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Just a quick note from Jan. 2011: When I first started writing these header pages in 2003 there was absolutely NO real information on exhaust header design available to the common motor head, I had to call experts in the field and talk to them to get relevant and useful information. I just did a Google search and there are dozens of web sites out there now, my how the world has changed!

Car Craft magazine has actually had some articles on header design and fabrication - magazines NEVER covered that topic for fear of pissing off some of their best advertisers - the off the shelf header mfg's. I am still disappointed when I read an engine article in a magazine or watch Horsepower on Sat. morning and they do a dyno test and give a complete description of the engine build and then just slap on a set of "dyno headers" that were lying around and never tell you what the header dimensions are and they almost NEVER try different primary or collector dia. or lengths to maximize HP and torque. It seems we still have a ways to go to get the media and the masses to understand the importance of header design to the overall engine combination.

By visiting this page and trying to get a handle on the information I hope your interest is raised and you continue to research the subject on the other web sites now available to you. There is still some misinformation out there so read a lot, learn for the real experts that have been in the business for a long time and have tried all the gimicks - not the forum loud mouths that parrot something they heard somewhere. The more you learn the more you will want to build your own custom header designed for YOUR engine and driving needs and stay away from off the shelf headers.

Here's the original article - have fun...

1. Where did I get my information?

a. Ed Henneman, 40+ years in custom header design/fabrication
     Headers by Ed, Inc.
     2710 16th Ave. So.
     Minneapolis   MN 55407

(612) 729-2802

b. Jere Stahl, 40+ years in header design and fabrication
     Stahl Headers
     1515 Mt. Rose Ave.
      York PA 17403

(717) 846-1632

c. Vince Roman, cutting edge header design and parts mfg.
    Burns Stainless
    1013 W. 18th St.
    Costa Mesa   CA 92627

(949) 631-5120

d. Mark Lelchook , cutting edge header design/mfg.
    Performance Welding
    5898 Melody Lane
    Foresthill CA 95631

(530) 367-4124

f. Scientific Design of Intake and Exhaust Systems

Phillip H. Smith and John C. Morrison

ISBN #0-8376-0309-9

HEADER DESIGN: ARE YOU GIVING AWAY HORSE POWER?

When most of us modify an engine for racing or performance street use we put a great amount of thought and effort into selecting a camshaft, determining what compression to run, and what head, valve train and intake modifications we will make. We double check the machine work and trial fit everything several times to ensure clearances are correct, all in an effort to ensure our engines will make the maximum horsepower possible and stay together. After installing our engine we bolt on an exhaust header(s) and fire it up for break-in without much thought about how the header(s) will complement the rest of our engine combination.

To many of us, exhaust header design is a science that we do not understand so we just buy a commercially manufactured header we think will work and don't put much more thought into it. We may check out our competition or engines similar to ours, and maybe ask around the pits to see what others are running on their cars, but that is usually the extent of our research into the subject. We may be leaving significant horsepower on the table without realizing it. Off-the-shelf production headers are not optimal for any engine unless it is configured exactly like the engine the header design was developed on.

   Headers can increase power two ways. First by reducing the restriction to exhaust gas flow caused by a production cast iron exhaust manifold. A header allows the exhaust gases to exit the combustion chamber more easily allowing more air/fuel mixture to enter. The second is by taking advantage of a pressure wave phenomenon that occurs inside the header tube and utilizing it to create a low pressure area in the exhaust port during the overlap period of the valve timing events.

Pressure wave tuning is discussed at great length in the book "Scientific Design Of Exhaust & Intake Systems" by Phillip H. Smith and John C. Morrison. When an exhaust valve opens and the exhaust gases begin to exit the combustion chamber under high pressure, a pressure wave is formed that travels at about 1400 feet per second; this is much faster than the exhaust gases, which move at about 300 feet per second. How that pressure wave is managed can greatly affect the performance of a header design.

    The conventional 4 into 1 header is the most common exhaust header found on performance cars. It is the easiest and least expensive to fabricate. By designing the header with the appropriate length primary tubes to match the pressure waves at a desired RPM, a negative pressure can be formed at the exhaust port at the optimal time to help pull more of the exhaust gases from the combustion chamber and fresh air/fuel mixture in.

     The 4-2-1, or "tri-Y" has gained favor in recent years. The 4-2-1 header is more expensive to manufacture than a 4-1 header but is often easier to fit due to fewer long tubes. Many people think that the exhaust gases from one cylinder help pull the exhaust gases from other cylinders as the gases pass the junction in the tubing.   Pressure tests have proven that this is not true. 4-2-1 headers rely on multiple pressure wave reflections combining together to lengthen the duration of the low pressure during cylinder scavenging and attenuate the pressure peaks. By having tubes from multiple cylinders join together well before the collector, multiple pressure wave reflections can be created. When the original pressure wave reaches the point where the tubes join, the wave will travel up the non-exhausting tube and reflect back, joining the normal pressure wave reflection returning from the end of the header. Correctly designed, these headers can produce gains in the midrange of the power band. The primary and secondary tube diameters and lengths are very critical for optimum horsepower and torque on performance engines. Modern 4-2-1 racing headers are a far cry from the old "tri-Y" headers of the '60s; they are designed using advanced engine simulation software and must then be tested on a dyno and at the track to fine tune them for maximum performance. Off-the-shelf "tri-Y" headers are often touted as producing a broader or flatter power curve than a 4-1 header. In some cases this has proven to be true but the curve is often much lower. Testing has shown that a properly designed 4-1 header can often equal or exceed the power curve of a production 4-2-1 header while generating more peak power.

The 4 into 1 stepped primary header has also gained popularity in recent years. They are more expensive than a basic 4-1 header but are supposed to give the benefits of a small tube primary header for low/mid power and large tube header for peak power. The step in the primary tubes also creates a secondary pressure wave reflection similar to a 4-2-1 header. According to Vincent Roman PE of Burns Stainless, the placement and sizing of the steps is critical for success and every engine combination is different so a new header must be designed to match each combination. Vince also said that having correct primary tube length and collector sizing was more critical than having a stepped header. With all factors equal, Jere Stahl owner of Stahl Headers has shown only a 1% gain in horsepower over a single diameter header. Stepped headers should be used on maximum effort racing engines and configured using advanced engine simulation software to ensure correct placement and sizing of the steps. Club racers and street performance engine builders should concentrate on correct primary tube diameter and length as well as collector sizing. Many people think stepped headers are a new development; however, Ed Henneman, owner of Headers By Ed, built a stepped header in 1962. Smith and Morrison also discussed stepped headers in the first edition of their book published that same year.

There are several different collector designs that have been developed over the years, some more effective than others. Most headers have a basic collector utilizing a large tube formed to fit over the primary tubes, with the center of the primary tubes having a gap that is plugged by a small flat piece of sheet metal. Most commercial headers are still made this way. Early in Ed's career he developed a "pinched primary" collector where the center of the primary tubes is heated red hot and pinched together to eliminate the gap before welding, resulting in less exhaust gas turbulence as the gases enter the collector, increasing performance. Merged collectors were first used in about 1964 but did not become popular until the '90s. Merged collectors are formed by carefully cutting mandrel bends on a band saw to form a smooth primary tube transition into the collector resulting in a pyramid shape in the center of the collector to smooth the gas flow. The pyramid also takes up volume in the collector, ensuring a more gradual change in volume as the exhaust gases transition from the primary tubes into the collector. This more gradual transition gives smaller pressure wave reflections. Venturi collectors are similar to the merged collector but have a smaller outlet that transitions to a megaphone shape, creating a venturi to speed up the exhaust gases and increase combustion chamber scavenging. The outlet diameter of the merged collector into the venturi transition is critical for maximum performance and is different for each engine combination.

How long should your primary tubes be? There are several different formulas to calculate a starting point for developing a header primary tube. One formula for primary tube length is: P= 850 x ED divided by RPM - 3, where

P= primary length, ED= 180 + the number of degrees before bottom dead center that the exhaust valve opens, and RPM = the RPM you want to tune for.

Here's an example: The exhaust valve opens at 62 degrees before BDC, ED = 242, I want my engine to focus power in the 5500 rpm range, so…. P = 850 x 242 = 20,570 divided by 5500 = 37.4 - 3 = 34.4. So my total primary length from the back of the exhaust valve to the collector should be 34.4".

This is not an absolutely concrete number and it should be fine tuned on the track or dyno to maximize your performance. Generally, longer primary tubes lower the RPM of your power band and shorter tubes raise the RPM of your power band.

Experience has shown that primary lengths must be as close to equal as possible, preferably within about an inch of the target length. Headers with large differences in primary tube length have been proven to cause tuning problems and will not produce as much power as equal length headers. Mark Lelchook of Performance Welding told me that he has found more benefit from using larger radii bends and keeping his primary lengths within an inch or so than using tighter bends and having the lengths exactly the same.

How do you know what RPM to tune for? You need to know the power band of your engine combination and your intended usage. You will need to make some decisions such as tuning for peak power on a straightaway or power to pull you out of a slow corner. There is no such thing as a header that can do it all; some compromises are always required. The key is to understand the compromises and minimize them for your application. Some header manufacturers make adjustable length headers for fine tuning or adjusting to track conditions. These headers have a removable collector and primary tube extension pieces that can be added or removed to change the length and adjust your power band up or down.

What diameter primary tubes does your engine really need? Just as with primary tube length, there are several formulas you could use, none of which is absolutely accurate. Here is a formula for primary size that will get you close:

_________

ID = \/ cc x 2.1, where

(P+3) x 2

ID = Primary tube inside diameter, CC = Cubic centimeter displacement of one cylinder, and P = Primary tube length

__________ _____

Here's an example: ID = \/ 500 = \/ 500 x 2.1 ID = 1.53"

(34.4+3) x 25 935

NOTE: This formula didn't come out very well on this page and I can't find the source of this formula anymore so here's a link to a page that has a calculator that will get you the number you are looking for: Header calculator page

You then pick the appropriate outside diameter primary tubing with a wall thickness to give you an ID close to the calculation. Example: 18 gage 1 5/8" OD tubing has a wall thickness of .049" and an ID of 1.527", which is pretty close to our 1.53" calculation.

Ed Henneman has determined that each size primary tube will support a range of power levels and includes a chart in his header design information packet. His chart shows that a 1.5" OD primary tube will support approximately 26 to 39 horsepower per cylinder and a 1 5/8" OD primary tube will support approximately 37 to 50 horsepower per cylinder. As you can see there is some overlap in the ranges. Dyno and track testing is the only way to validate your choice in primary tube diameter. Ed's testing has shown that if you use a primary tube 2 sizes larger than necessary you will lose significant power. Smaller ID primary tubes increase power at lower RPM and larger ID primary tubes increase power at higher RPM, as long as you don't go larger than necessary. Header designers/manufacturers that truly know what works through testing will tell you to use the smallest diameter primary tubes necessary to support the horsepower level of their engine. Mark Lelchook feels that "80% of the guys out there are running the wrong size header". Other builders made similar comments.

What diameter collector outlet should you use? Testing has shown that a straight collector outlet should have a 1.4 to 1.7 in/out relationship for best all around performance. Here's an example: Four 1 5/8" primary pipes have a total area of 8 sq. in. so: 8 divided by 1.4 = 5.71" and 8 divided by 1.7 = 4.7", so the collector outlet should have an area between 4.7" and 5.7". The closest tubing sizes to these are 2.5" and 2.75". To maximize your low and midrange power band, such as on a street car or on a track that pulls your RPM down, use a smaller collector outlet. For higher RPM power you can try a slightly larger collector outlet. The rule of thumb for merged venturi collectors used on 4 cylinder engines is the outlet should be two tubing sizes larger than the primary tubes then transition up to the normal size of a straight collector. Six and most eight cylinder engines do not have a 180 degree firing order and will have adjacent cylinders firing consecutively and require different venturi openings depending on the horsepower the engine is developing. The transition should have about a 14 degree taper. Interchangeable collectors can be a great tuning tool like adjustable primary lengths. Burns Stainless offers their DynoSYS adjustable venturi merged collector to allow easier R & D as well as their B-TEC tunable exhaust collector for track use. As with primary tube diameter and length selection, collector outlet diameter is a compromise and your specific use and requirements must be taken into account. Remember, bigger is not always better. Testing on the track will confirm which collector dimensions work best for your engine combination and conditions.

Many production headers have poorly designed collectors. A short collector with steep angle to the outlet will restrict flow and throw away power. The collector taper should be at least 4" to 5" to smooth the transition to the collector extension or tail pipe. The inside of the collector should be smooth to decrease turbulence that can reduce exhaust flow.

How long should your tail pipe/collector extension be? Trial and error is the only way to be sure you are getting the most from your combination. This makes a good case for interchangeable collectors and extensions.

Some of us must run mufflers at the track; sometimes the header to muffler length is not changeable. Straight-through glasspack type mufflers are seen by the exhaust gasses as pipe when it comes to exhaust tuning and may be used to decrease noise without altering your collector/extension tuning. Be sure the glasspack core is at least the same diameter as the inlet and outlet and the core is smooth and does not use jagged louvers. There are 2.5" inlet/outlet glasspack mufflers on the market with a 1 7/8" core tube! The modern "race" glasspack mufflers from vendors like Summit Racing or DynoMax have large, smooth core tubes. Burns Stainless offers their Ultra-Light race mufflers in a multitude of sizes to suit almost any race engine and can custom make mufflers to suit your needs if necessary. A box at the end of the collector/extension with a volume of at least 8 times the volume of one cylinder (12 to 15 times that volume is better) has been proven to simulate the exhaust gasses reaching the atmosphere. This causes anything after the chamber to have no effect on exhaust tuning as long as the pipes/mufflers can handle the volume of gasses the engine generates. These chambers can be hard to package in some cars but will allow a quiet exhaust system that has little or no effect on hp.

Ed Henneman, Jere Stahl, Mark Lelchook and Vince Roman, all said the most common mistake people make when it comes to headers is installing a header that is too large for their engine configuration and rpm requirements.

Very few racers experiment with header dimensions to optimize their engine's performance, it takes time and can be expensive but the improvements can be much larger than most of us realize. An exhaust header designed, tested and fine tuned specifically for YOUR engine combination and driving needs can give you an edge over your competition that just bolted on a production header and hoped for the best. Hope doesn't get you into the winners circle! On the street a custom header will give you the extra torque and horsepower to make your driving experience that much more enjoyable.

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