Back Pressure, too much or not enough?
Registered User
Joined: May 2001
Posts: 160
Re: Back Pressure, too much or not enough?
"you can NOT have too little amount of backpressure
the less the better
low backpressure will create low end torque as well as high rpm power
if you have something that will prove me wrong I would please like to see so. again just think of the meaning of word. from just the way the word is alone it is stating that you are putting pressure against the flow of the exhaust. how is putting pressure that your motor has to overcome to give you more low end torque
ideal would be a vac inside your exhaust ( that would have no pressure at all ) and with that it would help reduce pumping loss from the motor trying to force all your exhaust gas out. and also would help suck out a lot more of your exhaust which would create a high volumetric efficiency.
when you induce backpressure into the exhaust you are doing nothing more then making everything fight.
problems that come from backpressure is lower VE, pumping loss, high exhaust temps, high motor temps, more stress on your motor (small as it might be) and I am sure there are other things
just remember backpressure does not create a high velocity in the exhaust, and there is a difference between backpressure and velocity.
though by putting huge pipes on your car that is not really going to help you out either, it is not due to lack of backpressure though. what will go on if you put a large pipe on the car without the flow to back it up then the exhaust gas doesn't really have the walls of the pipe holding it into a controlled flow pattern instead the gas will flow around and have eddies in the system and try to turn back itself. if you get the right size pipes for your car you will reduce as much backpressure as low as you can but also keep the exhaust in check by not letting it just wander around but keep it all flowing in the right direction
and if backpressure was something that helped performance wouldn't ppl try to put something to create backpressure inside there intake also? instead ppl try to open up the intake, port it out, polish it make everything as smooth as they can to reduce the force the motor has to give to pull the air in. same thing on the exhaust side." QUOTE
This quote makes alot of sense.
the less the better
low backpressure will create low end torque as well as high rpm power
if you have something that will prove me wrong I would please like to see so. again just think of the meaning of word. from just the way the word is alone it is stating that you are putting pressure against the flow of the exhaust. how is putting pressure that your motor has to overcome to give you more low end torque
ideal would be a vac inside your exhaust ( that would have no pressure at all ) and with that it would help reduce pumping loss from the motor trying to force all your exhaust gas out. and also would help suck out a lot more of your exhaust which would create a high volumetric efficiency.
when you induce backpressure into the exhaust you are doing nothing more then making everything fight.
problems that come from backpressure is lower VE, pumping loss, high exhaust temps, high motor temps, more stress on your motor (small as it might be) and I am sure there are other things
just remember backpressure does not create a high velocity in the exhaust, and there is a difference between backpressure and velocity.
though by putting huge pipes on your car that is not really going to help you out either, it is not due to lack of backpressure though. what will go on if you put a large pipe on the car without the flow to back it up then the exhaust gas doesn't really have the walls of the pipe holding it into a controlled flow pattern instead the gas will flow around and have eddies in the system and try to turn back itself. if you get the right size pipes for your car you will reduce as much backpressure as low as you can but also keep the exhaust in check by not letting it just wander around but keep it all flowing in the right direction
and if backpressure was something that helped performance wouldn't ppl try to put something to create backpressure inside there intake also? instead ppl try to open up the intake, port it out, polish it make everything as smooth as they can to reduce the force the motor has to give to pull the air in. same thing on the exhaust side." QUOTE
This quote makes alot of sense.
Registered User
Joined: Sep 2002
Posts: 655
Re: Back Pressure, too much or not enough?
*gulp*
You're asking a novice to define tuned length amongst a group of skilled engine
tuners and engineers?
I'll give it my best shot using supported links and examples:
When valves open and close they create a pressure pulse of a certain length
based on the frequency of the valve opening/closing over time.
As RPM increases, frequencies of the valve opening/closing events increase as well.
The wavelength of the pulses can be calculated using complex formulas.
Temperature, atmospheric pressure, speed of sound within a tube, etc.
would all become a factor to determine the wavelength.
I have no idea how this is formulated, so I simply use a standard formula
which assumes perfect conditions.
Here's a simple calculator:
http://www.sengpielaudio.com/calculator-wavelength.htm
Frequency__Wavelength___ 1/4 Wavelength
200 Hz______ 5.65 ft________ 1.41 ft
100 Hz_____ 11.30 ft________ 2.83 ft
_80 Hz_____ 14.13 ft________ 3.53 ft
_60 Hz_____ 18.83 ft________ 4.81 ft
It is my understanding that engines exceeding 5000 RPM can produce frequencies
of 200Hz, or higher which allow exhaust pipe lengths of full, half and 1/4
wave lengths to be used.
If the pressure/sound wave is extremely long, and not practical to build
into an intake/exhaust system, you can use a 1/2 wavelength, or 1/4 wave
length tuned runner to achieve similar results.
If the reflected waves mix with the new incoming waves and are in phase,
or mix such that there is a gain, the resulting pressure wave will be stronger.
Use this link to see how phase relationships with two pulses can mix:
http://www.ling.udel.edu/idsardi/253/sinewave/
I am basing most of my theory on speaker port tuning and audio which I know
more about than exhaust tuning. I'm banking on the fact that audio tuning
is damn near close to exhaust/intake tuning because we're still dealing with
air pressure and sound.
http://www.hpt-sport.com/tunedpip.htm
http://headerdesign.com/extras/design.asp
(search for paragraph titled "The Exhaust Pulse"
http://www.nsxprime.com/FAQ/Miscella...austtheory.htm
You're asking a novice to define tuned length amongst a group of skilled engine
tuners and engineers?
I'll give it my best shot using supported links and examples:
When valves open and close they create a pressure pulse of a certain length
based on the frequency of the valve opening/closing over time.
As RPM increases, frequencies of the valve opening/closing events increase as well.
The wavelength of the pulses can be calculated using complex formulas.
Temperature, atmospheric pressure, speed of sound within a tube, etc.
would all become a factor to determine the wavelength.
I have no idea how this is formulated, so I simply use a standard formula
which assumes perfect conditions.
Here's a simple calculator:
http://www.sengpielaudio.com/calculator-wavelength.htm
Frequency__Wavelength___ 1/4 Wavelength
200 Hz______ 5.65 ft________ 1.41 ft
100 Hz_____ 11.30 ft________ 2.83 ft
_80 Hz_____ 14.13 ft________ 3.53 ft
_60 Hz_____ 18.83 ft________ 4.81 ft
It is my understanding that engines exceeding 5000 RPM can produce frequencies
of 200Hz, or higher which allow exhaust pipe lengths of full, half and 1/4
wave lengths to be used.
If the pressure/sound wave is extremely long, and not practical to build
into an intake/exhaust system, you can use a 1/2 wavelength, or 1/4 wave
length tuned runner to achieve similar results.
If the reflected waves mix with the new incoming waves and are in phase,
or mix such that there is a gain, the resulting pressure wave will be stronger.
Use this link to see how phase relationships with two pulses can mix:
http://www.ling.udel.edu/idsardi/253/sinewave/
I am basing most of my theory on speaker port tuning and audio which I know
more about than exhaust tuning. I'm banking on the fact that audio tuning
is damn near close to exhaust/intake tuning because we're still dealing with
air pressure and sound.
http://www.hpt-sport.com/tunedpip.htm
http://headerdesign.com/extras/design.asp
(search for paragraph titled "The Exhaust Pulse"
http://www.nsxprime.com/FAQ/Miscella...austtheory.htm
Registered User
Joined: Oct 2004
Posts: 236
From: Costa Mesa, CA no more!
Re: Back Pressure, too much or not enough?
Originally Posted by Big454blockchevy
What some people here are trying to get across is , that you never want "more" back pressure introduced into an exhaust system. Some comments here are completely wrong. Another example of a person not understanding any part of exhaust systems would be if they made a comment "hey , going to a 3 inch dual exhaust setup would be to much because there wouldn't be enough back pressure" (I heard this at exhaust shop) . Now if this person had substituted "exhaust velocity" in place of "back pressure" then he would have been correct.
The idea in a nutshell is that the previous exhuast pulse pulls away fast and creates a negative pressure zone behind it.
The faster the exhuast pulse pulls away, the HIGHER your back pressure. Pressure = (V^2/2g)(specific weight)
with too big of a pipe, you will not have a negative pressure, and it will be close to atmoshere.
again, Pressure differential = V^2/2g (specific weight)
Its the same reason why a huge intake runner on a small engine will fail until very very high rpm, and even then it can be not as effective.
Also, keep in mind that all 2/3/4/5/6/8/10/12 (however many cylinders) pulses from the engine have to work together. This usually means that they have to meet in the collecter in a manner to support each other. Broken down again, this means not colliding with each other but one exhaust pulse pulling another one.
To do this, the obvious answer might be "equal length!" but no, not at all. Equal radii are faaar more important. Check out some basic head losses in a fluids text book. the losses due to length are mooy's number* length/diamter- (check out moody's diagram is usually VERY small number) in a system around 20'' with multiple bends and intersection when compared to geometric losses (bends, twists, change of size, intersections etc).Air, even more so than most liquids, HATES bending. it goes all disort fairly quickly.
getting a bit more complex, as far as i know, back pressure pressure doesn't equal to a restiction, which i should have put in my original post on page one.
For example- wrapping your headers will create more backpressure, but no more restiction bringing us full scale to what mr. bernuilli said.
Backpressure/SW = V^2/2g + Change in elevation + head losses (effectively, all restrictions)
Flowrate (v) = V*cross sectional area. as you can tell, you want the highest backpressure without adding restrictions. High velocity is where its all at.
- thats as far as I can see, please feel free to correct.
Last edited by Boost It!; Mar 25, 2005 at 01:26 PM.
Registered User
Joined: Sep 2002
Posts: 655
Re: Back Pressure, too much or not enough?
I'm not in a position to correct anyone's post, but I have a challenge to the
following:
"The faster the exhuast pulse pulls away, the HIGHER your back pressure. Pressure = (V^2/2g)(specific weight)"
Bernoulli also said a fast moving fluid (gas/fluid in motion) in a tube, will also
create a low pressure in the tube.
From this theory, I can infer that a fast moving pulse will create low pressure
resulting in a lower "backpressure".
Keeing in mind, the pressure throughout the pipe will be different because
we are dealing with a pulse, and not a steady flow.
For instance at the end of the pipe the pressure may be +10 PSI, the middle
of the pipe may be +5 PSI and the entry of the pipe may be +1 PSI along the
same pressure wave.
"Flowrate (v) = V*cross sectional area. as you can tell, you want the highest backpressure without adding restrictions. High velocity is where its all at."
I feel this statement is contradicting itself. With backpressure, you impede
velocity.
It is our job to time the pulses to arrive at their respective valves at open
to ensure a low pressure area for exhaust, and a high pressure ramming
effect at the intake valve.
Using a specific length of pipe at a steady RPM, the valve timing can be
matched to open the valve to receive the reflected pulse.
For those that don't believe that subtle changes in exhaust affect performance,
ask all of those people that have posted here why they lose torque when
the pipes are dropped from the collectors and back.
following:
"The faster the exhuast pulse pulls away, the HIGHER your back pressure. Pressure = (V^2/2g)(specific weight)"
Bernoulli also said a fast moving fluid (gas/fluid in motion) in a tube, will also
create a low pressure in the tube.
From this theory, I can infer that a fast moving pulse will create low pressure
resulting in a lower "backpressure".
Keeing in mind, the pressure throughout the pipe will be different because
we are dealing with a pulse, and not a steady flow.
For instance at the end of the pipe the pressure may be +10 PSI, the middle
of the pipe may be +5 PSI and the entry of the pipe may be +1 PSI along the
same pressure wave.
"Flowrate (v) = V*cross sectional area. as you can tell, you want the highest backpressure without adding restrictions. High velocity is where its all at."
I feel this statement is contradicting itself. With backpressure, you impede
velocity.
It is our job to time the pulses to arrive at their respective valves at open
to ensure a low pressure area for exhaust, and a high pressure ramming
effect at the intake valve.
Using a specific length of pipe at a steady RPM, the valve timing can be
matched to open the valve to receive the reflected pulse.
For those that don't believe that subtle changes in exhaust affect performance,
ask all of those people that have posted here why they lose torque when
the pipes are dropped from the collectors and back.
Last edited by Zero_to_69; Mar 25, 2005 at 10:18 PM.
Registered User
Joined: Oct 2004
Posts: 236
From: Costa Mesa, CA no more!
Re: Back Pressure, too much or not enough?
pressure differentials are what DRIVE flow. more pressure differential = more velocity/flow (since Area is constant)
restrictions will drop the velocity are "turn it into" pressure. think sticking your hand out the window at 70mph. Stagnation pressure I beleive is the word.
restrictions will drop the velocity are "turn it into" pressure. think sticking your hand out the window at 70mph. Stagnation pressure I beleive is the word.
Registered User
Joined: Sep 2002
Posts: 655
Re: Back Pressure, too much or not enough?
pressure differentials are what DRIVE flow. more pressure differential = more velocity/flow (since Area is constant)
I have no problem understanding that.
IE:
Intake runner pressure = 5 PSI
Cyilnder chamber pressure = - 10 PSI
Flow moves from intake runner to chamber
Intake runner pressure = 5 PSI
Cyilnder chamber pressure = - 20 PSI
Flow moves from intake runner to chamber with an increase in cylinder filling
Intake runner pressure = 5 PSI
Cyilnder chamber pressure = 30 PSI
Flow moves from chamber to intake runner (reversion)
It`s the back pressure statement in your previous post that I`m questioning.
I have no problem understanding that.
IE:
Intake runner pressure = 5 PSI
Cyilnder chamber pressure = - 10 PSI
Flow moves from intake runner to chamber
Intake runner pressure = 5 PSI
Cyilnder chamber pressure = - 20 PSI
Flow moves from intake runner to chamber with an increase in cylinder filling
Intake runner pressure = 5 PSI
Cyilnder chamber pressure = 30 PSI
Flow moves from chamber to intake runner (reversion)
It`s the back pressure statement in your previous post that I`m questioning.
Registered User
Joined: Sep 2002
Posts: 655
Re: Back Pressure, too much or not enough?
Stagnation according to one source:
"The stagnation or total pressure, p_0, is the pressure measured at the point where the fluid comes to rest. It is the highest pressure found anywhere in the flowfield, and it occurs at the stagnation point. It is the sum of the static pressure (p_0), and the dynamic pressure measured far upstream. It is called the dynamic pressure because it arises from the motion of the fluid."
If backpressure is due to stagnate flow of the gasses exiting the exhaust
system, I can understand how this term applies.
The dynamic changes in pressure within the exhaust tube are meeting
atmospheric pressure causing the exhaust gas to slow down upon exiting
creating backpressure.
By another definitoin, stagnate pressure occurs once a fluid hits a surface
and comes to rest.
Stagnate fluid/gas according to one source:
"cease to flow; stand without moving; "Stagnating waters"; "blood stagnates in the capillaries"
"The fluid along the dividing, or "stagnation streamline'' slows down and eventually comes to rest without deflection at the stagnation point."
The above definitions to the best of my knowledge cannot occur in an open
ended dynamic environemt (exhaust tube). This would imply that there is a constant blockage of
flow due to a surface obstruction. The above definiton also implies that
there is no reflection, compression, or rarefaction of the gas pulses.
With a muffler in place, I can see where the fluid along the dividing, or "stagnation streamline'' slows down and eventually comes to rest without deflection at the stagnation point where the gas flow meets a baffle in a
muffler.
Even then, there is a reflection and continued flow around the baffle.
In either case, how can either scenario improve gas velocity in a tube?
That is my question and challenge.
I may be misunderstanding stagnate to mean a fixed static pressure, as opposed
to a dynamic impedence to flow?
Hopefully somebody with supporting information can help us out.
"The stagnation or total pressure, p_0, is the pressure measured at the point where the fluid comes to rest. It is the highest pressure found anywhere in the flowfield, and it occurs at the stagnation point. It is the sum of the static pressure (p_0), and the dynamic pressure measured far upstream. It is called the dynamic pressure because it arises from the motion of the fluid."
If backpressure is due to stagnate flow of the gasses exiting the exhaust
system, I can understand how this term applies.
The dynamic changes in pressure within the exhaust tube are meeting
atmospheric pressure causing the exhaust gas to slow down upon exiting
creating backpressure.
By another definitoin, stagnate pressure occurs once a fluid hits a surface
and comes to rest.
Stagnate fluid/gas according to one source:
"cease to flow; stand without moving; "Stagnating waters"; "blood stagnates in the capillaries"
"The fluid along the dividing, or "stagnation streamline'' slows down and eventually comes to rest without deflection at the stagnation point."
The above definitions to the best of my knowledge cannot occur in an open
ended dynamic environemt (exhaust tube). This would imply that there is a constant blockage of
flow due to a surface obstruction. The above definiton also implies that
there is no reflection, compression, or rarefaction of the gas pulses.
With a muffler in place, I can see where the fluid along the dividing, or "stagnation streamline'' slows down and eventually comes to rest without deflection at the stagnation point where the gas flow meets a baffle in a
muffler.
Even then, there is a reflection and continued flow around the baffle.
In either case, how can either scenario improve gas velocity in a tube?
That is my question and challenge.
I may be misunderstanding stagnate to mean a fixed static pressure, as opposed
to a dynamic impedence to flow?
Hopefully somebody with supporting information can help us out.
Last edited by Zero_to_69; Mar 26, 2005 at 10:10 AM.
Registered User
Joined: Sep 2002
Posts: 655
Re: Back Pressure, too much or not enough?
Oh my gawd! Somebody check hell, it must have frozen over; people are asking me for opinions! 
I have no clue about the specs on the LEx components, but I've read good
things about them. Seems like you have a nice free flowing exhaust to support
the cam and heads.
I have no clue about the specs on the LEx components, but I've read good
things about them. Seems like you have a nice free flowing exhaust to support
the cam and heads.
Registered User
Joined: May 2001
Posts: 160
Re: Back Pressure, too much or not enough?
There goes another quote that explains it a bit more
"Scavenging has to do with exhaust gas velocity. Momentum of moving exhaust pulses help to pull the gasses out of the engine. Since momentum is mass x velocity, high velocity is good. This is where the confusion comes in. High velocity comes from smaller exhaust tubing. So, assuming smaller tubes could support the required flow, they are good. However, small tubes can't flow the same volume as easily as larger tubes (frictional losses, etc). So, we need to be careful not to confuse velocity with backpressure. Yes, small tubes create both. Velocity is good, but backpressure is bad. Actually, I'd say backpressure is worse than velocity is good, if I have to pick sides. Ideally, you'd want the smallest tubing that will support the flow that you need. Not oversized (reduced velocity/momentum) and not undersized (can't flow enough"
"Scavenging has to do with exhaust gas velocity. Momentum of moving exhaust pulses help to pull the gasses out of the engine. Since momentum is mass x velocity, high velocity is good. This is where the confusion comes in. High velocity comes from smaller exhaust tubing. So, assuming smaller tubes could support the required flow, they are good. However, small tubes can't flow the same volume as easily as larger tubes (frictional losses, etc). So, we need to be careful not to confuse velocity with backpressure. Yes, small tubes create both. Velocity is good, but backpressure is bad. Actually, I'd say backpressure is worse than velocity is good, if I have to pick sides. Ideally, you'd want the smallest tubing that will support the flow that you need. Not oversized (reduced velocity/momentum) and not undersized (can't flow enough"
Registered User
Joined: Oct 2004
Posts: 236
From: Costa Mesa, CA no more!
Re: Back Pressure, too much or not enough?
Originally Posted by Zero_to_69
Oh my gawd! Somebody check hell, it must have frozen over; people are asking me for opinions!
I have no clue about the specs on the LEx components, but I've read good
things about them. Seems like you have a nice free flowing exhaust to support
the cam and heads.
I have no clue about the specs on the LEx components, but I've read good
things about them. Seems like you have a nice free flowing exhaust to support
the cam and heads.
since we are getting technical, its my opinion (not that anyone asked) that hooker/jethot headers suck. nowhere near equal length (not that its important), all kinds of different radii bends and different number of them. its all crazy!
even for trucks where packaging isn't a real issue, that problem persists
Registered User
Joined: May 2001
Posts: 160
Re: Back Pressure, too much or not enough?
Originally Posted by engineermike
I believe that quote to be refering to header primaries, not from the collector back.
