Exhaust Backpressure ???
03-18-2004, 06:31 AM
#31
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Join Date: Sep 2002
Posts: 655
If 'stars' taste as good as cookies, I'm a happy man!
OK...pipe volume after the collector does not affect gas velocity
as much as I figured.
1.) For sake of argument, let's state that our 350 CID engine needs
800 CFM to make all its power.
If we were to reduce the extension pipe down to 1.5" diameter x 4' long,
the exhaust gas velocity would increase at a lower RPM correct?
On the other hand, 1.5" x 4' would not be sufficient for flow at
wide open throttle?
I don't recall the formula, but I'm thinking 800 CFM would need at
least 2.5" diameter to flow unrestricted?
Basically, all we would need for maximum exhaust pipe diameter
is what the engine requires at the intake (charge in = charge out).
2.) With respect to wave tuning, the length of the extension pipe is
more a factor (at full wave lengths, or harmonic lengths) than the
diameter?
So...a 4' extension would be resonant at xxx RPM, where 3' would
be resonant at a "certain" higher RPM (xxx + xx RPM) .
3.) Lastly, regarding the wave travel through the "medium":
If the acoustic wave, or pressuve wave length is longer than the
pipe length; the effect of the pulse peak (which is occuring somewhere
beyond the extension pipe exit) is not interfered with the atmosphere
residing in the extension pipe.
On the flip side, if the extension pipe was longer than the acoustic
wave, or pressure wave, the peak of the pulse is occuring within
the pipe and creating more of an impact at the exhaust port.
Please correct my errors
Hopefully I am starting to grasp these points. Keeping in mind the
pipe lengths and diameters would need to change size to be 100% efficient
across the RPM range in the above examples.
OK...pipe volume after the collector does not affect gas velocity
as much as I figured.
1.) For sake of argument, let's state that our 350 CID engine needs
800 CFM to make all its power.
If we were to reduce the extension pipe down to 1.5" diameter x 4' long,
the exhaust gas velocity would increase at a lower RPM correct?
On the other hand, 1.5" x 4' would not be sufficient for flow at
wide open throttle?
I don't recall the formula, but I'm thinking 800 CFM would need at
least 2.5" diameter to flow unrestricted?
Basically, all we would need for maximum exhaust pipe diameter
is what the engine requires at the intake (charge in = charge out).
2.) With respect to wave tuning, the length of the extension pipe is
more a factor (at full wave lengths, or harmonic lengths) than the
diameter?
So...a 4' extension would be resonant at xxx RPM, where 3' would
be resonant at a "certain" higher RPM (xxx + xx RPM) .
3.) Lastly, regarding the wave travel through the "medium":
If the acoustic wave, or pressuve wave length is longer than the
pipe length; the effect of the pulse peak (which is occuring somewhere
beyond the extension pipe exit) is not interfered with the atmosphere
residing in the extension pipe.
On the flip side, if the extension pipe was longer than the acoustic
wave, or pressure wave, the peak of the pulse is occuring within
the pipe and creating more of an impact at the exhaust port.
Please correct my errors
Hopefully I am starting to grasp these points. Keeping in mind the
pipe lengths and diameters would need to change size to be 100% efficient
across the RPM range in the above examples.
Last edited by Zero_to_69; 03-18-2004 at 06:36 AM.
03-18-2004, 07:15 AM
#32
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Join Date: Oct 2002
Location: Upstate NY
Posts: 2,931
Originally posted by Zero_to_69
Basically, all we would need for maximum exhaust pipe diameter
is what the engine requires at the intake (charge in = charge out).
Basically, all we would need for maximum exhaust pipe diameter
is what the engine requires at the intake (charge in = charge out).
03-18-2004, 04:34 PM
#34
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Join Date: Oct 2001
Location: smog zone adjacent to a great lake
Posts: 1,462
Originally posted by Zero_to_69
Right...
Right...
Last edited by arnie; 03-18-2004 at 07:45 PM.
03-18-2004, 09:41 PM
#36
Registered User
Join Date: Feb 2002
Posts: 2,428
how extreme can back pressure get? a while back i was wondering if you could use a really radical cam that closed fast (i forget the official tech term for that). by closing it faster and at the right spot you could make tremendous back pressure, enough to pull air in fast enough to creat boost. it would probably rob a little power as stress on the valves to cam. is that even possible, i was so confident when i first thought of it, hahaah.
03-18-2004, 09:56 PM
#37
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Join Date: Oct 2001
Location: smog zone adjacent to a great lake
Posts: 1,462
Originally posted by Zero_to_69
Do you mean progressively reducing the pipe diameter as the
gas travels down the length of the exhaust?
Do you mean progressively reducing the pipe diameter as the
gas travels down the length of the exhaust?
03-18-2004, 09:57 PM
#38
Registered User
Join Date: Sep 2002
Posts: 655
RE: Number77
IMHO, any amount of backpressure is extreme as it will degrade
power.
As for your idea about the backpressure "ramming" more air to
pressurize the cylinder, it ain't happening.
Higher pressure in the exhaust (backpressure) will prevent air
from entering the cylinder because there is a blockage in flow
in the exhaust port.
The bottleneck will cause the exhaust gas to remain in the cylinder
not allowing fresh intake charge to enter.
IMHO, any amount of backpressure is extreme as it will degrade
power.
As for your idea about the backpressure "ramming" more air to
pressurize the cylinder, it ain't happening.
Higher pressure in the exhaust (backpressure) will prevent air
from entering the cylinder because there is a blockage in flow
in the exhaust port.
The bottleneck will cause the exhaust gas to remain in the cylinder
not allowing fresh intake charge to enter.
03-21-2004, 02:44 AM
#41
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Join Date: Mar 2002
Location: Baton Rouge, LA, USA
Posts: 2,743
I believe that backpressure can help hp/torque.
If you are overscavenging (allowing air/fuel to short-circuit though the chamber and out the exhaust during overlap) due to good low-lift flow, well tuned exhaust header, and well tuned intake runner, then backpressure can help hold the air/fuel in the chamber.
Granted, this seems to be a band-aid fix to a problem with the combination.
At any rate, nearly all of the modern high-performance street bikes have exhaust backpressure valves that adjust backpressure for the rpm/load. My theory is that they overscavenge in certain operating conditions, so the valve increases backpressure to hold the air in.
My '04 ZX-10R has a backpressure valve and makes 164 rwhp from 62 cid.
Mike
If you are overscavenging (allowing air/fuel to short-circuit though the chamber and out the exhaust during overlap) due to good low-lift flow, well tuned exhaust header, and well tuned intake runner, then backpressure can help hold the air/fuel in the chamber.
Granted, this seems to be a band-aid fix to a problem with the combination.
At any rate, nearly all of the modern high-performance street bikes have exhaust backpressure valves that adjust backpressure for the rpm/load. My theory is that they overscavenge in certain operating conditions, so the valve increases backpressure to hold the air in.
My '04 ZX-10R has a backpressure valve and makes 164 rwhp from 62 cid.
Mike
03-21-2004, 03:29 PM
#42
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Thread Starter
Join Date: Feb 2004
Location: Abbeville , LA
Posts: 291
At any rate, nearly all of the modern high-performance street bikes have exhaust backpressure valves that adjust backpressure for the rpm/load. My theory is that they overscavenge in certain operating conditions, so the valve increases backpressure to hold the air in.
========================================
maybe or maybe not ?
"exhaust backpressure valves that adjust backpressure for the rpm/load."
its very possible this adjustible valve is still really causing a "negative" wave at exhaust/chamber during overlap period
the adjustible valve will change exhaust gas temps and densities to different values along exhaust pipe lengths
one way to determine would be p-v diagram or pressure transducer readouts.
========================================
maybe or maybe not ?
"exhaust backpressure valves that adjust backpressure for the rpm/load."
its very possible this adjustible valve is still really causing a "negative" wave at exhaust/chamber during overlap period
the adjustible valve will change exhaust gas temps and densities to different values along exhaust pipe lengths
one way to determine would be p-v diagram or pressure transducer readouts.
03-21-2004, 03:41 PM
#43
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Thread Starter
Join Date: Feb 2004
Location: Abbeville , LA
Posts: 291
that one Chrysler SS Hemi engine with 140.0+% Ve on my dyno was the worst "overscavenging" example i've ever seen
but the majority of its problem was just solved with new Cam change
and would have probably benefitted more with different cylinder head porting
all that change in overscavenging ..."WITHOUT" changing anything in exhaust headers
you always want the headers to work or scavenge,
but just because the SS Hemi had the completely wrong Cam in it and was combined with extremely good "Low-Lift" flowing ports
produced a very bad combination...it really wasn't fault of headers
but the majority of its problem was just solved with new Cam change
and would have probably benefitted more with different cylinder head porting
all that change in overscavenging ..."WITHOUT" changing anything in exhaust headers
you always want the headers to work or scavenge,
but just because the SS Hemi had the completely wrong Cam in it and was combined with extremely good "Low-Lift" flowing ports
produced a very bad combination...it really wasn't fault of headers
03-22-2004, 07:08 AM
#44
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Join Date: Jan 2001
Location: houston, Tx
Posts: 293
If backpressure is helping power then the cam is in way wrong. At the most this could only used at lower speeds because it is not going to be there during the good part of the power band. Restricting the exhaust like restricting the intake is not going to ever help power in the fat part of the power band. This must be a bandaid from hell used in place of variable valve timing or something else but it is still interesting. The motorcycle market is famous for gimmicry that appears and then disappears like clockwork. If one of them does it then they will usually all do it so they can have that bell or whistle too. I know at least one of those systems is supposed to be worth some decent power if you remove it from what I've been told.
03-27-2004, 07:22 AM
#45
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Location: Mobile, Ala..USA
Posts: 1,935
My '04 ZX-10R has a backpressure valve and makes 164 rwhp from 62 cid.
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This a stock ZX-10? Damnit boy, I needed a FULL array of mods on a 2001 Hayabusa to make that power, anyway I used to be heavily into sportbikes at one time. I rode a Gixxer 1000 when it was new with the valve in the exhaust, shortly after he put a Hindle race pipe on. With the valve in place in second gear roll on from a low rpm it would pick the front tire up in a lower RPM range than it would with the Hindle race system. Seemed to pull harder in the upper ranges and lose some lower RPM grunt with the Hindle on it. WIth no "flapper valve" as we called it. Hindle race system seemed to have the same size primary pipe, maybe even slightly smaller.
Anyway, anyone see my post on exhaust in this section....no one has really answered. I would really like to know some thoughts on it.
David
AKA FASTFATBOY
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This a stock ZX-10? Damnit boy, I needed a FULL array of mods on a 2001 Hayabusa to make that power, anyway I used to be heavily into sportbikes at one time. I rode a Gixxer 1000 when it was new with the valve in the exhaust, shortly after he put a Hindle race pipe on. With the valve in place in second gear roll on from a low rpm it would pick the front tire up in a lower RPM range than it would with the Hindle race system. Seemed to pull harder in the upper ranges and lose some lower RPM grunt with the Hindle on it. WIth no "flapper valve" as we called it. Hindle race system seemed to have the same size primary pipe, maybe even slightly smaller.
Anyway, anyone see my post on exhaust in this section....no one has really answered. I would really like to know some thoughts on it.
David
AKA FASTFATBOY