Exhaust Header Tube Sizing and Length

There are a variety of data available online on how to select a set of headers for a specific application. Instead, this article will provide a simplified, yet detailed, and hopefully easy to understand explanation of what is important regarding header sizing and application. Headers and exhaust system modifications are the best improvement per dollar of any bolt-on modification for the majority of vehicles. To put all the magazine articles, manufacturer and dealer recommendations into perspective, three primary concerns exist:
  1. The size of the primary tube must meet the requirements of the engine and the vehicle's intended use.
  2. The length of the header tubes and collector size meets the RPM requirements and the use of the vehicle.
  3. The headers actually fit the vehicle without having to use a sawzall, cutting torch, or sledge hammer to persuade them into place.
This article will help explain the various choices available and how to select the correct header size for the intended application. Information provided here will sometimes challenge common misrepresentations of facts provided in other popular publications. Instead of manufacturer-sponsored "advertising" reviews, our articles utilize facts, real-world experience, and an understanding that our customers deserve exposure to real-world, factual information.
 
Header Primary Tube Size
 
The primary tube size is the OUTSIDE diameter of the header tubes. Common primary tube sizes include 1-1/2", 1-5/8", 1-3/4", 1-7/8", 2", 2-1/8", 2-1/4", and 2-3/8".
 
When considering the correct primary tube size, decisons must be based upon a multitude of factors. For example, engine size, horsepower output, average operating RPM, use of the vehicle, and if any other power-adders (nitrous or supercharger) are being used. What type of cylinder heads are being used? What is the bore and stroke of the application? What are the camshaft specifications (lobe centerline, exhaust opening, lobe lift), the rocker arm ratio, exhaust valve size, and more? What does the rest of the exhaust consist of (piping size, muffler type and how many)? What is the vehicle weight, gear ratios, and transmission type? The more data available, the more accurate the header size recommendation will be.
 
Smaller primary tube diameters keep the air velocity up for more torque, where a larger tube size is better for upper RPM (above 6,500 RPM) performance. If the headers are too small for the application they will cause restriction and increase heat in the engine. Too large a header will reduce torque output of the engine and increase the possibility of exhaust gas reversion---again increasing heat. Simply put, torque is related to air velocity (the faster the air/fuel get through the engine), and horsepower to the volume of air/fuel that processes through the engine. The faster the air enters and exists an engine the more torque the engine makes, and the more volume of air that passes through the engine, the more horsepower the engine will produce. A balance of both is necessary for typical street applications, where racing-specific engines are often designed within specific RPM range requirements.
 
Example: On a common small block V8 engine producing 300-400 horsepower and installed in a street rod that is going to spend most of its time operating stop light to stop light, a 1-1/2" primary tube header is perfectly effective. Add a supercharger to the same engine and a 1-5/8" to 1-3/4" header is better suited.
 
Take that same 400 horsepower on a racing application that spends all its time on a drag strip between 3,500 and 6,500 RPM and a 1-5/8" tube is the bare minimum, and a 1-3/4" header is the standard choice.
 
Using the example above, a small tube header is perfect, and the primary tube size would increase with engine size, horsepower output, and expected higher operating RPM range. Change the engine to a mild 396-454" big block Chevrolet and a minimum 1-5/8" to 1-3/4"  header is needed. Again, engine size, power output, and intended use determine the header primary tube size.
 
Most header manufacturers are fairly accurate with their header sizes when they are for specific vehicle applications. This is why they categorize their header options (passenger car, truck/SUV, RV, marine, and racing applications). This also holds true for all of the smog-legal direct-fit headers required for modern vehicles. These manufacturers, such as JBA, Gibson, Thorley, Dougs, Hedman, BBK and others have already completed the research and development work necessary to ensure a header offering matches a specific vehicle. More specfic selection questions occur with universal, racing, or custom fabricated headers.
 
The dyno chart below shows basic differences between a small tube and large tube header set. This big block Chevrolet engine was tested with both small and large tube headers. If this engine was installed in a vehicle where the majority of operation occured at low engine speeds (below 5,500 RPM), the small tube design would definitely perform the best in this application. Bigger is not always better!
 

Test Engine:
- 433 cubic inch big block Chevy
- 10.4:1 compression
- Holley 780cfm carburetor
- Oval port cylinder heads (2.09" intake and 1.88" exhaust valves)
- Solid lifter camshaft (250°/259° duration @ 0.050" lift)
 
For racing applications, highly specific data is required to select the proper header set. Decades ago, trial and error at the track was the true test of proper header size. The research occurred on the track after evaluating air and altitude values while staring at the time slip's Estimated Time and Miles Per Hour. Later, engine and chassis dynometers were used to develop and test virtually every possible engine component, including headers. This tried and true experience helped to develop the math, software, and applications we use today to select correct engine system components.
 
Again, engine size, operating RPM range, horsepower output, type of racing (drag, circle track, road racing, monster truck, tractor pulls, sprints, hill climb, etc.), and whether a single or multi-step header design (if allowed in a particular racing class) is best. Further down in this article we have provided specs for a properly configured header using modern calculation software. Data was entered into the software and corresponding outcome calculations take all the necessary parameters into consideration and then present a header design (tube size, length, etc.) that is matched specifically to the application. With those results the builder/car owner can eitther try and find an affordable  production header set that is "close" to these specs, or have a custom header set fabricated specifically for the application.

Primary Tube Length
 
The next, often confusing and mischaracterized topic addresses the debate over SHORTY versus LONG TUBE, and when Equal-Length headers are necessary.
 
We offer both styles to our customers, from a variety of header manufacturers, and both stles have their benefits and disadvantages. But, for those who bad-mouthing either design without attaining and understanding adequate information is severely limiting their options and knowledge of facts.
 
Shorty Headers:
PROS - Easy installation; variety of sizes available for popular applications; improved ground clearance; designed for space-limited installations; definite improvement over factory manifolds; inexpensive
CONS - Slight loss of torque in high power and high RPM applications; it is sometimes possible for the shorter design to be strained at the collector (increased heat) on high power applications
 
Full-Length Headers (Equal Length Primary)
PROS - Best for racing and high performance; improved torque in some applications; best for maximum power; more variety of sizes and lengths available to match specific engine, racing style, and operating RPM needs
CONS - Installation is more difficult; less clearance everywhere, including ground clearance; tube and other modifications is common to ensure proper fit; mini-starter required on many applications (see note on mini starters below); more expensive
 
A few words on mini starters:
Many headers require the use of a mini-starter to ensure adequate header to starter clearance. However, upgrading to a mini-starter is actually beneficial for many more reasons.
  1. Lighter weight
  2. Gear reduction design provides increased torque and faster cranking speeds
  3. Lower power drain
  4. Easier on flywheels and flexplates
  5. Infinitely adjustable clocking (many brands have this feature)
  6. Reduced negative affects from header/exhaust heat
  7. Smaller overall dimensions

For many classic vehicles using OE-style rebuilt starters purchased through common parts stores, it is difficult to ensure that the starter core provided by the store's electrical rebuilder is one that originally came on that specific vehicle. Rebuilders often combine numerous OE part numbers into one general (fits all these applications) part number. The starters may be of varying dimensions (length, width, diameter) or clocking, but somehow now use the same part number from the rebuilder. These combined numbers may fit a variety of vehicles using factory exhaust manifolds without any interference issues. However, that same starter may not work on the same vehicle once headers are installed. This is because header manufactures will design a header using a project vehicle that uses the correct original starter part number for that vehicle model/year. If the starter on that vehicle is not original or compatible with what the header manufacturer had on the project vehicle, a different, compatible starter is required. This is easily aleviated by upgrading to a mini-starter and the benefits it provides.

When choosing a mini-starter, compare manufacturer warranties and features (like unlimited clocking). Those with the best warranties often use the best armatures and other components.

In a final note, all modern vehicles come from the factory with smaller, more efficient gear-reduction starters. Why install a bulky, heavy, inefficient OE-style starter on an engine or vehicle that has already incurred thousands of dollars in meticulous improvements? Doing so could be compared to wanting solid rubber tires and wooden wheels on an otherwise fully modified classic street rod! Get the better mini-starter and know you've made a responsible decision!

The biggest complaint (or concern) we hear from our customers regarding header choice is that they are worried a shorty header is not going to make as much power as a full length header---or that their engine builder or the spec sheet included with their newly purchased crate engine states that they need X-size header. That is true in some cases. However, they often miss the fact that if they are looking at a shorty header in the first place it is NOT all about maximum horsepower. If the vehicle is not primarilly used for racing, has clearance issues, or simply trying to find a header that fits their vehicle is a concern, the short header is the way to go. If they are worried about the expense of either the full length or having to pay for someone to build a custom set, a shorty header is often the answer. Looking at the graph below, the shorty header performs better at RPMs where a street-driven vehicle spends most of its time---below 4,000 RPM. On this engine, the longer tube header offers benefits where expected---at RPM levels above 4,000 RPM.
 
In reviewing the dyno results below, this author's experience suggests that this engine would benefit from a different camshaft with a wider lobe centerline, or in the very least, high ratio rocker arms on both intake and exhaust valves. This change would flatten out the torque curves on either header length. However, I'd like more information on what intake manifold was used on this engine. (NOTE: This is the same big block Chevy engine used in the previous dyno graph, now used to compare different header lengths.)
 

Of course, if the vehicle is a racing or performance application where maximum power is of primary concern, we are going to advise the customer as to the best header choice for their application, which is typically a full length header.

But My Engine Builder (or Crate Engine Spec Sheet) Told Me I Need ____Size Header...

Every week we hear it. Yes, every week a customer tells us what header spec they were told to purchase  I often wonder if some engine builders understand more than maximum dyno numbers or ask their customers what vehicle the engine is going into and how they plan to use that vehicle. I can offer an example, in a direct quote from the installation guide for the popular GM ZZ383 crate engine #12498772
 
"Headers:
The ZZ383 High Performance engine should be installed with a pair of high performance headers for maximum performance. The headers used during development of the ZZ383 had 1.75" diameter primary tubes. Primary tubes were approximately 32.00" in length and had 3.00" diameter collectors. Using a similar combination in your application, along with a performance exhaust system with a balance tube (“H” pipe) and low restriction mufflers, will provide you with optimum performance from your ZZ383."
 
The interesting point of this quote is that they clearly state "1-3/4" headers, using equal length (32.0") tubes, and a 3.0" collector." However, when looking at the dyno graph from this engine, what can be determined from this basic graphic?
 
GM ZZ383 Dyno Chart
 
First, the torque curve is almost perfectly flat, meaning that acceleration is consistent, and easy to setup the vehicle to ~5,000 RPM. This makes gearing relatively easy and using an overdrive transmission with fairly conservative gearing offers plenty of streetable driveability. It's not "peaky" in this regard. However, what does it tell us about header size and dimensions?
 
Having 425 peak horsepower and 449 ft/lbs of torque under the hood is decent, but really, anything over 5,000 RPM in this example merely amplifies wear and tear on the engine. Torque is falling, and peak horsepower occurs just 400 RPM higher (900 RPM over peak torque). The big torque numbers occur between 3,000 and 4,500 RPM. Therefore, even if driving this engine hard (or racing it), we'd want the shift points to allow the engine to fall to roughly 3900-4,200 RPM for maximum continued acceleration (pull).
 
Using simple calculations (and 100% volumetric efficiency), I show that this engine at 5,400 RPM requires a minimum 1.60" primary tube diameter with a 2.78" collector. However, if I reduce the operating RPM to a more reasonable "daily use" value of 4,500 RPM I calculate a primary tube diameter of just 1.46" and a 2.54" collector. Of course, we want slightly more capability than what the calculations provide. This cushion is used to adapt for variations in air quality (altitude) and other factors. Please note that this calculation is an oversimplification. The more data provided in the calculation, the closer to optimum sizing the calculations provide. However, even using simple calculations, it is easy to see that a 1-3/4" header on the ZZ383 may be fine for racing or regular wide open throttle (WOT) use, but on a common street application (for example, somone's '32 Ford, '70 Chevelle, or 4WD truck), a smaller header is perfectly adequate and a better, more efficient choice.
 

Collector Size
 
Collector size is measured in both length and diameter. In a shorty, street rod type application the collector length is of no concern. In these headers the collector is merely a union point for the primary tubes, and a connection point for the vehicle's under chassis exhaust system. The collector diameter is important in that the application requires a diameter compatible with both the power output of the engine, and the under chassis exhaust piping size that completes the rest of the vehicle's exhaust system. Of course, most headers come with a collector reducer to meet the size of a smaller exhaust system, but with a shorty or mid-length header the collector is already manufactured using a standard sized complimentary collector for that header and the engine it fits. On these applications it is common (and cleaner) to have a one-piece down pipe fabricated instead of using the reducer. If someone does not have access to a muffler shop, tubing bender, or other fabrication equipment, the reducer provides a weld point for under chassis exhaust piping connection.With a full length header the collector diameter is usually fairly large in all but those designed for light duty passenger cars, trucks, SUVs and motorhomes. This is due to the fact that most full length header designs are for max power, and upper RPM use. Additionally, in racing applications we often use the collector length to slightly tune the torque output of the engine.
 
To reiterate, collector length and diameter selection are determined by the engine size, power output, and operating RPM. Too large a diameter and the exhaust gas velocity slows down, affecting torque output; and too small restricts airflow and increases heat.

 Step-Header Designs
 
It is possible that someone reading this article is unfamilliar with the term step-header. This is because step-headers are used primarilly in race-only applications. However, more and more header manufacturers and engine builders are seeing the potential this technology offers. A step-header design is that which has more than one size of primary tubing, sectioned in simple 1/8" increases over the length of each tube. The header begins at the cylinder head with a smaller tube diameter and then increases in usually one or two larger incremental sizes that occur at calculated distances from the cylinder head flange. Where the steps (increases in tube diameter) occur depends upon the engine size, operating RPM, exhaust timing, engine bore and stroke, and numerous other factors. New technology and research has also evolved into the development of stepped design shorty headers.
 
There are a few websites and articles that bad-mouth step-header designs as a waste of money. They claim that they do not work, the theory is bad, etc. It is important to ask why these people are condemning the step header design. Are they selling something different? If the step-header designs were as crappy as these sites/individuals describe, heavily funded race teams would NOT be using them on championship winning engines. It is this writer's opinion that there are uses and benefits from step-headers, but not every engine/vehicle application is going to show adequate gains to justify the expense. Have deep pockets and are looking for every last 1/4 horsepower at a relatively narrow RPM range? Then step headers are for you!
 
A step-header is built by starting with a slightly smaller tube, then going up in size in one more (2-step), or two more (3-step) sizes. Examining the exhaust port of most cylinder heads, it is easy to notice that they are typically NOT the same size as the header tubes. Header flanges are rarely the same size as the tube diameter, and often not a direct match in dimension to the exhaust port itself unless it is a race-only application that the user/builder modified---or again, someone looking to spend the time and expense to find every last available 1/4-horsepower. If the exhaust ports on the cylinder heads get too big, air velocity and torque suffers, and this also increases the possibility of reversion. As the exiting exhaust gases travel through the header tubes, expansion occurs until cool enough where expansion slows, reducing air speed and scavenging effects. The longer the exhaust velocity remains at or near its highest values, the better the scavenging of the cylinders, and the more power and torque the engine can provide.
 
In simple terms, by starting with a slightly smaller tube, the exhaust velocity stays up. As the gases expand, it hits the next size step. This allows for contained control of the expansion while still keeping velocity speed high. This would also continue to the third step, and then to the collector. This gradual increase in size provides the maximum balance of exhaust velocity and volume. In more complex terms, we look at the pressure waves within the header primary tubes. Engine pulses in an equal-length, tuned header create what is called rarefaction, or a negative, low-pressure wave that is partially compressed and redirected back up the primary tube toward the exhaust port. This wave reaches the exhaust valve about the time valve overlap occurs (when both intake and exhaust valves are open). It is easy to believe that these rarefactions are bad, or something we don't want. However, on an internal combustion engine we actually want to control them to our benefit. The animated image below (source: Wikipedia) shows a sound rarefaction. The official definition states, "a reduction or diminution of an item's density, opposite of compression, especially in air or a gas."
Rarefaction
Step headers actually create additional rarefactions, and the location and size of the steps (usually in 1/8" diameter increments with most headers) affect the strength of the negative pulses. Increasing the step too much (or having to many) can weaken the initial rarefaction while increasing corresponding negative pulses, which negatively affects the benefit. This is why most step headers have just one or two steps. Three-step step headers are not as common.
 
The data below was created using Meaux Racing's PIPEMAX software. This data uses a 500" engine with an optimum RPM (where the engine will spend the majority of time) of 9,000 RPM. It shows the best single stage, two step, and three step header dimensions.
 
Optimized for 499.665 CID engine at 9000 RPM
 
Single Primary Pipe Specs:
Diameter = 2.227 to 2.352
Length = 21.2 to 24.4 inches long
 
2-Step Primary Pipe Specs
1st Dia. = 2.227,  Length = 10.6 to 12.2
2nd Dia. = 2.352,  Length = 10.6 to 12.2
 
3-Step Primary Pipe Specs
1st Dia. = 2.227  Length = 10.6 to 12.2
2nd Dia. = 2.352  Length = 5.3 to 6.1
3rd Dia. = 2.477  Length = 5.3 to 6.1
 
Header Collector Specs  (Conventional Straight Tube)
Diameter = 4.028 to 4.278
Tuned Lengths = 12.3 best (and also 6.1 or 24.5)
 
Header Collector Specs  (Megaphone or Diffuser Cone Shape)
Diameter = 3.528 taper to 4.528
Megaphone/Diffuser Length = 12.3 inches

Total Exhaust System Tuned Lengths (Primary ends to TailPipe end)
Best HP/TQ Tuned Lengths = 12.3 , 24.5 , 49.1 , 98.1 inches long
Worst HP/TQ Loss Lengths = 18.4 , 36.8 , 73.6 , 147.2 inches long
 
Note -> measured from where the Primary Pipes end inside the Collector to the point the tailpipe exits into the atmosphere.
Note ->
all Pipe Diameters are OD and based-off .0625 inch Pipe thickness
 
The same people who orginally bad-mouthed step-header technology also condemned the software used to find these dimensions. PIPEMAX is only one program of many, and most engine builders and race teams use a variety of software simulations as a guide or baseline when testing and developing engine components and combinations. These programs are extremely detailed and intricate tools designed to help the industry professional. Oddly, many of the original detractors have since embraced step header technology.

Other Header References
 
Header Flange Thickness: Common header flange thicknesses are either 5/16" or 3/8". For many years all headers were manufactured using a 5/16" think flange. It was difficult and costly to manufacture flanges thicker than 5/16". Although this may have offered some benefit in a little weight savings, it was hell on flange gasket life and proper sealing (stable and consistent clamping force during thermal cycles). With the expansive properties that exhaust heat provides, header flanges are heated (stretched) and cooled (after engine shut down) repeatedly to the point that loosens bolts and makes gasket failure eminent. Finally, as manufacturing and matrial costs became more reasonable, header manufacturers started using the thicker 3/8" flanges to better control of thermal expansion and cycling. The thicker flange provides better clamping force and less movement, and header bolts are not exposed to as much movement. Therefore, proper sealing can last almost indefinitely.
 
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