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Old 09-02-2012, 06:43 PM   #1
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Alright I'm starting this thread to stop the thread jack in the 96-04 forum. So here's the question I keep asking and have yet to get a good answer for: Why is back-pressure good and what evidence do you have to prove that? I am of the mindset that it's NOT good for your engine so please prove me wrong.

Here is a good thread showing the benefits of having less back-pressure.
Myths about "back-pressure"
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Old 09-02-2012, 07:42 PM   #2
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Re: Back-pressure

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Originally Posted by JDF View Post
Alright I'm starting this thread to stop the thread jack in the 96-04 forum. So here's the question I keep asking and have yet to get a good answer for: Why is back-pressure good and what evidence do you have to prove that? I am of the mindset that it's NOT good for your engine so please prove me wrong.

Here is a good thread showing the benefits of having less back-pressure.
Myths about "back-pressure"
It's also more-so disproving that "back-pressure" is even a factor, really.
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Old 09-02-2012, 07:47 PM   #3
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It's all about the tune... Stock ECU's like some back pressure.. Only turbo's truly benefit from ZERO BP.
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Old 09-02-2012, 08:06 PM   #4
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Re: Back-pressure

I know with a chevelle i had,,when i had headers with mufflers at the ends,,it ran strong and sounded cool,,but i ran the mufflers a few feet from the headers and then went all the way out,,with the same mufflers,,and it acually ran faster,,but i dont know the physics of it but im sure someone will tell ya..Someone told me once it had something to do with smoothing out the pulses of the exhaust coming out ,,making it flow better,,but im not sure
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Old 09-02-2012, 08:35 PM   #5
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I know with a chevelle i had,,when i had headers with mufflers at the ends,,it ran strong and sounded cool,,but i ran the mufflers a few feet from the headers and then went all the way out,,with the same mufflers,,and it acually ran faster,,but i dont know the physics of it but im sure someone will tell ya..Someone told me once it had something to do with smoothing out the pulses of the exhaust coming out ,,making it flow better,,but im not sure
Exhaust flow is what you're looking for. They talk about it a lot in that link I posted.
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Old 09-02-2012, 10:06 PM   #6
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Re: Back-pressure

why do people still think back pressure exists...it's all about exhaust velocity :/

i actually got into a good debate with my GM over his stand on back pressure while i said it didn't exist
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Old 09-02-2012, 11:01 PM   #7
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Originally Posted by 1slosix
why do people still think back pressure exists...it's all about exhaust velocity :/

i actually got into a good debate with my GM over his stand on back pressure while i said it didn't exist
Doesn't exist?
Is this just your theory or do you have a creditable source for this statement?
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Old 09-03-2012, 03:18 PM   #8
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Re: Back-pressure

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Doesn't exist?
Is this just your theory or do you have a creditable source for this statement?
here's a simple read up on exhaust velocity vs the backpressure myth...if you've never read up on it before you'll find numerous studies. which have been around since i've been into cars so it's nothing new.


Back pressure: 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.
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Old 09-03-2012, 05:14 PM   #9
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Re: Back-pressure

I guess thats why you see the pencil exhaust on them from the factory..So the fart box on the end is just for noise..lol
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Old 09-03-2012, 05:15 PM   #10
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Quote:
Originally Posted by 1slosix

here's a simple read up on exhaust velocity vs the backpressure myth...if you've never read up on it before you'll find numerous studies. which have been around since i've been into cars so it's nothing new.

Back pressure: 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.
SO... Where in here does it say back pressure does not exist? The conclusion in fact states 'as little back pressure as possible.'

This just looks like a copy and paste off a message board somewhere.
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Old 09-03-2012, 06:14 PM   #11
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Quote:
Originally Posted by deadsp0t

SO... Where in here does it say back pressure does not exist? The conclusion in fact states 'as little back pressure as possible.'

This just looks like a copy and paste off a message board somewhere.
I was thinking the same thing. Don't get me wrong it was well written and at a "laymans" level of understanding. However, it didn't disprove the notion of back pressure. It did make me understand why I saw the loudest 13 second Honda at the drag strip the other night. Lol. Is there any other reference someone knows of to support the "no back pressure" thesis?
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Old 09-03-2012, 06:22 PM   #12
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back pressure exists. but it is just a by-product of even having an exhaust. using x&h pipes help increase velocity of the exhaust flow. this also works like a vacuum and help pull out more exhaust. but in between pulses the by product back pressure becomes existant. the key isnt to get back pressure. but to get a faster flowimg exhaust. and just switching exhaust pipes around doesnt always just give you more power. with different flowing exhausts the tune will have to proper for what the exhaust is doing. i dont have any links off the topp of my head, but have read many articles, and this is the best way i can explain in accordinace to how i understand whats going on.
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