Exhaust Questions...
07-30-2002, 11:48 AM
#51
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http://www.hondavision.com/showthrea...&threadid=6283
http://www.team-integra.net/sections...p?ArticleID=47
also if your a member at team integra heres an article, dont wanna spam but in the interest of imports and great articles and to help with those post this is the best i can do without writing for a couple hours on this topic.
http://www.team-integra.net/sections...p?ArticleID=47
also if your a member at team integra heres an article, dont wanna spam but in the interest of imports and great articles and to help with those post this is the best i can do without writing for a couple hours on this topic.
07-30-2002, 11:50 AM
#52
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Originally posted by knightridergs-r
http://www.hondavision.com/showthrea...&threadid=6283
http://www.hondavision.com/showthrea...&threadid=6283
07-30-2002, 12:46 PM
#53
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reno - its like improved cyllinder EMPTYING. it increses the speed of the exhaust gasses, forcing more out when the valve is closing.
and I resent the idea that you called me a ricer, and the fact that you dont know what you are talking about just makes it downright funny.
and I resent the idea that you called me a ricer, and the fact that you dont know what you are talking about just makes it downright funny.
07-30-2002, 01:01 PM
#54
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Originally posted by sinfestboy
and the fact that you dont know what you are talking about just makes it downright funny.
and the fact that you dont know what you are talking about just makes it downright funny.
aren't you the that so vehemently said that back pressure helps with nothing AND that it only hurts?
son, i do believe you've been
wned:.
07-30-2002, 01:56 PM
#55
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wow, so much misconception
Backpressure: The myth and why it's wrong.
I. Introduction
One of the most misunderstood concepts in exhaust theory is backpressure. People love to talk about backpressure on message boards with no real understanding of what it is and what it's consequences are. I'm sure many of you have heard or read the phrase "Hondas need backpressure" when discussing exhaust upgrades. That phrase is in fact completely inaccurate and a wholly misguided notion.
II. Some basic exhaust theory
Your exhaust system is designed to evacuate gases from the combustion chamber quickly and efficently. Exhaust gases are not produced in a smooth stream; exhaust gases originate in pulses. A 4 cylinder motor will have 4 distinct pulses per complete engine cycle, a 6 cylinder has 6 pules and so on. The more pulses that are produced, the more continuous the exhaust flow. Backpressure can be loosely defined as the resistance to positive flow - in this case, the resistance to positive flow of the exhaust stream.
III. Backpressure and velocity
Some people operate under the misguided notion that wider pipes are more effective at clearing the combustion chamber than narrower pipes. It's not hard to see how this misconception is appealing - wider pipes have the capability to flow more than narrower pipes. So if they have the ability to flow more, why isn't "wider is better" a good rule of thumb for exhaust upgrading? In a word - VELOCITY. I'm sure that all of you have at one time used a garden hose w/o a spray nozzle on it. If you let the water just run unrestricted out of the house it flows at a rather slow rate. However, if you take your finger and cover part of the opening, the water will flow out at a much much faster rate.
The astute exhaust designer knows that you must balance flow capacity with velocity. You want the exhaust gases to exit the chamber and speed along at the highest velocity possible - you want a FAST exhaust stream. If you have two exhaust pulses of equal volume, one in a 2" pipe and one in a 3" pipe, the pulse in the 2" pipe will be traveling considerably FASTER than the pulse in the 3" pipe. While it is true that the narrower the pipe, the higher the velocity of the exiting gases, you want make sure the pipe is wide enough so that there is as little backpressure as possible while maintaining suitable exhaust gas velocity. Backpressure in it's most extreme form can lead to reversion of the exhaust stream - that is to say the exhaust flows backwards, which is not good. The trick is to have a pipe that that is as narrow as possible while having as close to zero backpressure as possible at the RPM range you want your power band to be located at. Exhaust pipe diameters are best suited to a particular RPM range. A smaller pipe diameter will produce higher exhaust velocities at a lower RPM but create unacceptably high amounts of backpressure at high rpm. Thus if your powerband is located 2-3000 RPM you'd want a narrower pipe than if your powerband is located at 8-9000RPM.
Many engineers try to work around the RPM specific nature of pipe diameters by using setups that are capable of creating a similar effect as a change in pipe diameter on the fly. The most advanced is Ferrari's which consists of two exhaust paths after the header - at low RPM only one path is open to maintain exhaust velocity, but as RPM climbs and exhaust volume increases, the second path is opened to curb backpressure - since there is greater exhaust volume there is no loss in flow velocity. BMW and Nissan use a simpler and less effective method - there is a single exhaust path to the muffler; the muffler has two paths; one path is closed at low RPM but both are open at high RPM.
IV. So how did this myth come to be?
I often wonder how the myth "Hondas need backpressure" came to be. Mostly I believe it is a misunderstanding of what is going on with the exhaust stream as pipe diameters change. For instance, someone with a civic decides he's going to uprade his exhaust with a 3" diameter piping. Once it's installed the owner notices that he seems to have lost a good bit of power throughout the powerband. He makes the connections in the following manner: "My wider exhaust eliminated all backpressure but I lost power, therefore the motor must need some backpressure in order to make power." What he did not realize is that he killed off all his flow velocity by using such a ridiculously wide pipe. It would have been possible for him to achieve close to zero backpressure with a much narrower pipe - in that way he would not have lost all his flow velocity.
V. So why is exhaust velocity so important?
The faster an exhaust pulse moves, the better it can scavenge out all of the spent gasses during valve overlap. The guiding principles of exhaust pulse scavenging are a bit beyond the scope of this doc but the general idea is a fast moving pulse creates a low pressure area behind it. This low pressure area acts as a vacuum and draws along the air behind it. A similar example would be a vehicle traveling at a high rate of speed on a dusty road. There is a low pressure area immediately behind the moving vehicle - dust particles get sucked into this low pressure area causing it to collect on the back of the vehicle. This effect is most noticeable on vans and hatchbacks which tend to create large trailing low pressure areas - giving rise to the numerous "wash me please" messages written in the thickly collected dust on the rear door(s).
VI. Conclusion.
SO it turns out that Hondas don't need backpressure, they need as high a flow velocity as possible with as little backpressure as possible.
I. Introduction
One of the most misunderstood concepts in exhaust theory is backpressure. People love to talk about backpressure on message boards with no real understanding of what it is and what it's consequences are. I'm sure many of you have heard or read the phrase "Hondas need backpressure" when discussing exhaust upgrades. That phrase is in fact completely inaccurate and a wholly misguided notion.
II. Some basic exhaust theory
Your exhaust system is designed to evacuate gases from the combustion chamber quickly and efficently. Exhaust gases are not produced in a smooth stream; exhaust gases originate in pulses. A 4 cylinder motor will have 4 distinct pulses per complete engine cycle, a 6 cylinder has 6 pules and so on. The more pulses that are produced, the more continuous the exhaust flow. Backpressure can be loosely defined as the resistance to positive flow - in this case, the resistance to positive flow of the exhaust stream.
III. Backpressure and velocity
Some people operate under the misguided notion that wider pipes are more effective at clearing the combustion chamber than narrower pipes. It's not hard to see how this misconception is appealing - wider pipes have the capability to flow more than narrower pipes. So if they have the ability to flow more, why isn't "wider is better" a good rule of thumb for exhaust upgrading? In a word - VELOCITY. I'm sure that all of you have at one time used a garden hose w/o a spray nozzle on it. If you let the water just run unrestricted out of the house it flows at a rather slow rate. However, if you take your finger and cover part of the opening, the water will flow out at a much much faster rate.
The astute exhaust designer knows that you must balance flow capacity with velocity. You want the exhaust gases to exit the chamber and speed along at the highest velocity possible - you want a FAST exhaust stream. If you have two exhaust pulses of equal volume, one in a 2" pipe and one in a 3" pipe, the pulse in the 2" pipe will be traveling considerably FASTER than the pulse in the 3" pipe. While it is true that the narrower the pipe, the higher the velocity of the exiting gases, you want make sure the pipe is wide enough so that there is as little backpressure as possible while maintaining suitable exhaust gas velocity. Backpressure in it's most extreme form can lead to reversion of the exhaust stream - that is to say the exhaust flows backwards, which is not good. The trick is to have a pipe that that is as narrow as possible while having as close to zero backpressure as possible at the RPM range you want your power band to be located at. Exhaust pipe diameters are best suited to a particular RPM range. A smaller pipe diameter will produce higher exhaust velocities at a lower RPM but create unacceptably high amounts of backpressure at high rpm. Thus if your powerband is located 2-3000 RPM you'd want a narrower pipe than if your powerband is located at 8-9000RPM.
Many engineers try to work around the RPM specific nature of pipe diameters by using setups that are capable of creating a similar effect as a change in pipe diameter on the fly. The most advanced is Ferrari's which consists of two exhaust paths after the header - at low RPM only one path is open to maintain exhaust velocity, but as RPM climbs and exhaust volume increases, the second path is opened to curb backpressure - since there is greater exhaust volume there is no loss in flow velocity. BMW and Nissan use a simpler and less effective method - there is a single exhaust path to the muffler; the muffler has two paths; one path is closed at low RPM but both are open at high RPM.
IV. So how did this myth come to be?
I often wonder how the myth "Hondas need backpressure" came to be. Mostly I believe it is a misunderstanding of what is going on with the exhaust stream as pipe diameters change. For instance, someone with a civic decides he's going to uprade his exhaust with a 3" diameter piping. Once it's installed the owner notices that he seems to have lost a good bit of power throughout the powerband. He makes the connections in the following manner: "My wider exhaust eliminated all backpressure but I lost power, therefore the motor must need some backpressure in order to make power." What he did not realize is that he killed off all his flow velocity by using such a ridiculously wide pipe. It would have been possible for him to achieve close to zero backpressure with a much narrower pipe - in that way he would not have lost all his flow velocity.
V. So why is exhaust velocity so important?
The faster an exhaust pulse moves, the better it can scavenge out all of the spent gasses during valve overlap. The guiding principles of exhaust pulse scavenging are a bit beyond the scope of this doc but the general idea is a fast moving pulse creates a low pressure area behind it. This low pressure area acts as a vacuum and draws along the air behind it. A similar example would be a vehicle traveling at a high rate of speed on a dusty road. There is a low pressure area immediately behind the moving vehicle - dust particles get sucked into this low pressure area causing it to collect on the back of the vehicle. This effect is most noticeable on vans and hatchbacks which tend to create large trailing low pressure areas - giving rise to the numerous "wash me please" messages written in the thickly collected dust on the rear door(s).
VI. Conclusion.
SO it turns out that Hondas don't need backpressure, they need as high a flow velocity as possible with as little backpressure as possible.
07-30-2002, 02:25 PM
#56
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again, i reiterate myself by saying..."you want zero backpressure" i thank all those who know and realize this concept already. for the nonbelievers who support the "well, you need some backpressure." read on:
The most commonly talked about modification, there are many myths and speculation regarding exhausts, so let's start of clean shall we? Yeah that's right, anything you might have in your head about what makes a good exhaust system whether it be thoughts on pipe size or misunderstanding of backpressure just forget about it for now.
I Need A Little Bit of backpressure For Midrange Power
The Backpressure Myth:
You want zero backpressure instead of "some" backpressure, as you may sometimes hear on the street.
Stock backpressure is around 16 psi in a GSR. Good aftermarket exhausts yield 2-5 psi backpressure. "Bolt-ons only" engine packages, in the past, used exhausts with some backpressure, since there is this incorrect belief that having a little backpressure prevents the fresh air/fuel from shooting into the header at cam overlap (when both the opening intake valve & the closing exhaust valve are simultaneously, partially open). The backpressure supposedly "pushed" the fresh air/fuel back into the combustion chamber rather than having it go into the header. This shooting of fresh air/fuel from the intake manifold and intake port into the header cannot happen at cam overlap, since the pressure inside the header is already much higher than on the intake side , even when there is zero backpressure.
In reality, having more backpressure reduces the difference between the higher pressure in the head's exhaust port and lower pressure in the header and cat. You need this difference in pressure going from the head to the exhaust system or "pressure gradient" to keep the exhaust flow speed or energy at a high level. Having some backpressure during cam overlap and the exhaust stroke means that the exhaust gas must now push against something and therefore, this backwards force slows exhaust gas down.
This need for backpressure no longer exists when you have a properly tuned (timed) engine and a good stepped header. In fact, increased backpressure may lead to backwards flow or "reversion", where the exhaust gas travels backwards into the combustion chamber and dilutes the fresh intake charge at cam overlap. At the very least, it slows exhaust flow velocity or energy and prevents the creation of a vacuum for scavenging.
So please ignore the obsolete "you should have at least some backpressure" sales pitch. It's all about the creating high exhaust flow velocity/speed or energy leaving the exhaust port, in order for the header-cat-exhaust SYSTEM to do it's job properly (i.e. remove all the burnt exhaust gases and help pull in fresh intake charge by scavenging at cam overlap) and make power for you.
as eloquently stated by Larry at Endyn on the Backpressure issue:
quote:
--------------------------------------------------------------------------------
Just changing back pressure is a bogus way of trying to create
the "ideal" pressure in the system. The exhaust system should work like
a correctly conceived header. It should extract the exhaust
from the header, to minimize pumping pressures.
The only way to create a system that will serve as an extractor is to properly size the tubing to allow the flow velocity to create a sort of "vacuum" behind it.
Just as with headers, creating a sytem that will provide the best of
all worlds at all throttle positions and rpm ranges is impossible. It's all going to be a trade-off.
You can tune for the throttle positions and rpm ranges where you desire the greatest performance, but you'll sacrifice performance at the other end of the scale. -michael delaney
The exhaust is responsible for carrying and expelling waste gases away from the engine as well as silencing the engine's noise output. Gasses leave the combustion chamber under extreme pressure and enter into the exhaust manifold. After the exhaust manifold joins the gasses from all the cylinders together, the gasses enter the catalytic converter where they undergo a process discussed in our Catalytic Converter (coming soon) article. After the gasses exit the catalytic converter, they enter into the B-pipe where there is usually a resonator to help silence some of the noise caused by the escaping of the pressurized gasses. After that the gasses enter into a specialized muffler where the majority of the silencing occurs and then are expelled out into the air.
The Basics
This here is a simplified picture showing one of the four cylinders inside our engines. The piston in this picture has begun it's intake stroke and air is starting to enter the combustion chamber. So what does this have to do with exhausts? Well look at the exhaust valve, you can see that it is still open even though the piston already completed it's exhaust stroke. This is called valve overlap and is necessary for high revving engines such as Integras. At high RPMs, air is being pushed out the combustion chamber so fast that not all of the air can escape in time after the piston completes it's stroke. The overlapping period as you see here allows the momentum of the air to keep moving it out of the chamber even though the piston already has started creating a vacuum with it's intake stroke. This works great at high RPMs, but what about the low RPMs? The air is usually not moving fast enough for the overlapping period to be beneficial so if exhaust gasses aren't moved quickly through the exhaust, the vacuum created by the piston's intake stroke can actually suck some exhaust gas back into the combustion chamber. This is how aftermarket exhausts can cause a loss in low end power. Incorrectly sized exhaust pipes can disrupt the airflow, allowing exhaust gasses to be sucked back in during this valve overlap period. Obviously we don't want already burnt, noxious gasses to mix with our fresh air/fuel mixture so it is very important to have a properly tuned exhaust for the right application.
To understand how to properly tune an exhaust you must first understand exactly how the gasses will be flowing through it. Exhaust gasses do not flow in one continuous stream they get shot out in pulses. Each time a valve opens and gas is pushed out of the combustion chamber, it creates a pulse. As you can see from the picture, the head of the exhaust pulse remains at a high pressure after it is shoved out of the combustion chamber. The center of the pulse is closer to the ambient pressure of the exhaust system and the tail turns into a low pressure vacuum.
Now look at these exhaust pulses all lined up as they travel through the exhaust. This would be an exhaust system which was sized perfectly for the car at the right RPM. The pulses, to an extent, will move through the B-pipe to the muffler by themselves. But we want the exhaust gasses to travel as fast as they possibly can. How can we get them moving faster? Well you get them to line up like shown, and the pulses will then pull each other through. As with most naturally occurring phenomenon, opposites will attract. The high pressure heads are pulled into the low pressure, almost vacuum-like tails. So each pulse will follow after the other, effectively leading each other to the muffler and away from the engine.
Pipe Sizing
In a perfect exhaust system with the engine at the right RPM, all the heads would line up perfectly with the tails of the exhaust gas pulses. Of course in real life this can't always be that way but that's why we must choose our pipe diameter carefully. In the lower RPMs, pulses are smaller, and further apart. When you rev up into the high RPMs, pulses get bigger and closer together. So we want to keep the small, spread out exhaust pulses in line in the low RPMs but we also want to accommodate the larger, quicker pulses in the high RPMs. For this reason we have to decide what pipe size will give us the best trade-off of low-end vs. high-end power so we can get the highest total HP increase possible throughout the RPM range.
We see here a restrictive stock exhaust system. In the low RPMs, this tight formation of pulses becomes an advantage as it keeps the them close together so they can pull each other through the exhaust rather than lingering around. However as the RPMs rise, the pulses become closer and closer together, raising backpressure to restrict the piston in pushing out more exhaust gasses.
Here you can see a correctly sized exhaust system. The backpressure is minimized as much as possible while allowing pulses to line up nicely and provides a good trade-off between both low and high-end power. For most N/A applications this pipe size would be 2.25" in diameter. A common mistake sometimes talked about in "tuner" magazines is that you need some backpressure for the system to flow properly. This is a myth created by amateur testing methods. You can see the exhaust here is properly sized and will get the best performance out of any other sizes, bigger or smaller. So because it outperforms the bigger exhaust they just assume that backpressure was the key to success here. As you can see from this picture, they are wrong. The reason this exhaust performed the best was because the pules were perfectly in line and able to draw each other out of the exhaust at the highest velocity possible.
Now we are looking at the opposite end of the spectrum from advice commonly given by "tuner" magazines. "The bigger exhaust you have the better it flows and you have no backpressure anymore so you make tons more power." Yeah right. Funny how these "tuner" magazines will say something on one page and then say something opposite on the next. And yeah I keep putting "tuner" in quotes because I'm being sarcastic. I honestly thing most of those magazines are a joke. Anyway in an exhaust system that is too big, the pulses get disorganized and don't follow each other in line. Some of the pulses will bounce around, causing exhaust gasses to hang around in the exhaust pipe with little forward motion. These gasses are susceptible to being sucked back into the combustion chamber on the piston's intake stroke, diluting the fresh air/fuel mixture and ultimately causing power loss. So while you have minimized backpressure even further with an over-sized pipe, you did so at the price of disorganizing the exhaust pulses.
Forced Induction
Gasses exiting the combustion chamber of an N/A car are already under very high pressure, which is what creates the exhaust pulse in the first place. Turbos and Superchargers can raise that pressure dramatically, causing much bigger pulses. Because of the larger pulse sizes, you can see that having larger exhaust pipe sizes are okay. But still even turbo powered exhausts have their limit and the pulses can get disorganized just like an N/A car, just at a much larger degree.
Mufflers
This is a picture of an OEM muffler. These decrease noise by reflecting pulses into each other which causes "destructive interference". Basically if a sound wave runs into another sound wave of the same frequency and opposite phase, then the sound waves are cancelled out and no noise is heard. Of course no muffler can ever do this perfectly so there is still sound to be heard. The performance of these mufflers as you can guess is less than wonderful. The air is forced into many dead-end chambers, creating backpressure and making it difficult for the air to flow through smoothly. Sounds like the stock muffler sucks, and maybe just changing the muffler will give us good performance gains right? Wrong.
This would be another one for the Magazine Mechanics. "This muffler will reduce backpressure and you'll see at least 15-20HP with this ultimate free flowing design!" By now I hope we can see what a big fat load of crap that is. In the stock exhaust system, the pulses have already been jammed together, and the backpressure has already affected the engine. So you change your muffler to a better flowing one, so what? The amount of backpressure in the exhaust system remains unchanged. You're just putting a fire hose at the end of a garden hose, the flow remains unchanged. So unless your exhaust has reduced backpressure enough so that the muffler becomes a restriction, then changing the muffler to a "free flowing" design will have no effect on real-world performance.
That's about all there is to do with exhausts. I hope I could help you all learn a bit more our the cars which we love and make the correct purchasing decisions. Any questions please feel free to e-mail me at SurferX@team-integra.net alright? Till next time, peace.
****articles brought to you by the performance kings of team-integra.net****
if you still don't believe me, you don't know a head gasket from a head bolt.
you want ZERO backpressure!
The most commonly talked about modification, there are many myths and speculation regarding exhausts, so let's start of clean shall we? Yeah that's right, anything you might have in your head about what makes a good exhaust system whether it be thoughts on pipe size or misunderstanding of backpressure just forget about it for now.
I Need A Little Bit of backpressure For Midrange Power
The Backpressure Myth:
You want zero backpressure instead of "some" backpressure, as you may sometimes hear on the street.
Stock backpressure is around 16 psi in a GSR. Good aftermarket exhausts yield 2-5 psi backpressure. "Bolt-ons only" engine packages, in the past, used exhausts with some backpressure, since there is this incorrect belief that having a little backpressure prevents the fresh air/fuel from shooting into the header at cam overlap (when both the opening intake valve & the closing exhaust valve are simultaneously, partially open). The backpressure supposedly "pushed" the fresh air/fuel back into the combustion chamber rather than having it go into the header. This shooting of fresh air/fuel from the intake manifold and intake port into the header cannot happen at cam overlap, since the pressure inside the header is already much higher than on the intake side , even when there is zero backpressure.
In reality, having more backpressure reduces the difference between the higher pressure in the head's exhaust port and lower pressure in the header and cat. You need this difference in pressure going from the head to the exhaust system or "pressure gradient" to keep the exhaust flow speed or energy at a high level. Having some backpressure during cam overlap and the exhaust stroke means that the exhaust gas must now push against something and therefore, this backwards force slows exhaust gas down.
This need for backpressure no longer exists when you have a properly tuned (timed) engine and a good stepped header. In fact, increased backpressure may lead to backwards flow or "reversion", where the exhaust gas travels backwards into the combustion chamber and dilutes the fresh intake charge at cam overlap. At the very least, it slows exhaust flow velocity or energy and prevents the creation of a vacuum for scavenging.
So please ignore the obsolete "you should have at least some backpressure" sales pitch. It's all about the creating high exhaust flow velocity/speed or energy leaving the exhaust port, in order for the header-cat-exhaust SYSTEM to do it's job properly (i.e. remove all the burnt exhaust gases and help pull in fresh intake charge by scavenging at cam overlap) and make power for you.
as eloquently stated by Larry at Endyn on the Backpressure issue:
quote:
--------------------------------------------------------------------------------
Just changing back pressure is a bogus way of trying to create
the "ideal" pressure in the system. The exhaust system should work like
a correctly conceived header. It should extract the exhaust
from the header, to minimize pumping pressures.
The only way to create a system that will serve as an extractor is to properly size the tubing to allow the flow velocity to create a sort of "vacuum" behind it.
Just as with headers, creating a sytem that will provide the best of
all worlds at all throttle positions and rpm ranges is impossible. It's all going to be a trade-off.
You can tune for the throttle positions and rpm ranges where you desire the greatest performance, but you'll sacrifice performance at the other end of the scale. -michael delaney
The exhaust is responsible for carrying and expelling waste gases away from the engine as well as silencing the engine's noise output. Gasses leave the combustion chamber under extreme pressure and enter into the exhaust manifold. After the exhaust manifold joins the gasses from all the cylinders together, the gasses enter the catalytic converter where they undergo a process discussed in our Catalytic Converter (coming soon) article. After the gasses exit the catalytic converter, they enter into the B-pipe where there is usually a resonator to help silence some of the noise caused by the escaping of the pressurized gasses. After that the gasses enter into a specialized muffler where the majority of the silencing occurs and then are expelled out into the air.
The Basics
This here is a simplified picture showing one of the four cylinders inside our engines. The piston in this picture has begun it's intake stroke and air is starting to enter the combustion chamber. So what does this have to do with exhausts? Well look at the exhaust valve, you can see that it is still open even though the piston already completed it's exhaust stroke. This is called valve overlap and is necessary for high revving engines such as Integras. At high RPMs, air is being pushed out the combustion chamber so fast that not all of the air can escape in time after the piston completes it's stroke. The overlapping period as you see here allows the momentum of the air to keep moving it out of the chamber even though the piston already has started creating a vacuum with it's intake stroke. This works great at high RPMs, but what about the low RPMs? The air is usually not moving fast enough for the overlapping period to be beneficial so if exhaust gasses aren't moved quickly through the exhaust, the vacuum created by the piston's intake stroke can actually suck some exhaust gas back into the combustion chamber. This is how aftermarket exhausts can cause a loss in low end power. Incorrectly sized exhaust pipes can disrupt the airflow, allowing exhaust gasses to be sucked back in during this valve overlap period. Obviously we don't want already burnt, noxious gasses to mix with our fresh air/fuel mixture so it is very important to have a properly tuned exhaust for the right application.
To understand how to properly tune an exhaust you must first understand exactly how the gasses will be flowing through it. Exhaust gasses do not flow in one continuous stream they get shot out in pulses. Each time a valve opens and gas is pushed out of the combustion chamber, it creates a pulse. As you can see from the picture, the head of the exhaust pulse remains at a high pressure after it is shoved out of the combustion chamber. The center of the pulse is closer to the ambient pressure of the exhaust system and the tail turns into a low pressure vacuum.
Now look at these exhaust pulses all lined up as they travel through the exhaust. This would be an exhaust system which was sized perfectly for the car at the right RPM. The pulses, to an extent, will move through the B-pipe to the muffler by themselves. But we want the exhaust gasses to travel as fast as they possibly can. How can we get them moving faster? Well you get them to line up like shown, and the pulses will then pull each other through. As with most naturally occurring phenomenon, opposites will attract. The high pressure heads are pulled into the low pressure, almost vacuum-like tails. So each pulse will follow after the other, effectively leading each other to the muffler and away from the engine.
Pipe Sizing
In a perfect exhaust system with the engine at the right RPM, all the heads would line up perfectly with the tails of the exhaust gas pulses. Of course in real life this can't always be that way but that's why we must choose our pipe diameter carefully. In the lower RPMs, pulses are smaller, and further apart. When you rev up into the high RPMs, pulses get bigger and closer together. So we want to keep the small, spread out exhaust pulses in line in the low RPMs but we also want to accommodate the larger, quicker pulses in the high RPMs. For this reason we have to decide what pipe size will give us the best trade-off of low-end vs. high-end power so we can get the highest total HP increase possible throughout the RPM range.
We see here a restrictive stock exhaust system. In the low RPMs, this tight formation of pulses becomes an advantage as it keeps the them close together so they can pull each other through the exhaust rather than lingering around. However as the RPMs rise, the pulses become closer and closer together, raising backpressure to restrict the piston in pushing out more exhaust gasses.
Here you can see a correctly sized exhaust system. The backpressure is minimized as much as possible while allowing pulses to line up nicely and provides a good trade-off between both low and high-end power. For most N/A applications this pipe size would be 2.25" in diameter. A common mistake sometimes talked about in "tuner" magazines is that you need some backpressure for the system to flow properly. This is a myth created by amateur testing methods. You can see the exhaust here is properly sized and will get the best performance out of any other sizes, bigger or smaller. So because it outperforms the bigger exhaust they just assume that backpressure was the key to success here. As you can see from this picture, they are wrong. The reason this exhaust performed the best was because the pules were perfectly in line and able to draw each other out of the exhaust at the highest velocity possible.
Now we are looking at the opposite end of the spectrum from advice commonly given by "tuner" magazines. "The bigger exhaust you have the better it flows and you have no backpressure anymore so you make tons more power." Yeah right. Funny how these "tuner" magazines will say something on one page and then say something opposite on the next. And yeah I keep putting "tuner" in quotes because I'm being sarcastic. I honestly thing most of those magazines are a joke. Anyway in an exhaust system that is too big, the pulses get disorganized and don't follow each other in line. Some of the pulses will bounce around, causing exhaust gasses to hang around in the exhaust pipe with little forward motion. These gasses are susceptible to being sucked back into the combustion chamber on the piston's intake stroke, diluting the fresh air/fuel mixture and ultimately causing power loss. So while you have minimized backpressure even further with an over-sized pipe, you did so at the price of disorganizing the exhaust pulses.
Forced Induction
Gasses exiting the combustion chamber of an N/A car are already under very high pressure, which is what creates the exhaust pulse in the first place. Turbos and Superchargers can raise that pressure dramatically, causing much bigger pulses. Because of the larger pulse sizes, you can see that having larger exhaust pipe sizes are okay. But still even turbo powered exhausts have their limit and the pulses can get disorganized just like an N/A car, just at a much larger degree.
Mufflers
This is a picture of an OEM muffler. These decrease noise by reflecting pulses into each other which causes "destructive interference". Basically if a sound wave runs into another sound wave of the same frequency and opposite phase, then the sound waves are cancelled out and no noise is heard. Of course no muffler can ever do this perfectly so there is still sound to be heard. The performance of these mufflers as you can guess is less than wonderful. The air is forced into many dead-end chambers, creating backpressure and making it difficult for the air to flow through smoothly. Sounds like the stock muffler sucks, and maybe just changing the muffler will give us good performance gains right? Wrong.
This would be another one for the Magazine Mechanics. "This muffler will reduce backpressure and you'll see at least 15-20HP with this ultimate free flowing design!" By now I hope we can see what a big fat load of crap that is. In the stock exhaust system, the pulses have already been jammed together, and the backpressure has already affected the engine. So you change your muffler to a better flowing one, so what? The amount of backpressure in the exhaust system remains unchanged. You're just putting a fire hose at the end of a garden hose, the flow remains unchanged. So unless your exhaust has reduced backpressure enough so that the muffler becomes a restriction, then changing the muffler to a "free flowing" design will have no effect on real-world performance.
That's about all there is to do with exhausts. I hope I could help you all learn a bit more our the cars which we love and make the correct purchasing decisions. Any questions please feel free to e-mail me at SurferX@team-integra.net alright? Till next time, peace.
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you want ZERO backpressure!
07-30-2002, 02:37 PM
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you are prooving our point for us. I do agree that at higher RPM's, the less back pressure the better, but on the other hand, if you are at lower RPMs the larger piping will cause the exhuast gases to have less velocity at said RPMs...which is where the problem exists. Basically a larger diameter exahust would be too large for low RPMs, but just right for the higher end. If the piping could expand and contract in relation to the amount of gases escaping, you would have optimum piping diameter at all times, but since that isn't possible, we have to compromise. Loose a little low end, gain a little high end.
07-30-2002, 02:46 PM
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I saw that article when it was put up....this topic has gone way beyond what the owner was asking and I think I'm talking about something slightly different than eveyone else here regarding back pressure. I'm thinking more along the lines of what that article calls vaccum.
07-30-2002, 02:52 PM
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jgrotkier and thx, what you are describing has nothing to do with backpressure. What you refer to is all based on flow velocity and flow capacity.
This whole thread is one big misconception about what backpressure really is. Exhaust velocity is what makes 3" piping on a stock engine perform awful, not lack of backpressure. You are all on the right track, but you are saying its the lack of backpressure that causes the poor performance when its the lack of exhaust velocity that really causes it.
Backpressure is always bad
Exhaust velocity and Flow capacity have to be balanced for your engines output.
This whole thread is one big misconception about what backpressure really is. Exhaust velocity is what makes 3" piping on a stock engine perform awful, not lack of backpressure. You are all on the right track, but you are saying its the lack of backpressure that causes the poor performance when its the lack of exhaust velocity that really causes it.
Backpressure is always bad
Exhaust velocity and Flow capacity have to be balanced for your engines output.
07-30-2002, 03:02 PM
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ok, so it seems that there is just a misunderstanding of the correct phrases.
Backpressure is when the the exhuast can't expel the gases fast enough to keep up with the engine. Unforntunately, in order for the exhuast to expel the gases fast enough at a lower RPM, it takes a smaller diameter piping, which in turn can cause backpressure at higher RPMs...
SO thats where the idea that you need backpressure comes from. You are correct in saying that backpressure is bad...BUT in order to keep low end torque, you have to have a little backpressure at higher RPMs
Backpressure is when the the exhuast can't expel the gases fast enough to keep up with the engine. Unforntunately, in order for the exhuast to expel the gases fast enough at a lower RPM, it takes a smaller diameter piping, which in turn can cause backpressure at higher RPMs...
SO thats where the idea that you need backpressure comes from. You are correct in saying that backpressure is bad...BUT in order to keep low end torque, you have to have a little backpressure at higher RPMs