Backpressure: The MYTH and Why It's Wrong
03-24-2010, 07:48 AM
#1
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Backpressure: The MYTH and Why It's Wrong
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 Honda's don't need backpressure, they need as high a flow velocity as possible with as little backpressure as possible
Thanks to Sacicons over at HondaCivicForum for this write-up
Reason for write-up - I was getting tired of people saying a 3" exhaust would benefit from a stock motor.
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 Honda's don't need backpressure, they need as high a flow velocity as possible with as little backpressure as possible
Thanks to Sacicons over at HondaCivicForum for this write-up
Reason for write-up - I was getting tired of people saying a 3" exhaust would benefit from a stock motor.
03-24-2010, 08:14 AM
#2
I believe this writeup is leaving out the importance of exhaust length in the equation. Similar to intakes, exhausts have harmonic properties that impact performance at certain RPM ranges. Every exhaust (and intake) is going to have certain RPM ranges where they perform very well and very bad. You can change the harmonics of an exhaust and intake by changing diameter OR length.
03-24-2010, 11:10 AM
#3
Registered User
The last place for exhaust velocity to increase is at the collector of the header. Everything after that is a restriction. So if you have to run an exhaust, go as big as your ears will allow. This is why alot of people drop their exhaust or run a cut-out at the drag strip. Throw a 3" on their and call it a day. This is what a tuner with a degree in Engineering told me. I'd take his word over some forum whores anyday.
I laugh everytime I hear/read the turm "backpressure". LMAO
I laugh everytime I hear/read the turm "backpressure". LMAO
03-24-2010, 11:34 AM
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So if you have to run an exhaust, go as big as your ears will allow
Dropping your exhaust or running a cutout is drastically different than using a large diameter exhaust.
03-24-2010, 11:47 AM
#5
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^I'm not turning this thread into an arguemant. I'm just stating what a very knowlegable person told me and also what the dyno's have proven. 3" has made more power on EVERY Honda four cylinder dyno I've seen, near stock or not; if it was tuned obviously.
EDIT Sorry I have to point this out. You do realize that headers have a collector, right? The collector is by far the most important part of the whole exhaust system; length and diameter are both key factors. So you wouldn't have primaries going directly to an exhaust system LMAO. Like I said, the last place for exhaust velocity to increase is at the collector, not in the exhaust.
EDIT Sorry I have to point this out. You do realize that headers have a collector, right? The collector is by far the most important part of the whole exhaust system; length and diameter are both key factors. So you wouldn't have primaries going directly to an exhaust system LMAO. Like I said, the last place for exhaust velocity to increase is at the collector, not in the exhaust.
03-24-2010, 11:59 AM
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you want the highest mass flowrate through the exhaust (assuming resonance is a minimal effect outside of the header)
mass flowrate depends on cross sectional area of the piping, density of the gas (which is a constant in this case) and velocity of the flow. as you increase cross section, you decrease velocity, so there is a sweet spot where the curves intersect.
there are no simple rule that always applie (ie 3 in piping is always awesome). it is different for every engine at every throttle position and rpm and even ambient conditions like temperature, pressure and altitude.
engineers get paid good money for a reason!
mass flowrate depends on cross sectional area of the piping, density of the gas (which is a constant in this case) and velocity of the flow. as you increase cross section, you decrease velocity, so there is a sweet spot where the curves intersect.
there are no simple rule that always applie (ie 3 in piping is always awesome). it is different for every engine at every throttle position and rpm and even ambient conditions like temperature, pressure and altitude.
engineers get paid good money for a reason!
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03-24-2010, 12:04 PM
#8
Originally Posted by Momentum,Mar 24 2010, 11:34 AM
This is absolutely not true, I'm going to use a ridiculous example but the point applies most likely even to a 3" being to big for a honda (NA). If you have 1.75" primaries flowing into a 6" exhaust the loss in velocity will be huge causing a ton of turbulence and in turn back pressure at the exit of the collector.
Dropping your exhaust or running a cutout is drastically different than using a large diameter exhaust.
Dropping your exhaust or running a cutout is drastically different than using a large diameter exhaust.
One thing that is missing from the above is the fact that there is such a thing as too much exhaust velocity. This is especially relevant with high-overlap cams where too much velocity can actually cause the air/fuel mixture to bypass the cylinder alltogether and get sucked into the exhaust unburned.
03-24-2010, 12:08 PM
#9
Originally Posted by H22toF20,Mar 24 2010, 02:47 PM
3" has made more power on EVERY Honda four cylinder dyno I've seen, near stock or not; if it was tuned obviously.
03-24-2010, 12:30 PM
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Originally Posted by H22toF20,Mar 24 2010, 11:47 AM
^I'm not turning this thread into an arguemant. I'm just stating what a very knowlegable person told me and also what the dyno's have proven. 3" has made more power on EVERY Honda four cylinder dyno I've seen, near stock or not; if it was tuned obviously.
EDIT Sorry I have to point this out. You do realize that headers have a collector, right? The collector is by far the most important part of the whole exhaust system; length and diameter are both key factors. So you wouldn't have primaries going directly to an exhaust system LMAO. Like I said, the last place for exhaust velocity to increase is at the collector, not in the exhaust.
EDIT Sorry I have to point this out. You do realize that headers have a collector, right? The collector is by far the most important part of the whole exhaust system; length and diameter are both key factors. So you wouldn't have primaries going directly to an exhaust system LMAO. Like I said, the last place for exhaust velocity to increase is at the collector, not in the exhaust.
I also find it hard to believe that many of the dyno graphs you have seen compared that 3" exhaust to various other sizes. I have no doubt it would pick up power over stock but I'm certain that an in between size with the same muffler type would be more beneficial.
Also exhaust velocity decreases in merge collectors, the basic point of a merge collector is to minimize that effect but it certainly doesn't increase.