used.
Bends can be made in any degree, up to and including 180 degrees. During the bending process, even with a mandrel bender in which the inside of the tube is supported, a very slight and barely noticeable deformation of the inside of the bend occurs. To remove this minor irregularity, a wiper, positioned immediately behind the die, applies moderate pressure and wipes the inside of the bend smooth to regain roundness. The CNC operator must precisely adjust the wiper at an angle of 1 degree. Even a minor deviation of this angle can affect the finish quality. The wiper is located in a wiping die that is aligned with the entry of the bending die.
Although tubes can be bent to shape using a manually operated mandrel bender, high-performance header makers commonly take advantage of CNC mandrel bending machines. Once the program has been written for a specific primary tube, a section of straight tube is inserted into the machine, which is fitted with the proper die that matches the tube OD. The automated bending machine then performs the desired bends needed for that specific primary tube. Remember: Depending on the engine application, this requires four, six, eight, or more individual primary tubes, each with its own unique shape. Separate programs must be written for the CNC machine for each primary pipe.
Another method of pipe bending, hydroforming is often used by mass-production direct-replacement exhaust pipe makers that supply carmakers and the replacement aftermarket. These machines are very complex and cost is prohibitive for most header makers.
Hydroforming is another type of mandrel bending. Instead of using a metal mandrel inside the tube, high-pressured water is injected into the tube. This fills the tube and, because of the enormous pressure, prevents tube collapse at the bend areas. Hydroforming is also used to fabricate various shapes, turning a straight piece of tube into just about any shape required. The tube is placed into a two-piece die, the die halves lock together, and water pressure is injected, pushing (expanding) the tube into the die.
Once a design has been established, primary tubes are bent to shape and trimmed for length. In a positioning jig that holds the flange and all of a bank’s primary tubes in place, the mounting flange is welded to the entry of the primary tubes. After the welds are dressed (if necessary), the collectors are installed and welded. This is a very basic overview. When a header manufacturer designs a system for a specific engine and vehicle application, the prototyping takes place on an actual vehicle and engine. Various manufacturers use different approaches. Stainless Works begins with a flange and tack-welded bends at the entry area from the flange, and then works outward using a selection of straight tube sections connected by a tack-welded bendable bridge tab. Once the basic shape has been established to fit and clear the application, this pattern is then transferred to a coordinate-measuring machine (CMM), where a digital probe traces the flow of each primary assembly. The CMM program then calculates the bend radius, which is based on the chosen tube diameter at each deviation between the straight sections. A program is written for each primary pipe, which is then transferred to a CNC tube bender.
Once a set (for each engine bank) has been assembled, test fitting and (if necessary) additional adjustment is made. When the engineer is satisfied with the results, the same primary tubes can be produced at will on the CNC bender. Fabricating your own exhaust headers requires patience, the ability to measure, the ability to weld, and access to specialty tools. Even if you don’t have access to a tubing bender, you can fabricate your own headers by purchasing a selection of pipe sections bent to specific angles, then piecing/welding the assembly together. Luckily, all components are available separately, including flanges for all popular cylinder heads, collectors, collector flanges, and tubing in a variety of diameters, lengths, and bends.
Depending on the complexity of each header primary tube in terms of the number and degree of bends, a primary tube may be formed as a single piece. If the design involves a series of complex bends that simply cannot be achieved using a single continuous length of tube, a series of two or more sections may be required in order to complete a single primary tube. In a manufacturing facility, in the case of a V-8 engine application, once all four primary tubes for a given engine bank have been bent to shape and trimmed to length, the header flange and the primary tubes are placed onto a pattern jig in order to hold everything in place in proper orientation. All sections are then tack welded together by TIG welding.
The collector is placed into position to encapsulate the ends of the primary tubes and double-checked for fit. Additional TIG welding is then performed in all areas that are accessible on the jig. The header assembly is then removed from the jig to complete all required welds. If the header is being fashioned by a hobbyist who does not have access to a pattern jig and is fabricating a unique custom header, tube arrangement and routing must be determined with the engine in the vehicle.
Start by obtaining a pair of header flanges. Unless you’re dealing with an unusual cylinder head, flanges are readily available for all popular engines, pre-cut and ready to bolt on. Once you’ve decided on the primary tube diameter, the general approach is to begin by welding a short stub section of tube onto each flange port. This provides an initial attachment point for the remainder of each tube. By purchasing a selection of straight tubing and various pre-bent tubes (45-, 90-, 180-degree for example), you have enough material to complete each tube by cutting sections to length for the desired routing. This must be performed with the engine in the vehicle in order to obtain clearances for the tubes to run past or around framework, steering shaft, steering box, etc.
With the stub-tube-equipped flange mounted to the head, temporarily mount the collector in your desired location, secured with clamps or with tach-welded braces. Positioning the collector provides your target for the ends of the primary tubes. Without a collector rigidly positioned in place, you end up with a nightmare when trying to group the ends of the tubes together at a later point. Now you can begin to add tubing, starting at the header flange. For instance, you may want a 90-degree bend at the flange stub. In order to begin routing the tube at your desired location, cut a pre-formed 90-degree tube at the desired radius point in order to obtain a sharp or wide angle. If you need a sharper dive angle, you can trim the stub tube and the mating tube ends at matching angles to make the ends meet without huge gaps.
As each section of primary tube positioning is determined, only tack weld it in place. This makes it easier to remove one or more sections if corrections are needed. Once the tubes are all in place and you’re happy with the results, add a few tack welds to each section to make the assembly stable. Carefully move the header to a clean workbench and add a few seam welds, then re-install it to verify fit. Remove it again and continue to weld. Once all welding is complete, re-install the header and verify that it fits properly. You want to make sure that the fit is correct before you dress your welds and certainly before you paint the headers or send them out for coating.
This is a time-consuming and painstaking task, and really isn’t necessary unless a pre-made header is not available for your application.
Header Design, Fitting and Fabrication
A timesaving approach to header prototype design is to tack weld and assemble a series of tube sections in a specific vehicle, rather than making numerous test bends.
Once the primary tube sections have been tack welded together, the test pipe is then profiled on a CMM.
This view of the CMM monitor shows the sections of straight tube that were mocked together during the prototype design. The CMM software then calculates the necessary bends.
The CMM calculates the bend radius required for each bend area along the length of the primary tube.
A close-up of the computer monitor shows the bend radius calculated by the CMM for a specific bend.
Once the data has been acquired by the CMM, the data is transferred to the CNC bender and a test pipe is formed. Here a technician checks the test pipe with the CMM digital probe to verify that the tube meets the initial design. Once all primary tubes have been formed, the header set is test fitted to the vehicle again to check for proper fit and clearances. Any additional tweaking (if needed) is then performed. After the design is approved, the software data is ready to be used on the CNC tubing bender to produce the finished tubes.
Once the merge spike has been installed, the collector is positioned and carefully TIG welded.
A threaded bung may be installed on the collector for those applications that require the use of an EGT sender.
In some applications, because of the required clearance and fitment issues, the collector-to-primary tube group needs to be angled. Here a 4-into-1 collector entrance is anglecut while held in a precision jig.
This batch of stainless steel headers awaits full polishing. Makers of stainless steel headers often offer a natural (satin) finish or a full polish. Since stainless steel 304 provides an effective thermal barrier (the material does not absorb heat as much as steel tubes), there’s really no need for additional heat barrier coating.
Port Matching and Scavenging
Port matching the header primary tube entry to the cylinder head exhaust port exit isn’t as critical as when matching an intake manifold’s ports to the cylinder head, but it is important if you hope to extract maximum engine power. While it at first appears that exactly matching the cylinder head’s exhaust port to the primary tube entrance is a good idea, this isn’t necessarily the case.
The most important consideration is to avoid restricting the exhaust path. You need to be sure that the primary tube entrance does not block any portion of the cylinder head exhaust port. Simply put, don’t obstruct the head’s exhaust ports, with either the primary tube/ flange or the exhaust gasket. The primary tube inlet should be the same size or larger than the cylinder head’s exhaust port (don’t choke the head).
Ideally, the entrance of the primary tube should provide a mirror image of the cylinder head’s exhaust port. This port has the same dimensions and is either round, D-shaped, or square. This is critical for racers trying to extract every ounce of power. For performance street applications, the ports really don’t need to match exactly, as long as the entrance of the primary tubes don’t block any area of the cylinder head exhaust ports.
With that said, you need to consider exhaust reversion. This can occur when the exhaust pulse bangs around as it flows through the primary pipes and can bounce back toward the head’s exhaust port. As I mentioned earlier, ideally you want the primary tubes to extend as straight as possible from the exhaust port by 2 to 3 inches. A severe bend immediately from the cylinder head can result in the exhaust pulse hitting a “wall” and the exhaust reverting back toward the ports and exhaust valves. However, because of the realities of fitting the headers in the available underhood space, this isn’t always possible.
Preventing exhaust pulse reversion is one aspect of exhaust tuning. Another aspect is scavenging, where exhaust pulse is used to pull or scavenge exhaust from other cylinders. The scavenging can take place where two or more primary pipes merge onto a secondary pipe or main collector. The pulse or pressure wave of gases exiting one primary pipe can be used to pull/siphon exhaust gas out of an adjacent primary pipe, which is sort of like a turbocharger or supercharger action in reverse. This scavenging effect, if balanced properly, improves the exit of exhaust gas from all cylinders.
An analogy is to picture two streams of equal size that converge in a Y intersection. The energy from each stream helps to pull the other stream along. The opening of the exhaust valves generates the engine pulses and the length of time those valves are open. Excessive backpressure and/or pulse reversion tends to restrict the engine’s breathing, as spent gases being trapped or pushed back into the combustion chamber are contaminating the exhaust gas charge that it’s trying to spit out. One way to promote scavenging lies within the design of the collector (where the primary tubes enter the collector). A 4-into-1 collector is an open collector that simply accepts the bundle of four primary tubes, collecting them into a single larger-diameter pipe.
A merge collector is similar, but had a tapered merge surface that promotes a smoother gas flow. This features a center spike point, with tapered walls that serve to extend the outlets of the primary pipes before the gas enters the collector. This merging effect helps to speed up the exhaust gas velocity. A tri-Y collector features two banks of two short secondary pipes where two primary tubes enter one pair and the other two primary pipes enter the second pair. Both pairs of secondary tubes enter a common collector. This speeds up the exhaust gas even more, for increased low-end torque.
A merge spike may be added to the bundle of primary pipes before the installation of the collector. This is a tapered four-sided spike that also promotes an extended and smoother transition from the end of the primary tubes into the collector. The merge spikes used by Stainless Works, for example, also feature a slight twist, which creates a vortex spin of exhaust gas inside the collector. This improves scavenging, promoting a vacuum effect to help to pull the exhaust gas through the system.
Gaskets and Fasteners
Sealing the exhaust header flanges to the cylinder head is critical. A leaking exhaust flange produces annoying exhaust noise, as well as visible burn/scorch marks on the heads and can even result in burning the exhaust valves. A gasket is the ticket here. Although various low-clampingload fastener applications (such as valve covers, timing covers, and thermostat housings) may be accommodated by common Grade-5 bolts, nuts, or studs, critical fastener applications absolutely demand the use of the highest-quality fasteners. Examples of high-stress applications include main cap bolts or studs, connecting rod bolts, cylinder head bolts or studs, flywheel bolts, crankshaft balancer bolts, supercharger housing fasteners, etc.
Every threaded fastener on the vehicle must provide the proper tensile strength required for any given application. Components that are exposed to extreme stresses simply cannot be compromised with the use of questionable fasteners. Avoid generic or bargain fasteners for critical high-stress applications. Use only high-quality fasteners from firms such as ARP, A1 Fasteners, AEBS, and other high-performance fastener manufacturers who specialize in high-stress automotive fasteners. In terms of threaded fasteners for exhaust systems, keep in mind that bolts or studs must be able to handle the required tightening and clamping loads. They are also exposed to higher levels of heat and routinely encounter periods of thermal expansion and contraction. Never use any bolt or stud that is rated less than a Grade 8.
Specifically for exhaust manifold or header flange to cylinder head mounting, it’s best to use fasteners that are specifically made for that purpose. Again, firms such as ARP, A1, and Totally Stainless offer them. My preference is to use the highest-quality threaded fasteners for every application, regardless of the desired torque load. There’s an old adage that applies to this approach: “For lack of a nail, the horseshoe was lost. For lack of the horseshoe, the horse was lost. For lack of the horse, the king was lost, and for lack of the king, the battle was lost.” I’m paraphrasing here, but you get the point. Whether you use bolts or nuts, the correct tightening force is critical in order to achieve the required clamping force, to both secure the header flange and to prevent gasket leaks.
To obtain a proper seal, the two mating surfaces (the cylinder head’s exhaust deck and the header flange) must be flat and parallel. Any warping in either surface is a leak waiting to happen. Various gasket materials are available, including fiber, composite (perforated metal/fiber), MLS (multi-layer steel), and beadformed copper. While the use of an RTV to aid sealing may at first seem crude and viewed as a “backyard” fix, high-temperature RTV is available that can enhance exhaust sealing for problem-leak situations. One example is Permatex’s Ultra Copper RTV. Adding a thin bead of ultra-high-temperature RTV to an exhaust gasket can eliminate troublesome leaks. I’m sure that some people disagree with this, but I’ve successfully used Ultra Copper for problem leaks on several occasions.
Believe it or not, ultra-hightemperature RTV can also be used as the sole means of sealing (without a gasket) the header flange to the cylinder head. Before installing the header flange gaskets, test fit them to your cylinder heads. If you plan to use a gasket, make sure that the gasket ports don’t block any portion of the cylinder head exhaust ports. If they do, don’t use those gaskets. Find gaskets that match (or are slightly larger than) the cylinder head exhaust ports. If you purchase fiber gaskets and the fit is too tight (where the gasket opening is smaller than the cylinder head exhaust port), you can try trimming the gasket opening using a new and very sharp razor knife. In addition, test fit the gasket to the header flange.
If any gasket material overhangs into the exhaust path, trim the gasket accordingly. The point is to avoid any gasket material sticking out into the path for the exhaust flow. Either studs or bolts secure the header flange to the cylinder heads, but which to use depends on the application or your fastener preference. I prefer using studs when possible, but in some cases, studs don’t clear primary tube bends.
Studs offer two distinct benefits. They provide an instant flange location when positioning the header on the cylinder head, making it easier to place the header and gaskets in position. In addition, they allow you to hang the header onto the head while installing the nuts. In the case of aluminum cylinder heads, the use of studs reduces wear and tear on the cylinder heads’ female threaded holes in those cases where the headers are removed and reinstalled on a regular basis.
If you decide to use studs, the choice of bullnose/radius-tip studs provides easier nut installation. The exposed stud tip has a radiused-nose profile above the threads. This allows easier nut thread engagement, with far less chance of cross threading (the smooth tips provide an initial guide for the nuts). When installing studs, there is no need to force-tighten them into the heads. Install the studs finger-tight (or at most, with a very slight preload of maybe 5 to 8 ft-lbs). The clamping action takes place by tightening the nuts. There is no need to double-nut each stud to severely tighten the stud into the head.
If you decide to use bolts, you have several choices regarding head size and drive style, including 6-point hex, 12-point hex, and SHCS (socket head cap screws). Aside from appearance preferences, this is dictated by fastener head clearance and wrench access. Depending on header design, regular-size hex head bolts may not clear tightly spaced primary tubes. Reduced-size hex and 12-point bolts are available with smaller heads for improved tube clearance. For example, a traditional 3/8-inch bolt typically requires the use of a 9/16-inch wrench. A reduced-size 3/8-inch bolt is available that features a smaller hex head, requiring the use of a 3/8-inch wrench. Depending on space restrictions, you may be able to use a socket wrench or you might be restricted to an open-end or box wrench. Another option is the use of SHCS, which feature a round head perimeter and require the use of a male hex wrench. Again, it all depends on the clearances (or lack thereof) in any given application.
Always follow the torque value provided by the header maker or by the cylinder head maker. Begin tightening at the center-most fasteners, working your way outward toward the front and rear ends. Do this in several steps; don’t fully torque any given fastener before proceeding to the next. Tighten in gradual steps in order to spread the clamping load evenly across the header flange. For instance, if the torque specification is 30 ft-lbs, first tighten (in the proper sequence) to 10 ft-lbs, then 20 ft-lbs, and then 30 ft-lbs.
Granted, there may be situations when it is not practical to use a torque wrench or socket wrench, due to potential access problems. If a straight socket wrench doesn’t fit because of primary tube obstruction, you may be able to use an offset box wrench on your torque wrench tool. Keep in mind that whenever you use an offset wrench (where you are effectively extending the overall length of the torque wrench body), you need to compensate for the additional leverage. If an adapter is used that effectively lengthens the wrench (such as a box wrench extension or crow’s-foot wrench), a calculation must be made to achieve the desired torque value. The use of an offset adapter changes the calibration of the torque wrench, which makes it necessary to calculate the correct torque settings.
Following are the formulas for calculating this change: The adapter makes the wrench longer:
TW = L ÷ (L + E) x desired TE
The adapter makes the wrench shorter:
TW = L ÷ (L – E) x desired TE
TW = torque setting on the torque wrench
L = lever length of the wrench (from center of the wrench drive to the center of the adapter’s grip area)
E = effective length of extension, measured along the centerline of the torque wrench.
TE = torque applied by the extension to the fastener
For example, if the fastener torque value is specified as 30 ft-lbs, but a 2-inch-long box-wrench extension is added to your torque wrench, you need to adjust your torque wrench setting to a lower value; otherwise you overtighten the fastener. Let’s say that your torque wrench length (measured from the center of the torque wrench grip to the center of the torque wrench drive) is 14 inches. You’re adding a 2-inch-long wrench extension.
Here, you use the formula where the adapter makes the wrench longer.
TW = L ÷ (L + E) x desired TE 14 ÷ (14 + 2) x 30 = 26.25 ft-lbs
With the 2-inch extension in place and the torque wrench set at 26.25 ft-lbs, the applied torque is 30 ft-lbs. If you want to know where to set the torque wrench when using an adapter that alters the effective length of the wrench, you must calculate to compensate for the adapter. If the distance from the wrench drive to the center of the bolt makes the wrench longer, the final wrench setting must be adjusted to a lower value in order to compensate. If the distance from the wrench drive to the bolt center makes the wrench shorter, the wrench must be set to a higher value to compensate.
Once all header flange fasteners have been tightened properly, you’re not done. Plan to re-tighten all fasteners a few times in the beginning after engine heat cycling. Bring the engine to full operating temperature, allow the engine to cool, and re-tighten. Repeat this process at least two or three times, allowing the gasket and fasteners to stabilize. Re-tightening after heat cycles helps to ensure a leak-free installation.
Design Your Own Headers
If you want to make your own custom exhaust headers, modeling kits are available from the innovators at Icenengineworks. These kits are akin to a Lego set and are designed to allow you to design your own tubular primary pipes in a very precise manner. As I’ve mentioned earlier, custom-making your own headers is simply not necessary if you have a production vehicle. Shorty headers are readily available for specific engines to accommodate engine swaps into custom or street rod applications.
Today’s header manufacturers offer ready-to-install headers for most popular vintage and modern vehicles, and even for retrofit applications such as the installation of a late-model engine into a vintage vehicle. The only instance for which a custom header may be required is in the case of custom purpose-built race cars or highly modified street rods. If off-the-shelf headers are available for your application, it’s foolish to invest the time and money for custom headers, especially when you consider the needed specialized tooling and welding equipment that most hobbyists don’t have access to.
Header Modeling Kits
The kits are available in a variety of diameters and consist of a collection of 1-inch-long round plastic blocks that snap together to create a length of pipe. These include straight pieces as well as a variety of radius bend blocks. Once you have an exhaust flange bolted to the cylinder head, with a short section of straight metal tubing welded to each port on the flange, you simply begin by attaching the plastic modeling blocks to create each primary tube’s shape and length. This allows you to mock-up the complete primary set either with the engine on a stand or in the vehicle (depending on your application needs).
Once the blocks are snapped together, each block can also be rotated for fine-tuning adjustment (for example, rotating a curved block changes tube direction). Once the bank of tubes has been mocked up to your satisfaction, you have your template as a guide when reproducing each tube in your choice of metal (steel or stainless steel). Each block is labeled with several indexing arrows and matchmarks that are placed 30 degrees apart that can be used as references. In order to keep everything organized before you disassemble the lock-up, it’s a good idea to create your own reference marks. Using a felt-tip pen (Sharpie, etc.), you can label each block according to cylinder number and location from the header flange.
For instance, at cylinder number 1, mark C-1 on each modeling block. Then mark the first block (closest to the head) as “A.” Mark the next block on that cylinder’s run as “B,” etc. In order to provide a reference regarding clock position on each block (handy in case you accidentally rotate the blocks during handling), place matchmarks from block to block. Now you’re ready to duplicate each primary tube model in metal. You can duplicate the model by piecing together sections from readily available stock of straight, 45-degree, 90-degree, and 180-degree mandrel-formed pieces. These are available from a variety of performance exhaust raw-parts makers and speed-equipment retailers. By having a selection of pre-formed sections, you’re able to cut and trim to achieve a variety of shapes and angles.
In a professional fabricating shop, headers are constructed by using a CMM that uses a digital probe to trace the entire length of the tube model. The data collected is then transferred to a CNC pipe bender. However, even a CNC bender may not be able to form extremely complex bends from a single tube, so the tube may have to be formed in sections that are subsequently welded together. Because most of us don’t have access to such high-tech equipment, the process can be handled manually. It may be time-consuming, but it’s relatively easy.
First, purchase a collection of metal tubing in your desired material, diameter, and wall thickness (straight sections, J-bends, and U-bends). You can then cut the metal tubes to mimic sections of the model. It’s imperative to make your cuts perpendicular to the centerline reference mark on the model in order to achieve a flush mating of each section. To provide a precision cut, Icengineworks offers a pivoting table (this works with any vertical bandsaw) that allows you to properly index each cut. Once you cut a section, label it for cylinder number and location. For instance, if you cut a length of tubing that mimics blocks C-1A, C-1B, C-1C, C-1D, label that piece of tubing accordingly so that you know exactly where this section is to be placed. Also, be sure to indicate the flow direction, to avoid welding a section upside-down. You can either draw an arrow to indicate flow direction (from head to tube outlet) or you can label the cylinder numbers and position numbers in order (exactly the order marked on the modeling blocks).
After all of the metal pieces have been cut, you need to weld them together. Rather than driving yourself nuts by trying to hold the pieces together by hand (which requires three hands), take advantage of the special tube clamps from companies such as Icengineworks. These positioning clamps are unique and are designed specifically for this job. Each clamp consists of two clamping straps that are connected by a slotted/ adjustable-length crossbar. One clamp strap attaches to each adjacent tube, and the space between the straps is adjusted to provide welding access. Once the clamp is tightened, the two sections of mated tubes are held securely in position. Continue to assemble the pre-cut metal tube sections by simply reproducing your previous plastic-block model. Once the entire header bank has been fully assembled using the special clamps, you have the opportunity to slightly loosen each clamp to rotate tube sections as necessary to finalize your shapes
When you’re satisfied that you’ve duplicated the model and you’re happy with the fit, tack weld the tube joints as needed to lock all pieces together. Once the header bank is securely tack welded together, remove the clamps and remove the header from the engine. Place the header on a clean, safe workbench and finish weld all joints. Just to be safe, reinstall the header to verify fit. At this point, you can dress the welds (if necessary) and apply the appropriate coating (such as a ceramic coating or high-temp paint).
Using a Header Fabrication Kit
Each modeling block is exactly 1-inch long and features handy indexing arrows that indicate the centerline. Blocks are also color coded, indicating tube OD. The block shown here is for 1.625-inch OD tubing. (Photo Courtesy Icengineworks)
Both straight and radiused blocks are included in the kit, allowing you to piece together your exact tube shapes. The two blocks shown here are orange (labeled for 1.750-inch-diameter tubing). Notice that the curved block is labeled as having a 3-inch centerline radius (CLR). (Photo Courtesy Icengineworks)
Here’s an example of a header bank mocked up using modeling blocks. Each block can be rotated to fine-tune tube angles and clearances. (Photo Courtesy Icengineworks)
Icengineworks’ collector mockup tool allows you to position the exit ends of the tube package together to simulate a planned collector. (Photo Courtesy Icengineworks)
Modeling blocks allow you to mock and test-fit the planned tubing even in the tightest confines. The adaptability and ease of assembly speeds up the design process significantly, without the need to fabricate metal tubing for the design stage. (Photo Courtesy Icengineworks)
In this example, tubing must clear the steering shaft and requires the use of creative bend shapes in order to achieve tube-to-tube clearance and fit. By simply adding or removing straight or radius blocks, and by rotating radius blocks, it’s easy to explore the various possibilities. Once the final shapes and lengths of all tubes have been determined, each tube package is replicated in the metal of choice (steel or stainless steel). (Photo Courtesy Icengineworks)
Special tack-weld clamps provide a gap between the clamp bands, allowing access for pipe tack welding. (Photo Courtesy Icengineworks)
Worksheets provided in the kit allow you to keep track of each segment and each bend radius. (Photo Courtesy Icengineworks)
This bank was mocked up using plastic blocks. Shown here is one metal primary tube that was replicated using the modeling blocks, clamped together and tack welded. The collector is attached, keeping the tubes in the correct location. (Photo Courtesy Icengineworks)
One by one, each primary tube is replicated and replaced with a steel counterpart, terminated in the collector, and tacked together. Once all sections have been tacked together, the entire bank is removed from the engine and finish-welded. (Photo Courtesy Icengineworks)
A mock-up collector template from Icengineworks aids in accurately determining primary tube termination points. (Photo Courtesy Icengineworks)
Here’s an example of a model-block mockup of a bank of primary tubes fitted to a collector guide. The guide mimics the anticipated collector ensuring that all modeling tubes gather for accurate placement into a planned collector. (Photo Courtesy Icengineworks)
Due to the availability of various-diameter modeling blocks, a stepped tube is easy to design. Here you see 1.625-inch blocks (blue) stepped up to 1.750-inch blocks (orange). By purchasing sections of tubing in both diameters, a stepped design can be obtained as easily as a constant-diameter design. The special tack-welding clamps are adjustable to accommodate the adjacent diameters. (Photo Courtesy Icengineworks)
Written by Mike Mavrigian and Posted with Permission of CarTechBooks