Exhaust Backpressure ???
03-12-2004, 02:45 AM
#1
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Exhaust Backpressure ???
What does Exhaust Backpressure mean to you ????
1- actual total pressure in PSI in exhaust system
2- the pressure influence of returning (Back) positive acoustical wave
3- other (state description)
1- actual total pressure in PSI in exhaust system
2- the pressure influence of returning (Back) positive acoustical wave
3- other (state description)
Last edited by MaxRaceSoftware; 03-12-2004 at 02:48 AM.
03-12-2004, 08:22 AM
#3
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My thoughts:
I think of it as the average (not instantaneous) pressure in the exhaust measured just before the muffler(s) with a mechanical pressure gage, like a fuel pressure gage. That would be closest to choice 1.
There are lots of pressure variations upstream in the headers and collectors as in your choice 2. Measuring these variations would require real time gaging with pretty quick response time.
In my somewhat convoluted mind, "back pressure" relates to the overall mechanical flow of the gasses, not the acoustical tuning pressures, even though there are certainly "backpressure" pulses, and well as "negativepressure" pulses. How about "compressions" and "rarefractions" in the flow?
I guess the terminology could get confusing much like Dynamic Compression Ratio which is often just a fixed calculated value for a given engine combination, and Effective Compression Ratio which is what the engine sees during opoeration and varies a lot, probably with VE (which means Volumetic Efficiency to some and Valve Events to others.
)
Larry, what's your answer?
I think of it as the average (not instantaneous) pressure in the exhaust measured just before the muffler(s) with a mechanical pressure gage, like a fuel pressure gage. That would be closest to choice 1.
There are lots of pressure variations upstream in the headers and collectors as in your choice 2. Measuring these variations would require real time gaging with pretty quick response time.
In my somewhat convoluted mind, "back pressure" relates to the overall mechanical flow of the gasses, not the acoustical tuning pressures, even though there are certainly "backpressure" pulses, and well as "negativepressure" pulses. How about "compressions" and "rarefractions" in the flow?
I guess the terminology could get confusing much like Dynamic Compression Ratio which is often just a fixed calculated value for a given engine combination, and Effective Compression Ratio which is what the engine sees during opoeration and varies a lot, probably with VE (which means Volumetic Efficiency to some and Valve Events to others.
)Larry, what's your answer?
03-12-2004, 10:21 AM
#4
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I'll let you know after reading my book.
For now, all I can say is that it's a nasty term and I can't find any
benefit of having backpressure in the exhaust system at this point
in reading (chapter 4).
My non-scientific response would be the amount of "fluid pressure"
built up between the exhaust valve and restriction point(s) as the
piston tries to force more charge out of the cylinder at higher RPM.
The restrictions may be pipe design (shape;volume (length & diameter); pipe material; etc), catalytic flow, muffler flow and atmospheric pressure.
For now, all I can say is that it's a nasty term and I can't find any
benefit of having backpressure in the exhaust system at this point
in reading (chapter 4).
My non-scientific response would be the amount of "fluid pressure"
built up between the exhaust valve and restriction point(s) as the
piston tries to force more charge out of the cylinder at higher RPM.
The restrictions may be pipe design (shape;volume (length & diameter); pipe material; etc), catalytic flow, muffler flow and atmospheric pressure.
03-12-2004, 10:51 AM
#5
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Larry, what's your answer?
=======================
i just wanted to see what majority thought meaning was to them
number 1 will probably be the majority
-----------------------------------
like you said about DCR terms can cause a little confusion
same with "Back Pressure"
------------------------------------
i look at Back Pressure as total exhaust system pressure in psi
and what psi might exists near or at exhaust valve, especially during overlap period.
there is no way an increase exhaust back pressure in psi
during overlap period or exhaust stroke, that will help HP/Torque
more exhaust psi should cause more exhaust dilution of mixture during overlap , along with less dense mixture from increased heat, and reduced intake port velocity along with reduced ram-effect near intake valve closing point (Ve Loss)
if you flow 1 5/8 headers -vs- 1 3/4 -vs- 1 7/8 -vs- 2 inch on FlowBench there isn't but a few CFM differences between all these sizes ..yet substantial HP/TQ differences
if you flow test a 1 3/4 dia header , one 20 inches long -vs- another at 34 inches long ...on the Flowbench there isn't much CFM difference ..but in live engine there can be huge differences at certain RPMs
so when changing/testing header pipe diameters and lengths
you are changing wave tuning
what you misinterupt as "Back Pressure" changing is really wave tuning changes..and not changes in system psi
some racer might say, i made a pass without mufflers with just open headers and slowed down ..i guess the engine needed the "back pressure" ?
or another racer might say , my car went faster with collector add on length..i guess it needed the "backpressure"
reality=> wave tuning was cause of effects, not backpressure
in all research papers i've read, back pressure always hurts HP/Torque
=======================
i just wanted to see what majority thought meaning was to them
number 1 will probably be the majority
-----------------------------------
like you said about DCR terms can cause a little confusion
same with "Back Pressure"
------------------------------------
i look at Back Pressure as total exhaust system pressure in psi
and what psi might exists near or at exhaust valve, especially during overlap period.
there is no way an increase exhaust back pressure in psi
during overlap period or exhaust stroke, that will help HP/Torque
more exhaust psi should cause more exhaust dilution of mixture during overlap , along with less dense mixture from increased heat, and reduced intake port velocity along with reduced ram-effect near intake valve closing point (Ve Loss)
if you flow 1 5/8 headers -vs- 1 3/4 -vs- 1 7/8 -vs- 2 inch on FlowBench there isn't but a few CFM differences between all these sizes ..yet substantial HP/TQ differences
if you flow test a 1 3/4 dia header , one 20 inches long -vs- another at 34 inches long ...on the Flowbench there isn't much CFM difference ..but in live engine there can be huge differences at certain RPMs
so when changing/testing header pipe diameters and lengths
you are changing wave tuning
what you misinterupt as "Back Pressure" changing is really wave tuning changes..and not changes in system psi
some racer might say, i made a pass without mufflers with just open headers and slowed down ..i guess the engine needed the "back pressure" ?
or another racer might say , my car went faster with collector add on length..i guess it needed the "backpressure"
reality=> wave tuning was cause of effects, not backpressure
in all research papers i've read, back pressure always hurts HP/Torque
03-12-2004, 10:57 AM
#6
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^ I get in debates about that all the time hence the creation of
the "Backpressure vs. Resonance Tuning" thread I started a couple
of months ago.
Now with all the help from this forum and the book, I'm going to
annihilate the next person that tries to tell me backpressure is beneficial!
the "Backpressure vs. Resonance Tuning" thread I started a couple
of months ago.
Now with all the help from this forum and the book, I'm going to
annihilate the next person that tries to tell me backpressure is beneficial!
03-12-2004, 08:09 PM
#7
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Location: Indiana
Posts: 587
From the perspective of the OEMs, backpressure is entirely #1. We measure backpressure at a tap just in front of the catalyst. Some groups meausure static backpressure only and some like static + dynamic using a Kiel probe. Either way, they like to see peak average pressure at WOT. The pulse tuning you refer to isn't really brought up that I've ever heard for normal cars.
I messed around with some sampling hardware and transducers years ago that were just about fast enough to see the individual pulses. There wasn't any real need for that type of resolution at the OEM level, so we stopped pursuing it.
This is a whole different class.
I messed around with some sampling hardware and transducers years ago that were just about fast enough to see the individual pulses. There wasn't any real need for that type of resolution at the OEM level, so we stopped pursuing it.
This is a whole different class.
03-12-2004, 08:21 PM
#8
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Location: BTR, Louisiana
Posts: 735
Zero, 
In some cases back pressure is beneficial. Keep reading. Cut all of the exhaust off a 5.3L truck and they don't run any faster. Those motors are all ready lacking in the low-end department.
http://www.pacific-audio.com/perform...iscussion.html
Go make a post here or search.
Ben T.
In some cases back pressure is beneficial. Keep reading. Cut all of the exhaust off a 5.3L truck and they don't run any faster. Those motors are all ready lacking in the low-end department.
http://www.pacific-audio.com/perform...iscussion.html
Go make a post here or search.
Ben T.
03-12-2004, 10:17 PM
#9
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Posts: 655
i"m going to disagree...but I'm not going to debate because my terminology is not up to par with many who post here.
The "backpressure" everyone refers to is misunderstood.
The engine requires some sort of medium to channel the 'fluid' out
of the chamber.
I'll let the big boys debate for me. Any scientific article that I have
read does not favor any sort of backpressure to make power/torque.
The "backpressure" everyone refers to is misunderstood.
The engine requires some sort of medium to channel the 'fluid' out
of the chamber.
I'll let the big boys debate for me. Any scientific article that I have
read does not favor any sort of backpressure to make power/torque.
03-13-2004, 08:30 AM
#13
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Location: Upstate NY
Posts: 2,931
Originally posted by Zero_to_69
Oscar Wilde the poet?
What does he know about engine technology?
Oscar Wilde the poet?
What does he know about engine technology?
LOL! Like many of us Old Farts, Oscar finally realized that sometimes with age comes wisdom.
The downside to that is: "Sometimes Age shows up alone."
03-13-2004, 08:36 AM
#14
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Location: houston, Tx
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"Sometimes Age shows up alone."
This happens more than I'd expect! Sometimes people's experiences are more limited than they guess and all this supposed "experience" is basically the same things happening over and over again so it becomes the truth to this particular old codger. In turn then they become close minded and jump to conclusions. Even worse are those with NO experience that have strong opinions however!
This happens more than I'd expect! Sometimes people's experiences are more limited than they guess and all this supposed "experience" is basically the same things happening over and over again so it becomes the truth to this particular old codger. In turn then they become close minded and jump to conclusions. Even worse are those with NO experience that have strong opinions however!
03-13-2004, 11:05 AM
#15
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Location: Upstate NY
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Originally posted by racer7088
"Sometimes Age shows up alone."
This happens more than I'd expect! Sometimes people's experiences are more limited than they guess and all this supposed "experience" is basically the same things happening over and over again so it becomes the truth to this particular old codger. In turn then they become close minded and jump to conclusions.
Should I resemble that remark?
Even worse are those with NO experience that have strong opinions however!
That happens about as much as I expect!
"Sometimes Age shows up alone."
This happens more than I'd expect! Sometimes people's experiences are more limited than they guess and all this supposed "experience" is basically the same things happening over and over again so it becomes the truth to this particular old codger. In turn then they become close minded and jump to conclusions.
Should I resemble that remark?
Even worse are those with NO experience that have strong opinions however!
That happens about as much as I expect!