Exhaust Header Tube Sizing and Length
There is a variety of data available on how to select a set of headers for your vehicle application. I am going to provide you a simplified explanation here that may help you make the correct choice and debunk some inaccurate information. Headers and exhaust system modifications are the best improvement per dollar of any bolt-on modification which you can do to your vehicle. To put all the magazine articles, header manufacturer and performance part dealer recommendations into perspective, you have three primary questions to ask:
- Does the size of the primary tubes meet the requirements of the engine?
- Does the length of the primary header tubes and collector size meet the use of the vehicle?
- Do the headers actually fit your vehicle without having to use a sawzall, cutting torch, or sledge hammer to persuade them into place?
As I mentioned above, this article will help explain the choices available and how to select the correct header for your application. You will also find other information that has been misrepresented in other publications. I will do my best to make this presentation of facts simple, informative, and precise.
Primary Tube Size
The primary tube size is the measurement of the OUTSIDE diameter of the header tubes. Common header primary tube sizes are 1-1/2", 1-5/8", 1-3/4", 1-7/8", 2.0", 2-1/8", 2-1/4", & 2-3/8".
When you are considering primary tube size you are going to base it upon certain factors. You need to know, or you "should" be asked by the person trying to sell you a set of headers:
- What is the engine size?
- What is the engine's horsepower output?
- What is the intended use of the vehicle (street, street/strip, racing, RV or towing)?
- What is the average operating RPM range (where the engine will spend most of its time)?
- Are there any power-adders such as nitrous oxide or a supercharger being used?
- How much space is available - what headers (if any) are going to fit?
The smaller diameter primary tubes provide a faster air velocity for more torque, where the larger diameter tube size is better for upper RPM performance. If your headers are too small for the application they can cause restriction and increase heat in both the engine and the exhaust. Too large of a header will reduce the torque output of the engine and increase the possibility of exhaust gas reversion. Simply put, you can relate torque to air velocity, and horsepower to the volume of air flow. The faster the air enters and exists your engine the more torque you are going to produce, and the more volume of air you get through the engine, the more horsepower the engine will produce. You want a balance of both in your selection, but we need to address a few more concerns from the list above.
Using a stock V8 engine up to about 350 cubic inches, and producing about 300 horsepower installed in your street rod that is going to spend most of its use going stop light to stop light, limited bursts of power when you want to show off. For this type of application we are perfectly suited to use a 1-1/2" primary tube size header. If we were to add a supercharger to the same vehicle and close engine configuration we would now need a 1-5/8" to 1-3/4" primary tube header. This is because we are now pushing considerably more air through the engine, effectively increasing the cubic inch volume of the engine.
Most header manufacturers are quite accurate with their manufactured header sizes when they are for specific vehicle applications. This holds true for all of the smog-legal direct fit headers for modern model vehicles. These manufacturers, such as JBA and Gibson, have already performed the research and development work for you, allowing them top offer a product that fits your vehicle perfectly and performs as advertised. Where you run into more complex selection questions begins with universal or custom fabricated headers.
Small Tube versus Large Tube Designs
I mentioned above that I will debunk a few inaccuracies in header advertising and knowledge we have all read or been presented. One of the most common asked questions is, "Won't a small tube header reduce the torque output of my engine?" The quick answer is a definitive "NO!" for the majority of vehicle applications! I am asked this question a few times a week, and even though the default answer is often accurate, the first thing I must do is qualify the customers application. This means that I have to ask more questions - those questions are listed above.
The dyno chart below from Sanderson Headers (the original source and testing was done for "Chevy Action! Magazine") shows a basic difference between a small tube and large tube header set. This engine was tested with both small and large tube headers. If this was your engine and a majority of the driving was done at low engine speeds, the small tube design would be your best choice. Bigger is not always better!
Header Tube Dyno Chart: Small Diameter vs Large Diameter Primary Tubes
- 433 cu. in. big block Chevy
- 10.4:1 compression
- Holley 780cfm carburetor
- Oval-port heads with 2.09" intake valves, and 1.88" exhaust valves
- High performance mechanical camshaft, specs 250°/259° duration at 0.050 lift
As with the street rod example I provided previously, a small tube header was perfect, and the primary tube size selection would increase with engine size, horsepower output, and if the use was driven more aggressively or raced. If you changed that small block engine to a mild 396-454" big block Chevrolet, a 1-5/8" to 1-3/4" header primary tube size is needed. Again, engines size, power output, and intended use must be most important underlying factors in deciding primary tube size.
When we discuss racing applications there is even more data, specific data that is required to select a correctly-sized header set. The information necessary to select a racing header is similar to a non-racing header, but with a few different and extra concerns. Racing header selection is based upon these factors:
- Engine size
- Operating RPM range
- Horsepower output
- Type of racing (drag, circle track, road racing, monster truck, tractor pulls, sprints, hill climb, or something else)
- Type of fuel
- Any power-adders such as nitrous oxide or a supercharger being used
- Whether we want to use a single or multi-step header design
Further down in this article I have provided specs for a properly configured header using modern calculation software from Meaux Racing. Their software takes all of these parameters and many more into consideration, then presents the user a header design that is matched specifically to their engine. With that knowledge you can either have a custom header built specifically for your vehicle, or purchase a production header set that is "close" to the recommended dimensions.
Primary Tube Length
I will jump right into this next topic, with the debate over SHORTY versus LONG TUBE, and the benefits (or lack thereof) from Equal Length headers. We sell headers in each of these styles (depending on the brand and application), and each style header provides various benefits as well as disadvantages. For anyone who has an opinion and wants to start by bad-mouthing any one of these header designs without all the facts is severely limiting their options and knowledge of what the facts really are.
PROS - Easy installation; a variety of sizes available for popular applications; improved ground clearance; designed for space-limited installations; definite and proven improvement in power, torque, and efficiency over factory manifolds; relatively inexpensive.
CONS - Slight loss of torque in high power/high RPM applications; it is sometimes possible for the shorter design to be strained and not provide adequate airflow at the collector on larger/high power/high RPM applications.
PROS - Better for racing and high performance; improved torque in some applications; better suited if hoping to gain maximum power; more variety of sizes and lengths available to match specific needs.
CONS - Installation is more difficult; less clearance everywhere; modifications to the header or chassis is common; a mini-starter is required on many applications; expensive.
Equal-Length Primary Tube Headers:
PROS - Best for all-out racing and high performance; improved torque in some applications; best suited when trying to squeeze every last bit of power out of the application; variety of sizes and lengths available for specific (popular) vehicle and engine combinations, increases power at more specific RPM ranges.
CONS - Installation is often times considerably more difficult; less clearance everywhere; common ground clearance problems; sometimes interferes with aftermarket oil pans, linkage, and other component-specific conflicts; modifications to the header or chassis is common; a mini-starter is required on most applications; very expensive.
Another one of the biggest complaints (or concerns) we hear from our customers is that they are worried that a shorty header is not going to make as much power as a full length header. That actually is a fact, but the part many people miss is that in most cases, if they are looking at a shorty header in the first place it is NOT all about horsepower. If they are not racing the vehicle they are wanting to install headers on, and they have issues with clearances or even getting a header to fit at all, a shorty header is probably the best choice. Additionally, if they are worried about the expense of the full length header or the costs associated in paying someone to build a custom set, a shorty header is again the best solution. If you look at the graph below, you will see that there is not really much loss of power with a shorty header, and in this particular engine the short tube header makes more power at lower operating RPMs. (This is also a graph from Sanderson Headers with the original source and testing done for "Chevy Action! Magazine." The specs for this test engine were provided above.
Of course, if the vehicle is primarily used for racing or high performance where maximum power is of primary concern, we are going to tell you that your best choice is a full length and/or equal length header design.
Collector size is measured in both length and diameter. In a shorty, direct-fit newer car model, or street rod type application, the collector length is of no concern. For this type of header the collector is merely a union point for the primary tubes, and a connection point for your under chassis exhaust system. The collector diameter is important in that you need a diameter compatible with both the power output of the engine, large enough for the primary tubes to merge, and to match (or be close using a reducer) the tubing size for the rest of your under chassis exhaust piping. It is OK to use a reducer to mate the size of a smaller exhaust system to a larger collector, but with a shorty or mid-length header the collector is typically of a standard size combination for vehicle and the engine it fits. With a full length header you will typically find that the collectors are fairly large in all but those designed for torque-focused applications such as pickup trucks, SUVs and motorhomes. This is due to the fact that most full length headers are designed for maximum power gains, and mostly upper RPM use. In racing applications (without under chassis exhaust piping) you can use the collector length to slightly tune the torque output of the engine. A longer collector (to a point found through testing) offers increased torque output at the cost of upper RPM horsepower. Trial and error experimentation is necessary to find the proper balance.
Repeating what has been mentioned above, collector length and diameter selection are also controlled by the engine size, power output, and operating RPM. Too large and you slow down the exhaust gas velocity, and too small restricts flow and increases heat.
Many of you who are reading this article may never have heard of a step-header. This is because step-headers are used almost exclusively in race-only applications. A step-header design is that which has more than one size (diameter) of primary tubing. You start with a smaller header tube diameter at the cylinder head, and then have one or two larger sizes which begin at various distances from the flange and previous step. Where the steps (increases in tube diameter) are placed depends on the engine type, engine size, operating RPM, and other factors.
You will find a few websites that bad-mouth step-header designs as a waste of money. They claim that they do not work, the theory is bad, no one uses them, etc. You have to decide why these people are condemning this design and look for any bias in this source. Are they selling something different? Well, if the step-header designs were as crappy as these websites describe, heavily funded race teams would NOT be using them on championship winning engines. It is my opinion that there are uses and benefits from step-headers, but not every engine is going to show gains in using the design. Over the years I have talked many people out of step headers as a waste "for their specific application." I have also recommended step headers those who felt they did not need them.
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. If you look at the exhaust port of your cylinder heads you will notice that they typically are NOT of the same size as your header tubes. You won't even find a header flange that is a direct match in dimension to your exhaust port unless it is a race only application that has been user/builder modified, or someone looking to spend the time and expense to fine every available horsepower. If the exhaust ports on your cylinder heads get too big you lose air velocity and torque, and take the chance of increasing the possibility reversion. As the exiting exhaust gases travel through the headers they are expanding until they cool enough to where expansion stops. The longer you can keep the exhaust velocity up, the better the scavenging of the cylinders, and the more power and torque the engine can make.
By starting with a slightly smaller tube at the cylinder head flange. This step is necessary to ensure the exhaust velocity stays UP. As the gases expand, they get to the next size, or the second step. This allows for a contained (managed) control of the expansion of the spent gases while still keeping an efficient exhaust velocity. The manufactured deign may include a third step (increase in tubing size), and then the tubes connect to the collector. This gradual increase in size provides the maximum balance of exhaust velocity and volume. The data below was created using Meaux Racing's PIPEMAX software. You can see that this is based upon a 500" (8.19L) engine with an optimum speed (where the engine will spend the majority of time) of 9,000 RPM. The example below shows the best single stage, two step, and three step header dimensions.
|Results Optimized for a 499.665 CID (8.19L) 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 tubing diameters are OD (outside diameter) and based-off 0.0625" inch tubing thickness
The same people bad-mouthing the step-headers also condemn the software used to find these dimensions. PIPEMAX is only one program. Most engine builders and race teams use a variety of software simulations to develop a guide or baseline when testing and developing engine components and combinations. These programs are extremely detailed and intricate tools designed to help the professional.
Other Header References
Header Flange Thickness:
Common header flange thicknesses are either 5/16" or 3/8". For many years all you could find was a 5/16" thick flange. This was great for a little weight savings, but it was hell on flange gasket life. With the expansive properties that exhaust heat provides, the header flanges are heated (stretched) and cooled repeatedly to the point of loosening the flange bolts, causing an imminent gasket failure. The header manufacturers started using the thicker 3/8" flanges to better control thermal expansion. This thicker flange provides better clamping force and offers less thermal movement. The header bolts are exposed to less movement, and thus the gaskets and clamping force lasts almost indefinitely.