Technical ramblings..exhaust scavenging, LSA, lift, duration, X pipes(LONG)
02-14-2003, 12:14 PM
#1
Le Mans Master
Thread Starter
Technical ramblings..exhaust scavenging, LSA, lift, duration, X pipes(LONG)
I posted a topic a couple days ago about adding an X pipe to your exhaust to help exhaust scavenging and got a couple of good replys but just need more information. I went out and found a book which covers alot of the technical end of building horsepower and here is what I found.....If I am wrong please help me see clearly. :)
What is exhaust scavenging?(vacuum behind exhaust pulses)
Exhaust scavenging is the ability of the exhaust to create high velocity situtation where while overlap is ocurring the combustion chamber can be completely emptied and actually filled with fresh air from the intake. This can only happen while the piston is at or near TDC and the intake and exhaust valves are open. How does this help? Lets look at 2 models:
Engine 1: No exhaust scavenging
Good flowing exhaust clears out the combustion chamber but doesnt create a vacuum behind the exhaust pulse. When the exhaust valve closes the combustion chamber is basically empty as the piston draws down in the bore it draws air and gas in for the combustion phase. The air in the bore is equal to how much air was pulled through the intake side.
Engine 2: Good exhaust scavenging
Great flowing headers clears out the combustion chamber and creates a vacuum behind the exhaust pulse. This vacuum pulls fresh air from the intake valve into the combustion chamber and out the exhaust port (while overlap is occuring). As the piston hits TDC and starts to fall the exhaust valve closes and intake valve opens BUT the combustion chamber is already full of fresh air and as the piston draws down in the bore it pulls more air into the bore. So the air in the bore is equal to the air drawn in by the piston + the size of the combustion chamber in the cylinder head. The engine can actually uses more air(thinks it bigger) because when the piston draws down in the bore it already has 60cc's worth of air from the exhaust scavenging.
Why will an X pipe(tangentially siamsed junction) help right after the headers?
Adding an X pipe after the headers will help synchronize the exhaust pulses from both sides of the motor which reduces back pressure, cancels sound, and provides additional scavaging.
Choosing a cam:
The 2 most important characterics of the cam are the Lobe Seperation Angle or Lobe centerline angle and Lift when determining power.
Lift:
First choose the lift of the cam, the lift of the cam should be equal to the flow data of the cylinder head. If your heads stop flowing at .500 lift no sense in having a cam which has .600 lift and vice-versa. If your heads flow the best at a .600 lift than shoot for a cam with .600 or slightly more.
Lobe seperation angle:
Lobe seperation angle is the centerline angle between the 2 lobes. This measurement will basically determine overlap degrees(overlap is the amount in degrees that the intake valve AND exhaust valve are open). Generally the choices for LSA or LCA are:
110: 7-10 degrees of overlap per cylinder depending on intake and exhaust duration
112: 3-6 degrees of overlap per cylinder depending on intake and exhaust duration
114: (-1)-2 degrees of overlap per cylinder depending on intake and exhaust duration
Basically a smaller LSA gives you rougher idle but with more power where a wider LSA will be more driveable with slightly less power.
Duration:
Duration should be chosen depending on RPM range and cylinder head flow data. If your shooting to shift your car at 5900 than I would stick to something between 215 and 222 on the duration. More duration moves the power up in the rpm band. If your cylinder head has a exhaust port efficiency that is less than 70% of the intake port than it might be a good idea to add a couple of degrees in duration to the exhaust lobe.(220 intake-226 exhaust). If your exhaust port flows 70-75% of the intake port and you have an excellent flowing exhaust system than you might consider more duration on the intake lobe. (230-224) I'll probably ramble later, post any questions or corrections.
Phillip :leaving:
[Modified by Phil97SVT, 5:20 PM 2/14/2003]
[Modified by Phil97SVT, 5:21 PM 2/14/2003]
What is exhaust scavenging?(vacuum behind exhaust pulses)
Exhaust scavenging is the ability of the exhaust to create high velocity situtation where while overlap is ocurring the combustion chamber can be completely emptied and actually filled with fresh air from the intake. This can only happen while the piston is at or near TDC and the intake and exhaust valves are open. How does this help? Lets look at 2 models:
Engine 1: No exhaust scavenging
Good flowing exhaust clears out the combustion chamber but doesnt create a vacuum behind the exhaust pulse. When the exhaust valve closes the combustion chamber is basically empty as the piston draws down in the bore it draws air and gas in for the combustion phase. The air in the bore is equal to how much air was pulled through the intake side.
Engine 2: Good exhaust scavenging
Great flowing headers clears out the combustion chamber and creates a vacuum behind the exhaust pulse. This vacuum pulls fresh air from the intake valve into the combustion chamber and out the exhaust port (while overlap is occuring). As the piston hits TDC and starts to fall the exhaust valve closes and intake valve opens BUT the combustion chamber is already full of fresh air and as the piston draws down in the bore it pulls more air into the bore. So the air in the bore is equal to the air drawn in by the piston + the size of the combustion chamber in the cylinder head. The engine can actually uses more air(thinks it bigger) because when the piston draws down in the bore it already has 60cc's worth of air from the exhaust scavenging.
Why will an X pipe(tangentially siamsed junction) help right after the headers?
Adding an X pipe after the headers will help synchronize the exhaust pulses from both sides of the motor which reduces back pressure, cancels sound, and provides additional scavaging.
Choosing a cam:
The 2 most important characterics of the cam are the Lobe Seperation Angle or Lobe centerline angle and Lift when determining power.
Lift:
First choose the lift of the cam, the lift of the cam should be equal to the flow data of the cylinder head. If your heads stop flowing at .500 lift no sense in having a cam which has .600 lift and vice-versa. If your heads flow the best at a .600 lift than shoot for a cam with .600 or slightly more.
Lobe seperation angle:
Lobe seperation angle is the centerline angle between the 2 lobes. This measurement will basically determine overlap degrees(overlap is the amount in degrees that the intake valve AND exhaust valve are open). Generally the choices for LSA or LCA are:
110: 7-10 degrees of overlap per cylinder depending on intake and exhaust duration
112: 3-6 degrees of overlap per cylinder depending on intake and exhaust duration
114: (-1)-2 degrees of overlap per cylinder depending on intake and exhaust duration
Basically a smaller LSA gives you rougher idle but with more power where a wider LSA will be more driveable with slightly less power.
Duration:
Duration should be chosen depending on RPM range and cylinder head flow data. If your shooting to shift your car at 5900 than I would stick to something between 215 and 222 on the duration. More duration moves the power up in the rpm band. If your cylinder head has a exhaust port efficiency that is less than 70% of the intake port than it might be a good idea to add a couple of degrees in duration to the exhaust lobe.(220 intake-226 exhaust). If your exhaust port flows 70-75% of the intake port and you have an excellent flowing exhaust system than you might consider more duration on the intake lobe. (230-224) I'll probably ramble later, post any questions or corrections.
Phillip :leaving:
[Modified by Phil97SVT, 5:20 PM 2/14/2003]
[Modified by Phil97SVT, 5:21 PM 2/14/2003]
02-14-2003, 01:05 PM
#3
Safety Car
Re: Technical ramblings..exhaust scavenging, LSA, lift, duration, X pipes(LONG) (jawski)
nice summary, thanks
02-14-2003, 01:07 PM
#4
Melting Slicks
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Re: Technical ramblings..exhaust scavenging, LSA, lift, duration, X pipes(LONG) (jawski)
Expanding on that. Here is a bit of information on header theory which ties into this. This is why header from LGM and Wheel to Wheel are better than some of the shorter long tubes out there. GM tried many lengths of header out there, and found that 32" was the best length for primaries. As stated, you can see why your entire system (intake, camshaft, heads, and exhaust ) need to be a matched system for best performance...
http://www.burnsstainless.com/TechAr...ry/theory.html
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.
For one of the best studies you’ll ever see on tuned exhaust system phenomena, with extremely informative gas pressure graphs from measurements on a Lycoming 4 cylinder engine, check out:
Aircraft Exhaust Systems IV by Brien A. Seeley and Ed Vetter.
You need the Adobe Acrobat reader to view it.
http://www.cafefoundation.org/research.htm
[Modified by J-Rod, 1:08 PM 2/14/2003]
[Modified by J-Rod, 1:09 PM 2/14/2003]
http://www.burnsstainless.com/TechAr...ry/theory.html
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.
For one of the best studies you’ll ever see on tuned exhaust system phenomena, with extremely informative gas pressure graphs from measurements on a Lycoming 4 cylinder engine, check out:
Aircraft Exhaust Systems IV by Brien A. Seeley and Ed Vetter.
You need the Adobe Acrobat reader to view it.
http://www.cafefoundation.org/research.htm
[Modified by J-Rod, 1:08 PM 2/14/2003]
[Modified by J-Rod, 1:09 PM 2/14/2003]
02-14-2003, 01:53 PM
#5
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St. Jude Donor '05
Re: Technical ramblings..exhaust scavenging, LSA, lift, duration, X pipes(LONG) (Phil97SVT)
So the short-short version:
Header diameter: smaller= low-end power, bigger= HP
Header length: longer=low-end power, shorter= HP
LSA: smaller= more power, larger= better idle
Yes?
Header diameter: smaller= low-end power, bigger= HP
Header length: longer=low-end power, shorter= HP
LSA: smaller= more power, larger= better idle
Yes?
02-14-2003, 07:39 PM
#6
Melting Slicks
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Re: Technical ramblings..exhaust scavenging, LSA, lift, duration, X pipes(LONG) (Phil97SVT)
Why will an X pipe(tangentially siamsed junction) help right after the headers?
Adding an X pipe after the headers will help synchronize the exhaust pulses from both sides of the motor which reduces back pressure, cancels sound, and provides additional scavaging.
Adding an X pipe after the headers will help synchronize the exhaust pulses from both sides of the motor which reduces back pressure, cancels sound, and provides additional scavaging.
I think this is why people used to put H pipes on the exhaust in front of the cats. Which goes along with your thoughts.
Does anyone know what's really going on?
:cheers:
02-14-2003, 10:45 PM
#7
Le Mans Master
Thread Starter
Re: Technical ramblings..exhaust scavenging, LSA, lift, duration, X pipes(LONG) (MMarquez)
Calculation for Exhaust pipe length in inches:
X = (Cam duration @.006 / 360) x (277,500 / peak RPM)
Diameter of pipe:
x = sqrt( (cubic inch x 1900) / (x * peak rpm) )
1.75 and 32/33 are right on the money
Phillip
X = (Cam duration @.006 / 360) x (277,500 / peak RPM)
Diameter of pipe:
x = sqrt( (cubic inch x 1900) / (x * peak rpm) )
1.75 and 32/33 are right on the money
Phillip
02-14-2003, 11:15 PM
#8
Safety Car
Re: Technical ramblings..exhaust scavenging, LSA, lift, duration, X pipes(LONG) (Phil97SVT)
This is all good stuff that requires calculations based on real world head specs and rpm ranges. I, like others have been making decisions based on incomplete assumptions and specsmanship of suppliers.
Thanks, now where is my slide rule?
[Modified by see5, 10:16 PM 2/14/2003]
Thanks, now where is my slide rule?
[Modified by see5, 10:16 PM 2/14/2003]
02-15-2003, 11:16 AM
#9
Le Mans Master
Thread Starter
Re: Technical ramblings..exhaust scavenging, LSA, lift, duration, X pipes(LONG) (see5)
This is all good stuff that requires calculations based on real world head specs and rpm ranges. I, like others have been making decisions based on incomplete assumptions and specsmanship of suppliers.
Thanks, now where is my slide rule?
[Modified by see5, 10:16 PM 2/14/2003]
Thanks, now where is my slide rule?
[Modified by see5, 10:16 PM 2/14/2003]
Here is how to calculate overlap:
x = overlap in degrees
x = ((((exhaust + intake duration @ .006) / 4) - LSA) * 2)
daily driven low rpm performance 30-55degs overlap
hot street performance 50-75 degs overlap
bracket/oval track racing 70-95degs overlap
dragster/comp eliminator engines 90-115 degs overlap
Phillip
[Modified by Phil97SVT, 4:17 PM 2/15/2003]
