When are 2" headers necessary?

[Inferno LSX]

Originally Posted by blazzin1

Actually.....Big Flow Exhaust says on their website they have 2-1/8" headers "coming soon". However, it has said that for a long time now. Not saying they're better or anything, but yes, they may be available soon.

Sweet! SOLD!

[FTI-EDC]

Burns Technology

Before we delve into the dark art of exhaust theory, let's take a quick journey through the exhaust system from the perspective of the exhaust gases.

As the piston approaches top dead center, the spark plug fires igniting a fireball just as the piston rocks over into the power stroke. The piston transfers the energy of the expanding gases to the crankshaft as the exhaust valve starts to open in the last part of the power stroke. The gas pressure is still high (70 to 90 p.s.i.) causing a rapid escape of the gases (blowdown). A pressure wave is generated as the valve continues to open. Gases can flow at an average speed of over 350 ft/sec, but the pressure wave travels at the speed of sound (and is dependent on gas temperature). Expanding exhaust gases rush into the port and down the primary header pipe. At the end of the pipe, the gases and waves converge at the collector. In the collector, the gases expand quickly as the waves propagate into all of the available orifices including the other primary tubes. The gases and some of the wave energy flow into the collector outlet and out the tail pipe.

Based on the above visualization, two basic phenomenon are at work in the exhaust system: gas particle movement and pressure wave activity. The absolute pressure differential between the cylinder and the atmosphere determines gas particle speed. As the gases travel down the pipe and expand, the speed decreases. The pressure waves, on the other hand, base their speed on the speed of sound. While the wave speed also decreases as they travel down the pipe due to gas cooling, the speed will increase again as the wave is reflected back up the pipe towards the cylinder. At all times, the speed of the wave action is much greater than the speed of the gas particles. Waves behave much differently than gas particles when a junction is encountered in the pipe. When two or more pipes come together, as in a collector for example, the waves travel into all of the available pipes - backwards as well as forwards. Waves are also reflected back up the original pipe, but with a negative pressure. The strength of the wave reflection is based on the area change compared to the area of the originating pipe.

This reflecting, negative pulse energy is the basis of wave action tuning. The basic idea is to time the negative wave pulse reflection to coincide with the period of overlap - this low pressure helps to pull in a fresh intake charge as the intake valve is opening and helps to remove the residual exhaust gases before the exhaust valve closes. Typically this phenomenon is controlled by the length of the primary header pipe. Due to the 'critical timing' aspect of this tuning technique, there may be parts of the power curve where more harm than good is done.

Gas speed is a double edged sword as well, too much gas speed indicates that that the system may be too restrictive hurting top end power, while too little gas speed tends to make the power curve excessively 'peaky' hurting low end torque. Larger diameter tubes allow the gases to expand; this cools the gases, slowing down both the gases and the waves.

Exhaust system design is a balancing act between all of these complex events and their timing. Even with the best compromise of exhaust pipe diameter and length, the collector outlet sizing can make or break the best design. The bottom line on any exhaust system design is to create the best, most useful power curve. All theory aside, the final judgement is how the engine likes the exhaust tuning on the dyno and on the track.

Various exhaust designs have evolved over the years from theory, but the majority are still being built from 'cut & try' experimenting. Only lately have computer programs like X-design or high end engine simulation programs been able to help in this process. Practical tools like adjustable length primary pipes and our B-TEC and DynoSYS adjustable collectors allow quicker design changes on the dyno or in the car. When considering a header design, the following points need to be considered:

1) Header primary pipe diameter (also whether constant size or stepped pipes).
2) Primary pipe overall length.
3) Collector package including the number of pipes per collector and the outlet sizing.
4) Megaphone/tailpipe package.

There are many ideas about header pipe sizing. Usually the primary pipe sizing is related to exhaust valve and port size. Header pipe length is dependent on wave tuning (or lack of it). Typically, longer pipes tune for lower r.p.m. power and the shorter pipes favor high r.p.m. power. The collector package is dependent on the number of cylinders, the engine configuration (V-8, inline 6, etc.), firing order and the basic design objectives (interference or independence). The collector outlet size is determined by primary pipe size and exhaust cam timing.

For more detail on the specifics of header theory read ‘The Scientific Design of Exhaust and Intake Systems' by Phillip H. Smith’. For those that prefer quicker results, Burns Stainless makes designing a racing exhaust header easy. Our revolutionary X-design parametric exhaust modeling program provides you with the perfect starting point for any header project. Just fill out the Race Engine Specification Form, send it to us, and we will do the rest.

After a proper header design is constructed, the fine tuning can be done on the dyno with adjustable pipe sections (typically in 2" increments) and our innovative B-TEC and DynoSYS adjustable collector systems.

Disclaimer: Yes, we use Burn products on customer race cars but it's because their header philosophy works!

[andredmg]

Originally Posted by Sawyer

703 N/A??

Yes sir it is a NA motor, huge cam and heads LS3 pump gas.

[Ovenmitt]

Originally Posted by wylde1

I shold preface my post by saying I'm not an expert, and only a hobbyist. That said, I've always been of the impression that you should match your primaries as close as possible to the size of the exhaust ports on your cylinder head. Pressure and velocity are inversely proportional, so evacuating into a larger pipe increases the pressure and decreases the gas velocity. The thing is though, sometimes the benefits of being able to extract more volume outweighs the benefits of higher velocity. So only going 1/8" diameter larger than your exhaust ports may help more than hinder, but going 1/4" may be the point where the higher pressure becomes counter productive since the stock motor might not be as efficient at overcoming it.

With high hp numbers and/or forced induction you are running higher cylinder pressures, which helps to overcome the expansion in the headers/exhaust, so you see better returns with bigger primaries even though you may not change the exhaust port size.

IMHO the primary is for extracting exhaust at the highest possible/most efficient velocity, and then the rest of your exhaust is for extracting volume.

The stock exhaust port is BARELY 1 5/8.

[GQ4Life]

I checked into all this when doing my car. It is about a 1.1% increase in power over the 1 7/8"
It worked out perfect when doing my car made 6 more HP then exact same car with 1 7/8"
It is just 2" usually cost more and at some point the same increase just might not be worth the extra money to most.
Also bigger then 2" runner would be extremely hard to make and fit. But yeah there is a point where there is too big for the HP. And the lack of back pressure will be noticable at like 2000rpms

[NicKey]

NicKey Performance tested on the Wegner NASCAR certified engine dyno with both the LS3 and LS7. We use 2" on most all LS7 builds and 2" on most LS3 / LSA FI builds and 1 7/8ths on L99/LS3 N/A builds up to 600hp.

Also, your exhaust system after the header collector can have an impact on the overall power results as well.

[Moto-Mojo]

Originally Posted by FTI-EDC

Burns Technology

Before we delve into the dark art of exhaust theory, let's take a quick journey through the exhaust system from the perspective of the exhaust gases.

As the piston approaches top dead center, the spark plug fires igniting a fireball just as the piston rocks over into the power stroke. The piston transfers the energy of the expanding gases to the crankshaft as the exhaust valve starts to open in the last part of the power stroke. The gas pressure is still high (70 to 90 p.s.i.) causing a rapid escape of the gases (blowdown). A pressure wave is generated as the valve continues to open. Gases can flow at an average speed of over 350 ft/sec, but the pressure wave travels at the speed of sound (and is dependent on gas temperature). Expanding exhaust gases rush into the port and down the primary header pipe. At the end of the pipe, the gases and waves converge at the collector. In the collector, the gases expand quickly as the waves propagate into all of the available orifices including the other primary tubes. The gases and some of the wave energy flow into the collector outlet and out the tail pipe.

Based on the above visualization, two basic phenomenon are at work in the exhaust system: gas particle movement and pressure wave activity. The absolute pressure differential between the cylinder and the atmosphere determines gas particle speed. As the gases travel down the pipe and expand, the speed decreases. The pressure waves, on the other hand, base their speed on the speed of sound. While the wave speed also decreases as they travel down the pipe due to gas cooling, the speed will increase again as the wave is reflected back up the pipe towards the cylinder. At all times, the speed of the wave action is much greater than the speed of the gas particles. Waves behave much differently than gas particles when a junction is encountered in the pipe. When two or more pipes come together, as in a collector for example, the waves travel into all of the available pipes - backwards as well as forwards. Waves are also reflected back up the original pipe, but with a negative pressure. The strength of the wave reflection is based on the area change compared to the area of the originating pipe.

This reflecting, negative pulse energy is the basis of wave action tuning. The basic idea is to time the negative wave pulse reflection to coincide with the period of overlap - this low pressure helps to pull in a fresh intake charge as the intake valve is opening and helps to remove the residual exhaust gases before the exhaust valve closes. Typically this phenomenon is controlled by the length of the primary header pipe. Due to the 'critical timing' aspect of this tuning technique, there may be parts of the power curve where more harm than good is done.

Gas speed is a double edged sword as well, too much gas speed indicates that that the system may be too restrictive hurting top end power, while too little gas speed tends to make the power curve excessively 'peaky' hurting low end torque. Larger diameter tubes allow the gases to expand; this cools the gases, slowing down both the gases and the waves.

Exhaust system design is a balancing act between all of these complex events and their timing. Even with the best compromise of exhaust pipe diameter and length, the collector outlet sizing can make or break the best design. The bottom line on any exhaust system design is to create the best, most useful power curve. All theory aside, the final judgement is how the engine likes the exhaust tuning on the dyno and on the track.

Various exhaust designs have evolved over the years from theory, but the majority are still being built from 'cut & try' experimenting. Only lately have computer programs like X-design or high end engine simulation programs been able to help in this process. Practical tools like adjustable length primary pipes and our B-TEC and DynoSYS adjustable collectors allow quicker design changes on the dyno or in the car. When considering a header design, the following points need to be considered:

1) Header primary pipe diameter (also whether constant size or stepped pipes).
2) Primary pipe overall length.
3) Collector package including the number of pipes per collector and the outlet sizing.
4) Megaphone/tailpipe package.

There are many ideas about header pipe sizing. Usually the primary pipe sizing is related to exhaust valve and port size. Header pipe length is dependent on wave tuning (or lack of it). Typically, longer pipes tune for lower r.p.m. power and the shorter pipes favor high r.p.m. power. The collector package is dependent on the number of cylinders, the engine configuration (V-8, inline 6, etc.), firing order and the basic design objectives (interference or independence). The collector outlet size is determined by primary pipe size and exhaust cam timing.

For more detail on the specifics of header theory read ‘The Scientific Design of Exhaust and Intake Systems' by Phillip H. Smith’. For those that prefer quicker results, Burns Stainless makes designing a racing exhaust header easy. Our revolutionary X-design parametric exhaust modeling program provides you with the perfect starting point for any header project. Just fill out the Race Engine Specification Form, send it to us, and we will do the rest.

After a proper header design is constructed, the fine tuning can be done on the dyno with adjustable pipe sections (typically in 2" increments) and our innovative B-TEC and DynoSYS adjustable collector systems.

Disclaimer: Yes, we use Burn products on customer race cars but it's because their header philosophy works!

That's one comprehensive explanation of the science behind exhaust flow and what takes place "in the tubes." You sound like you know what you're talking about.

[Pro Stock John]

^Ed C is sharp, knows a lot about engines, performance and racing.

[Unreal]

For reference, I made 1090 on 1 7/8" and 1116 on 2" with no other changes, but did lose low end below 4k. Gave up a solid 20+ftlbs until over 4k.