Id like someone to explain (and in some cases verify or clarify) the following issues

Interference vs. independence
Is one where the exhuast pulses from each primary help each other and interference where all the pulses/vacuum on one bank work co-operativly? I'm kinda confused here.


Primary length vs. pressure
...Longer primaries create longer periods and higher peaks of the waves. Shorter primaries do the exact opposite... but what does this all mean and how does it effect anything???
Is shorter better for a turbo due to smaller constant pulses?

Please explain "critical length"- I’m lost here

Also, what are the advantages of
Siamese ports, an interference branch and 4-2-1 headers?

 

The Exhaust Pulse
To gain a more complete understanding of how mufflers and headers do their job, we must be familiar with the dynamics of the exhaust pulse itself. Exhaust gas does not come out of the engine in one continuous stream. Since exhaust valves open and close, exhaust gas will flow, then stop, and then flow again as the exhaust valve opens. The more cylinders you have, the closer together these pulses run.

Keep in mind that for a "pulse" to move, the leading edge must be of a higher pressure than the surrounding atmosphere. The "body" of a pulse is very close to ambient pressure, and the tail end of the pulse is lower than ambient. It is so low, in fact, that it is almost a complete vacuum! The pressure differential is what keeps a pulse moving. A good Mr. Wizard experiment to illustrate this is a coffee can with the metal ends cut out and replaced with the plastic lids. Cut a hole in one of the lids, point it toward a lit candle and thump on the other plastic lid. What happens? The candle flame jumps, then blows out! The "jump" is caused by the high-pressure bow of the pulse we just created, and the candle goes out because the trailing portion of the pulse doesn't have enough oxygen-containing air to support combustion. Neat, huh?

Ok, now that we know that exhaust gas is actually a series of pulses, we can use this knowledge to propagate the forward-motion to the tailpipe. How? Ah, more of the engineering tricks we are so fond of come in to play here.

Just as Paula Abdul will tell you that opposites attract, the low pressure tail end of an exhaust pulse will most definitely attract the high-pressure bow of the following pulse, effectively "sucking" it along. This is what's so cool about a header. The runners on a header are specifically tuned to allow our exhaust pulses to "line up" and "suck" each other along! Whoa, bet you didn't know that! This brings up a few more issues, since engines rev at various speeds, the exhaust pulses don't always exactly line up. Thus, the reason for the Try-Y header, a 4-into-1 header, etc. Most Honda headers are tuned to make the most horsepower in high RPM ranges; usually 4,500 to 6,500 RPM. A good 4-into-1 header, such as the ones sold by Gude, are optimal for that high winding horsepower you've always dreamed of. What are exhaust manifolds and stock exhaust systems good for? Besides a really cheap boat anchor? If you think about it, you'll realize that since stock exhausts are so good at restricting that they'll actually ram the exhaust pulses together and actually make pretty darn good low-end torque! Something to keep in mind, though, is that even though an OEM exhaust may make gobs of low-end torque, they are not the most efficient setup overall, since your engine has to work so hard to expel those exhaust gasses. Also, a header does a pretty good job of additionally "sucking" more exhaust from your combustion chamber, so on the next intake stroke there's lots more fresh air to burn. Think of it this way: At 8,000 RPM, your Integra GS-R is making 280 pulses per second. There's a lot more to be gained by minimizing pumping losses as this busy time than optimizing torque production during the slow season.

General Rules of Thumb with Headers
You will undoubtedly see a variety of headers at your local speed shop. While you won't be able to determine the optimal power range of the headers by eyeballing them, you'll find that in general, the best high-revving horsepower can be had with headers utilizing larger diameter, shorter primary tubes. Headers with smaller, longer primaries will get you
slightly better fuel economy and better street driveability. With four cylinder engines, these are also usually of the Tri-Y design, such as the DC Sports and Lightspeed headers.

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.

heres >>http://naca.larc.nasa.gov/reports/1949/naca-tn-1446/

 

then theres this>>
Exhaust Cycle Part One


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Last months article described the basic internal workings of the rotary engine. The next several articles will break this down into separate cycles, and descirbe them in detail. I will begin with the exhaust cycle because it has the greatest effect on the power output of the engine. If the engine cannot exhaust itself completely, further modifications will result in very little improvement. This is true of naturally aspirated, and turbocharged engines. This first article will explain a few basic terms and concepts. Next months article will present some more new information, and then describe how all of this comes together to affect the complete exhaust cycle.

When attempting to increase the power output of the rotary engine, there are three basic aspects that can be improved upon. Volumetric efficiency, combustion efficiency, and reduction of pumping losses. As most of you know, the rotary engine has four separate cycles. Intake, compression, expansion, and exhaust. Of the four, only the expansion cycle contributes to the power output of the engine by exerting force on the output shaft. The other three cycles actually reduce horsepower by resisting the rotating force. This reduction in power is referred to as pumping loss. Pumping losses occur in both the intake and exhaust cycles. This article, and the next will deal with the importance of reducing pumping losses during the exhaust cycle.


Blowdown Period


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Early internal combustion engines opened the exhaust valve at BDC of the expansion cycle. This required the piston to pump, or physically force the exhaust gasses from the cylinder during the period from BDC to TDC. The force required to pump the gasses from the cylinder considerably reduced the power output of the engine. As performance, and rpm requirements increased, it was discovered that by opening the exhaust valve before BDC the residual combustion pressure could be used to help evacuate the cylinder at the beginning of the exhaust cycle. This is referred to as the blowdown period, and is responsible for approximately half of the exhaust flow. In theory, this will reduce thermal efficiency by releasing pressure that is still applying force to the crankshaft. In practice however it was determined that the reduction in pumping losses far outweighed the loss of pressure at the end of the expansion cycle. Since most of the useful work is done in the first third of the expansion cycle, the pressure loss caused by early exhaust valve opening is minimal. This also applies to the rotary engine. Referring to last months article you can see that the exhaust port of a stock engine opens approximately 75 degrees before BDC.


Pressure Wave Tuning


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Pressure wave phenomena is probably the least understood aspect of exhaust tuning. Right now I am thinking that it is also the hardest to explain! Entire books have been written on this subject, but I will try to boil it down to a few paragraphs.

Any time there is a pressure change in an elastic meduim (like air for instance) a series of resonances or vibrations will occur. Any time you hear a sound, it is the result of a pressure disturbance in the air. For instance, if someone across the room claps their hands together, the air pressure between their hands will increase. This rise in pressure will be transferred from one group of molecules to the next (at the speed of sound of course) until it finally reaches your ear. While this energy transfer is invisible, you can easily picture it by dropping a stone into an undisturbed pool of water. Pressure waves radiate outward from the center of the disturbance. This same thing happens in the exhaust system, but because of the higher pressures involved it is more like an elephant doing a belly flop in your swimming pool.

The main difference between the swimming pool analogy, and the exhaust system is that the pressure waves cannot travel outward in all directions from the source of the pressure disturbance, beacause they are enclosed by the tubing itself. In the case of the exhaust system, the initial pressure wave, or pulse caused by the exhaust port opening will travel towards the open end of the tube.

So far I have only referred to pressure waves as being positive, or caused by an increase in pressure. In fact, pressure waves can be negative, or caused by a decrease in pressure. Picture a wave in the ocean with the highest point of the wave being positive, or above sea level, and the trough between two waves being negative, or below sea level. This is analogous to the pressure waves in the exhaust system. These waves can also be referred to as high pressure, and low pressure.

These pressure waves can be used to our advantage because they have the effect of moving gas particles along with them. A positive, or high pressure wave will propel gasses in the same direction that it is travelling. A negative, or low pressure wave will propel gasses in the opposite direction that it is travelling. Take a moment to let this sink in, because this simple fact is at the heart of exhaust system tuning. Although the pressure wave is moving at the speed of sound, it will propel the gasses at a much slower speed. An example of this is a boat that catches a wave from another boat that is motoring by. As the wave passes it will propel the boat in the same direction the wave is travelling, but at a much slower speed, and the wave will eventually pass the boat completely. This is the same thing that happens to the gas molecules in the exhaust system as a pressure wave passes through them.

These pressure waves respond in an interesting manner when they reach a sudden area change in the pipe. An example of a sudden area change is the collector, where the two pipes empty into a larger diameter pipe, a megaphone, or the end of the exhaust where the pipe empties into the atmosphere. When a pressure wave reaches a larger cross sectional area, it will reverse its sign (positive becomes negative, and negative becomes positive) and its direction. For instance, when the exhaust port first opens, a strong positive wave will travel to the end of the pipe, change to a negative wave, and travel back to the exhaust port. This is called a reflection. Both the positive wave travelling towards the end of the pipe, and the negative wave travelling towards the exhaust port will propel exhaust gasses towards the end of the exhaust system which is exactly where we want them to go. The amount of time that this cycle takes is dependant on the total distance that the wave has to travel.

By changing the length of the header pipes, you can time the cycle so that the negative return wave arrives at the exhaust port at the end of the exhaust cycle where it is most beneficial. Assuming that the negative return wave is timed correctly for a given engine at 6000 rpm, lengthening the headers will further delay the return wave so that it is timed appropriately for a lower rpm, and shortening the headers will time the return wave so that it is timed appropriately for a higher rpm. The key to header length tuning is simply timing the low pressure return wave to give the greatest benefit for a given rpm.

This is a VERY basic description of pressure waves, and how they affect the exhaust system of an internal combustion engine. For a more detailed analysis, I would suggest researching two stroke exhaust system design. There is a great deal of information in print, and much of it can be found at public, or university libraries.


Velocity


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Velocity refers to the speed at which the exhaust gasses are travelling. The exact speed is not important to this discussion, but an uderstanding of how velocity affects exhaust flow is. There are two ways that velocity can be increased. One, by decreasing the cross sectional area of the orifice that the gasses are flowing through. (Making the headers or exhaust ports smaller) Two, by increasing the volume of air that is flowing through the orifice. (Increasing engine rpm) Velocity will increase proportionally with an increase in rpm. In other words, if you double the rpm, the velocity will also double. Velocity is inversely proportional to an increase in cross sectional area. Doubling the cross sectional are will halve the velocity, and halving the cross sectional area will double the velocity.

Velocity is important for one simple reason. Inertia. Websters dictionary describes inertia as "The property of matter by which it retains its state of rest or velocity so long as it is not acted upon by an external force." In other words, once it is moving, it will continue to move until some external force stops it. If you apply this theory to the gasses in the exhaust system you can see that once they have been accelerated by the pressure in the combustion chamber, It will take a given amount of energy to stop them, and even more to cause them to reverse direction. Since energy equals mass times velocity squared, you can see that doubling the velocity of the gasses will quadruple the amount of energy required to stop them. This is important because the flow of exhaust gasses is not steady. During each exhaust cycle, the gasses are accelerated, and decellerated rapidly. Often in the forward and reverse direction.



gotta love goggle serches

 

thanks ! now my brain is now thinking about exhaust pulses all day instead of naked women.

 

You should check out "Scientific Design of Exhaust & Intake Systems". Even thought this book is very old, sometimes in the 70s, it is still very good for understanding the way things work. It is very technical, so it may take a few times of reading to understand it, but I highly recommend it.

 

It sounds like you got a blurb of "Scientific design of Intake and Exhaust Syetems" by Philip H. Smith and John C . Morrison, as HR Hawk mentioned in his cut 'n paste. mastdrver read it too!

If you are really interested, you should buy the book (Third Edition). It's published by Bentley Publishers and may be at Barnes & Noble. About $23 in soft cover. Old Brits are fun to read. It was new in '62 when I was in school and first read it.

To paraphrase (I didn't read all that HRH posted from the RX-7 website...sorry if I'm repeating.):

Independence tuning: think 4 into 1 headers

Interference tuning; think 4-2-1 (Tri-Y) headers.

That's perhaps the opposite of what you said...maybe.

Siamese cylinders (2 exhausts emptying into one port) are bad, but have been done on old Brit 4 bangers and some early V8's (think Ford Flathead) for convenience, at the expense of power. Buggers up tuning, but there are ways around it.

4-2-1's are about all the is used on Nextel(?) Cup and ProStock engines...the guys at the top of the food chain. Cup guys have been using them for 5-6 years. 4-2-1 isn't new, it's about the first and oldest tuned exhaust for 4 or 8 cylinder engines. Smith goes into great detail.

Primary pipe length mainly determines rpm where best tuning occurs. Short for high rpm, long for lower rpm. Think organ pipe and pitch (or frequency).

"Critical length". Hmmm. Not sure here. Even Cup headers don't have equal length primary pipes due to packaging problems. I believe they do lots of tuning with the secondary pipes ( the '2' in 4-2-1 headers). If there is a critical length it might be there. Drag race cars usually aren't so space constricted so primaries can be closer to equal. Length depends on engine rpm.

You didn't ask about primary pipe diameter and torque peak rpm...but that's another subject.

All this tuning is about non-turbo engines. For turbos you need to get the gasses into the turbo with as much energy as possible, IMO. That means short, direct, low restriction plumbing which holds in the heat. Think thick wall, heat resistant tubing. Mark Stielow, in his most recent engine, a twin turbo SBC, I think, used schedule 40 stainless pipe fittings. They are about 1/8 inch wall thickness. FWIW, a Cup engine probably has about .050-.035 wall thickness headers, depending on how exotic the material is. Weight is the critical factor. Pounds (and ounces) count in that business.


My $.02