Backpressure-
Prelussion
12-01-2001, 10:47 PM
A lot of people have different thoughts on backpressure, and often confuse it with Velocity and Delta Pressure...
I will now post a colaboration of posts from Purehonda.com
"THE MYTH OF BACKPRESSURE"
…is probably the most widely misunderstood concept in engine tuning. IMO, the reason this concept is so hard to get around lies in the engineering terms surrounding gas flow. Here's the most impotant ones you need to be aware of to understand the things I'm about to say:
BACKPRESSURE: Resistance to air flow; usually stated in inches H2O or PSI.
DELTA PRESSURE (aka delta P): Describes the pressure drop through a component and is the difference in pressure between two points.
One other concept needs to be covered too, and that's the idea of air pressure vs. velocity. When a moving air column picks up speed, one of the weird things that happens is it’s pressure drops. So remember through all this that the higher the air velocity for a given volume of gas, the lower it's internal pressure becomes. And remember throughout all of this that I’m no mechanical engineer, simply an enthusiast who done all the reading he can. I don’t claim that this information is the absolute truth, just that it makes sense in my eyes.
Ok, so as you can see, backpressure is actually defined as the resistance to flow. So how can backpressure help power production at any RPM? IT CAN'T. I think the reason people began to think that pressure was in important thing to have at low RPM is because of the term delta pressure. Delta pressure is what you need to produce good power at any RPM, which means that you need to have a pressure DROP when measuring pressures from the cylinder to the exhaust tract (the term "pressure" is what I think continually confuses things). The larger the delta P measurement is, the higher this pressure drop becomes. And as earlier stated, you can understand that this pressure drop means the exhaust gas velocity is increasing as it travels from the cylinder to the exhaust system. Put simply, the higher the delta P value, the faster the exhaust gasses end up traveling. So what does all this mean? It means that it's important to have gas velocity reach a certain point in order to have good power production at any RPM (traditional engine techs sited 240 ft/sec as the magic number, but this is likely outdated by now).
The effect of having larger exhaust pipe diameters (in the primary, secondary, collector and cat-back exhaust tubes) has a direct effect on gas velocity and therefore delta P (as well as backpressure levels). The larger the exhaust diameter, the slower the exhaust gasses end up going for a given amount of airflow. Now the ***** of all this tech is that one exhaust size will not work over a large RPM range, so we are left with trying to find the best compromise in sizing for good low RPM velocity without hindering higher RPM flow ability. It doesn't take a rocket scientist to understand that an engine flows a whole lot more air at 6000 RPM than at 1000 RPM, and so it also makes sense that one single pipe diameter isn't going to acheive optiaml gas velocity and pressure at both these RPM points, given the need to flow such varying volumes.
These concepts are why larger exhaust piping works well for high RPM power but hurts low RPM power; becuase is hurts gas velocity and therefore delta P at low RPM. At higher RPM however, the larger piping lets the engine breath well without having the exhuast gasses get bundled up in the system, which would produce high levels of backpressure and therefore hurt flow. Remember, managing airflow in engines is mainly about three things; maintaining laminar flow and good charge velocity, and doing both of those with varying volumes of air. Ok, so now that all this has been explained, let's cover one last concept (sorry this is getting so long, but it takes time to explain things in straight text!).
This last concept is why low velocity gas flow and backpressure hurt power production. Understand that during the exhaust stroke of a 4 stroke engine, it's not only important to get as much of the spent air/fuel mixture out of the chamber (to make room for the unburnt mixture in the intake system), it's also important that these exhaust gasses never turn around and start flowing back into the cylinder. Why would this happen? Because of valve overlap, that's why. At the end of the exhaust stroke, not only does the piston start moving back down the bore to ingest the fresh mixture, but the intake valve also opens to expose the fresh air charge to this event. In modern automotive 4 stroke engines valve overlap occurs at all RPM, so for a short period of time the exhaust system is open to these low pressure influences which can suck things back towards the cylinder. if the exhaust gas velocity is low and pressure is high in the system, this will make everything turn around and go the opposite direction it's supposed to. If these gasses reach the cylinder they will dilute the incoming mixture with unburnable gasses and take up valuable space within the combustion chamber, thus lowering power output (and potentially pushing the intake charge temp beyond the fuel’s knock resistance). So having good velocity and therefore low pressure in the system is absolutely imperative to good power production at any RPM, you just have to remember that these concepts are also dependent on total flow volume. The overall volume of flow is important because it is entirely possible to have both high velocity and high pressure in the system, if there is simply not enough exhaust piping to handle the needed airflow.
It’s all about finding a compromise to work at both high and low RPM on most cars, but that’s a bit beyond the scope of this post. All I am trying to show here is how the term backpressure is in reference to a bad exhaust system, not one that creates good low RPM torque. You can just as easily have backpressure at low RPM too, which would also hurt low RPM cylinder scavenging and increase the potential for gas reversion. And understand that these tuning concepts will also affect cam timing, though that is again probably beyond the scope of this post. At any rate, hope this helps, peace. "
-here's a reply to the above post-
"I've been seeing a resurgence of the backpressure misnomer, but didn't have the time or inclination to write it up. So, again, thanks.
There is one thing I'd like to add to texan's work:
Exhaust Scavenging
In essence, this is the opposite of the exhaust reversion that texan describes.
Reversion: at the beginning of the intake stroke during cam overlap, exaust gas in the header is under high pressure (negative delta P) and is pushed back into the cylinder, diluting the new air/fuel charge.
Scavenging: at the beginning of the intake stroke during cam overlap, the momentum of the exiting exhaust gasses creates a brief vacuum (positive delta P) in the header, pulling out the remaining exhaust gases from the combustion chamber, and allowing the new air/fuel charge to be full-strength.
Scavenging is also the reason for differently shaped headers (4-2-1, 4-1) and collectors. We use the momentum of exiting exhaust from one cylinder to scavenge exhaust from another that is next in the firing order! The different shapes allow for this to happen at different airflow velocities thus at different RPM bands.
Scavenging takes advantage of the momentum of the exiting gasses. In essence, the fast moving exhaust pulse pulls a vacuum behind it. Momentum is mass times velocity. So not only do we need to keep the velocity high to prevent reversion - but it greatly improves the scavenging effect.
Thus we have a balancing act (as others have pointed out). We want to minimize friction to lower the backpressure as much as possible - larger pipes have less friction because they have less surface area per unit volume. But we want to increase the delta P as much as possible to prevent reversion and increase scavenging effects - smaller pipes increase delta P because they increase velocity.
There are lots of tricks to try to widen the useful RPM band (stepped headers) or to increase the overall effiency (ceramic coated exhausts), but it's still subject to this basic tradeoff:
Friction vs. Velocity
AKA: Backpressure vs. Delta Pressure
You want low friction and high velocity.
You want low backpressure and high positive delta pressure. "
Credit given to Texan and Fritz for their info on this topic.
I will now post a colaboration of posts from Purehonda.com
"THE MYTH OF BACKPRESSURE"
…is probably the most widely misunderstood concept in engine tuning. IMO, the reason this concept is so hard to get around lies in the engineering terms surrounding gas flow. Here's the most impotant ones you need to be aware of to understand the things I'm about to say:
BACKPRESSURE: Resistance to air flow; usually stated in inches H2O or PSI.
DELTA PRESSURE (aka delta P): Describes the pressure drop through a component and is the difference in pressure between two points.
One other concept needs to be covered too, and that's the idea of air pressure vs. velocity. When a moving air column picks up speed, one of the weird things that happens is it’s pressure drops. So remember through all this that the higher the air velocity for a given volume of gas, the lower it's internal pressure becomes. And remember throughout all of this that I’m no mechanical engineer, simply an enthusiast who done all the reading he can. I don’t claim that this information is the absolute truth, just that it makes sense in my eyes.
Ok, so as you can see, backpressure is actually defined as the resistance to flow. So how can backpressure help power production at any RPM? IT CAN'T. I think the reason people began to think that pressure was in important thing to have at low RPM is because of the term delta pressure. Delta pressure is what you need to produce good power at any RPM, which means that you need to have a pressure DROP when measuring pressures from the cylinder to the exhaust tract (the term "pressure" is what I think continually confuses things). The larger the delta P measurement is, the higher this pressure drop becomes. And as earlier stated, you can understand that this pressure drop means the exhaust gas velocity is increasing as it travels from the cylinder to the exhaust system. Put simply, the higher the delta P value, the faster the exhaust gasses end up traveling. So what does all this mean? It means that it's important to have gas velocity reach a certain point in order to have good power production at any RPM (traditional engine techs sited 240 ft/sec as the magic number, but this is likely outdated by now).
The effect of having larger exhaust pipe diameters (in the primary, secondary, collector and cat-back exhaust tubes) has a direct effect on gas velocity and therefore delta P (as well as backpressure levels). The larger the exhaust diameter, the slower the exhaust gasses end up going for a given amount of airflow. Now the ***** of all this tech is that one exhaust size will not work over a large RPM range, so we are left with trying to find the best compromise in sizing for good low RPM velocity without hindering higher RPM flow ability. It doesn't take a rocket scientist to understand that an engine flows a whole lot more air at 6000 RPM than at 1000 RPM, and so it also makes sense that one single pipe diameter isn't going to acheive optiaml gas velocity and pressure at both these RPM points, given the need to flow such varying volumes.
These concepts are why larger exhaust piping works well for high RPM power but hurts low RPM power; becuase is hurts gas velocity and therefore delta P at low RPM. At higher RPM however, the larger piping lets the engine breath well without having the exhuast gasses get bundled up in the system, which would produce high levels of backpressure and therefore hurt flow. Remember, managing airflow in engines is mainly about three things; maintaining laminar flow and good charge velocity, and doing both of those with varying volumes of air. Ok, so now that all this has been explained, let's cover one last concept (sorry this is getting so long, but it takes time to explain things in straight text!).
This last concept is why low velocity gas flow and backpressure hurt power production. Understand that during the exhaust stroke of a 4 stroke engine, it's not only important to get as much of the spent air/fuel mixture out of the chamber (to make room for the unburnt mixture in the intake system), it's also important that these exhaust gasses never turn around and start flowing back into the cylinder. Why would this happen? Because of valve overlap, that's why. At the end of the exhaust stroke, not only does the piston start moving back down the bore to ingest the fresh mixture, but the intake valve also opens to expose the fresh air charge to this event. In modern automotive 4 stroke engines valve overlap occurs at all RPM, so for a short period of time the exhaust system is open to these low pressure influences which can suck things back towards the cylinder. if the exhaust gas velocity is low and pressure is high in the system, this will make everything turn around and go the opposite direction it's supposed to. If these gasses reach the cylinder they will dilute the incoming mixture with unburnable gasses and take up valuable space within the combustion chamber, thus lowering power output (and potentially pushing the intake charge temp beyond the fuel’s knock resistance). So having good velocity and therefore low pressure in the system is absolutely imperative to good power production at any RPM, you just have to remember that these concepts are also dependent on total flow volume. The overall volume of flow is important because it is entirely possible to have both high velocity and high pressure in the system, if there is simply not enough exhaust piping to handle the needed airflow.
It’s all about finding a compromise to work at both high and low RPM on most cars, but that’s a bit beyond the scope of this post. All I am trying to show here is how the term backpressure is in reference to a bad exhaust system, not one that creates good low RPM torque. You can just as easily have backpressure at low RPM too, which would also hurt low RPM cylinder scavenging and increase the potential for gas reversion. And understand that these tuning concepts will also affect cam timing, though that is again probably beyond the scope of this post. At any rate, hope this helps, peace. "
-here's a reply to the above post-
"I've been seeing a resurgence of the backpressure misnomer, but didn't have the time or inclination to write it up. So, again, thanks.
There is one thing I'd like to add to texan's work:
Exhaust Scavenging
In essence, this is the opposite of the exhaust reversion that texan describes.
Reversion: at the beginning of the intake stroke during cam overlap, exaust gas in the header is under high pressure (negative delta P) and is pushed back into the cylinder, diluting the new air/fuel charge.
Scavenging: at the beginning of the intake stroke during cam overlap, the momentum of the exiting exhaust gasses creates a brief vacuum (positive delta P) in the header, pulling out the remaining exhaust gases from the combustion chamber, and allowing the new air/fuel charge to be full-strength.
Scavenging is also the reason for differently shaped headers (4-2-1, 4-1) and collectors. We use the momentum of exiting exhaust from one cylinder to scavenge exhaust from another that is next in the firing order! The different shapes allow for this to happen at different airflow velocities thus at different RPM bands.
Scavenging takes advantage of the momentum of the exiting gasses. In essence, the fast moving exhaust pulse pulls a vacuum behind it. Momentum is mass times velocity. So not only do we need to keep the velocity high to prevent reversion - but it greatly improves the scavenging effect.
Thus we have a balancing act (as others have pointed out). We want to minimize friction to lower the backpressure as much as possible - larger pipes have less friction because they have less surface area per unit volume. But we want to increase the delta P as much as possible to prevent reversion and increase scavenging effects - smaller pipes increase delta P because they increase velocity.
There are lots of tricks to try to widen the useful RPM band (stepped headers) or to increase the overall effiency (ceramic coated exhausts), but it's still subject to this basic tradeoff:
Friction vs. Velocity
AKA: Backpressure vs. Delta Pressure
You want low friction and high velocity.
You want low backpressure and high positive delta pressure. "
Credit given to Texan and Fritz for their info on this topic.
Prelussion
12-01-2001, 10:48 PM
"Needing Backpressure - Myth or Reality?
The goal of any exhaust system is to efficiently remove burnt gases from the combustion chamber, prevent reversion at overlap, and by enhancing exhaust gas velocity leaving the chamber, create a vacuum to help draw or scavenge in more intake charge volume at cam overlap.
The key is maintaining exhaust gas velocity or energy as the gases leave the exhaust port when the exhaust valve opens.
So as the exhaust gas leaves the exhaust port in a 4 stroke engine , it creates a series of pressure waves travelling at the speed of sound that move towards the exhaust tip (or forwards) and then some reflects back. Like the water waves coming onto the beach, forward and back, forward and back. The main overall direction is forwards but there is some reflection back to the exhaust port (reversion).
Simple enough...everyone knows this. So what's new and groovy?
The problem is at cam overlap (when both the exhaust valve and intake valve are both partially open and when the pressure in the chamber is greater than in the intake port).
If a high pressure wave is reflecting back and arrives at the exhaust port at the wrong time (i.e. when burnt gases still need to leave), it blocks the flow out. You see these instances when a high pressure wave is reflected back at the wrong time as dips in the torque curve AT REGULAR INTERVALS (usually in the midrange rpms).
If a low pressure wave is reflecting back at the correct time at the exhaust port it actually helps pull burnt gases out of the chamber and also helps pull in more intake air/fuel at overlap. You see these favourable low pressure reflected waves occurring on your torque curve as small torque increases AT REGULAR INTERVALS.
Now here's the first bone of contention and a source of debate between exhaust makers.
1. Is a reflected high pressure wave always bad?
Most of the experienced people I speak to and read on the various boards say YES! You never want backpressure and you want it as low as possible for as long as possible. The low backpressure assists in maintaining that high exhaust gas velocity. They then design anti-reversion chambers and/or place steps (increases in diameter at various proprietary points along the length of the header) to prevent the reflected waves from travelling back to the head.
There are also some pretty smart people who believe slightly differently ...They believe that if you have a high pressure reflected wave arriving a few milliseconds before exhaust valve closure, you prevent the loss of intake air:fuel out the exhaust valve at cam overlap. The exhaust backpressure at this crankshaft degree in the exhaust stroke prevents leaking out or bleeding out of you intake charge into the header and ensures all of it goes into the chamber for combustion.
However, these people do NOT use the exhaust diameter as a way to create this backpressure. That would be too crude or less precise, since the backpressure would exist at all times and they only want this backpressure over the few crankshaft degrees when the exhaust valve is just about to close ,when the intake valve is opening further, and the piston has reached TDC and starts downward for the intake stroke. Using an exhaust just to have backpressure then is like cutting butter with a chain saw.
The people who agree with this will often tell you that combustion chamber and intake port pressures are higher than the pressure in the exhaust just before exhaust valve closure . So some intake flow into the chamber can get pushed out the closing exhaust valve by the higher combustion chamber pressures.
So all you guys that say backpressure is a good thing...I don't believe so...not at all crankshaft degrees which is what you get with a restrictive diameter exhaust. You don't want to have too big a diameter (actually it's cross-sectional area) that will slow or kill velocity or energy. But no backpressure most (99%) of the time is good.
2. How do we get low pressure waves and high pressure wave to arrive at the correct time?
The conventional way to get the exhaust gas harmonic to do this dance of low pressure to pull in more intake charge and high pressure to prevent bleeding off all at the right time is by changing the tube layout on the header: using lengths, diameters, collectors with various merge angles. But these are limited to one harmonic or exhaust gas speed.
So some Japanese engineers at Yamaha (figures, it's always some genius engineer at some bike manufacturer that comes up with these wild ideas) thought: "What if you have an exhaust throttle valve (located in the header collector or at the entrance to the secondary tubes in the first merge collector) that could control the pressure wave behaviour?".
The throttle valve angle would vary as the speed of the exhaust gases changed to control the reflected waves. In an 11,000 rpm bike, the valve opens progressively as the rpms climb as the tubes are "in step" with the engine harmonics and less reflected waves occur but at around 7000 rpm, the valve is closed down to 40-60% of wide open when the harmonic is "out of step" with the engine and at 8500 rpm the exhaust throttle valve is progressively opened. How much to change the throttle angle is based on crankshaft angle input or ignition signal input to an ECU with then controls the throttle valve angle knowing the harmonics of the engine.
We see these in the Mercedes McLaren F1 car. If you think this is somebody's Frankenstein pipe dream then guess again. The new Suzuki GSXR1000, Honda Fireblade, and Yamaha R1 already have these. And those are today's street bikes! Can the new RSX and Civic Si be that far away from the next stage forward for more power? The impetus will not be performance oriented but the drive to bring this to the market place will likely be more practical, as this throttle valve (the first one was called the Exup or Exhaust Ultimate Power by Yamaha in the late 80's) gains better emissions and lower exhaust noise (less pollution is good ...admit it Kyoto is the right thing to do).
So the new toy for exhaust makers will be like variable valve timing and variable cam timing...the mating of electronics to optimise exhaust harmonics at each rpm as the harmonics change with the rpms climbing. It won't be just cut and try any longer...it will be cut try and reprogram. Welcome to the new millenium. "
Credit Given to Tuan at SHO. Happy now? :flipa:
The goal of any exhaust system is to efficiently remove burnt gases from the combustion chamber, prevent reversion at overlap, and by enhancing exhaust gas velocity leaving the chamber, create a vacuum to help draw or scavenge in more intake charge volume at cam overlap.
The key is maintaining exhaust gas velocity or energy as the gases leave the exhaust port when the exhaust valve opens.
So as the exhaust gas leaves the exhaust port in a 4 stroke engine , it creates a series of pressure waves travelling at the speed of sound that move towards the exhaust tip (or forwards) and then some reflects back. Like the water waves coming onto the beach, forward and back, forward and back. The main overall direction is forwards but there is some reflection back to the exhaust port (reversion).
Simple enough...everyone knows this. So what's new and groovy?
The problem is at cam overlap (when both the exhaust valve and intake valve are both partially open and when the pressure in the chamber is greater than in the intake port).
If a high pressure wave is reflecting back and arrives at the exhaust port at the wrong time (i.e. when burnt gases still need to leave), it blocks the flow out. You see these instances when a high pressure wave is reflected back at the wrong time as dips in the torque curve AT REGULAR INTERVALS (usually in the midrange rpms).
If a low pressure wave is reflecting back at the correct time at the exhaust port it actually helps pull burnt gases out of the chamber and also helps pull in more intake air/fuel at overlap. You see these favourable low pressure reflected waves occurring on your torque curve as small torque increases AT REGULAR INTERVALS.
Now here's the first bone of contention and a source of debate between exhaust makers.
1. Is a reflected high pressure wave always bad?
Most of the experienced people I speak to and read on the various boards say YES! You never want backpressure and you want it as low as possible for as long as possible. The low backpressure assists in maintaining that high exhaust gas velocity. They then design anti-reversion chambers and/or place steps (increases in diameter at various proprietary points along the length of the header) to prevent the reflected waves from travelling back to the head.
There are also some pretty smart people who believe slightly differently ...They believe that if you have a high pressure reflected wave arriving a few milliseconds before exhaust valve closure, you prevent the loss of intake air:fuel out the exhaust valve at cam overlap. The exhaust backpressure at this crankshaft degree in the exhaust stroke prevents leaking out or bleeding out of you intake charge into the header and ensures all of it goes into the chamber for combustion.
However, these people do NOT use the exhaust diameter as a way to create this backpressure. That would be too crude or less precise, since the backpressure would exist at all times and they only want this backpressure over the few crankshaft degrees when the exhaust valve is just about to close ,when the intake valve is opening further, and the piston has reached TDC and starts downward for the intake stroke. Using an exhaust just to have backpressure then is like cutting butter with a chain saw.
The people who agree with this will often tell you that combustion chamber and intake port pressures are higher than the pressure in the exhaust just before exhaust valve closure . So some intake flow into the chamber can get pushed out the closing exhaust valve by the higher combustion chamber pressures.
So all you guys that say backpressure is a good thing...I don't believe so...not at all crankshaft degrees which is what you get with a restrictive diameter exhaust. You don't want to have too big a diameter (actually it's cross-sectional area) that will slow or kill velocity or energy. But no backpressure most (99%) of the time is good.
2. How do we get low pressure waves and high pressure wave to arrive at the correct time?
The conventional way to get the exhaust gas harmonic to do this dance of low pressure to pull in more intake charge and high pressure to prevent bleeding off all at the right time is by changing the tube layout on the header: using lengths, diameters, collectors with various merge angles. But these are limited to one harmonic or exhaust gas speed.
So some Japanese engineers at Yamaha (figures, it's always some genius engineer at some bike manufacturer that comes up with these wild ideas) thought: "What if you have an exhaust throttle valve (located in the header collector or at the entrance to the secondary tubes in the first merge collector) that could control the pressure wave behaviour?".
The throttle valve angle would vary as the speed of the exhaust gases changed to control the reflected waves. In an 11,000 rpm bike, the valve opens progressively as the rpms climb as the tubes are "in step" with the engine harmonics and less reflected waves occur but at around 7000 rpm, the valve is closed down to 40-60% of wide open when the harmonic is "out of step" with the engine and at 8500 rpm the exhaust throttle valve is progressively opened. How much to change the throttle angle is based on crankshaft angle input or ignition signal input to an ECU with then controls the throttle valve angle knowing the harmonics of the engine.
We see these in the Mercedes McLaren F1 car. If you think this is somebody's Frankenstein pipe dream then guess again. The new Suzuki GSXR1000, Honda Fireblade, and Yamaha R1 already have these. And those are today's street bikes! Can the new RSX and Civic Si be that far away from the next stage forward for more power? The impetus will not be performance oriented but the drive to bring this to the market place will likely be more practical, as this throttle valve (the first one was called the Exup or Exhaust Ultimate Power by Yamaha in the late 80's) gains better emissions and lower exhaust noise (less pollution is good ...admit it Kyoto is the right thing to do).
So the new toy for exhaust makers will be like variable valve timing and variable cam timing...the mating of electronics to optimise exhaust harmonics at each rpm as the harmonics change with the rpms climbing. It won't be just cut and try any longer...it will be cut try and reprogram. Welcome to the new millenium. "
Credit Given to Tuan at SHO. Happy now? :flipa:
VTEC_boi
12-02-2001, 12:08 AM
Thanks for stating that early in the forming of this forum man.
Now, if Fritz would get here and "stickify" it...:)
peace
Now, if Fritz would get here and "stickify" it...:)
peace
texan
12-03-2001, 10:05 AM
You're not going to give any credit on those writings? Ok, I will: the first part of the first post is mine, the second part of the first post is fritz's, but whose wrote the second post? It's been awhile since I've been to Purehonda.
Kohhal
12-03-2001, 10:47 AM
Originally posted by texan
...but whose wrote the second post? ...
It was Originally posted by Tuan at SHO.org
Maybe Prelussion or a mod can update the original post to include credits?
...but whose wrote the second post? ...
It was Originally posted by Tuan at SHO.org
Maybe Prelussion or a mod can update the original post to include credits?
texan
12-03-2001, 11:52 AM
Ok, thought I recognized that.
DVSNCYNIKL
12-03-2001, 12:17 PM
Originally posted by texan
Ok, thought I recognized that.
There you go Texan. I made an additional comment at the bottom of the first post giving credit.:D
Ok, thought I recognized that.
There you go Texan. I made an additional comment at the bottom of the first post giving credit.:D
Prelussion
12-04-2001, 06:41 PM
Well, if people paid attention i didn't exactly give credit to texan, but it was noted that the second part of my post said it was in reply to the post above, and fritz noted the post by texan....but whatever sorry i didn't come back soon enough to change it :rolleyes: good post though, you prolly know more than i do.
madtownhonda
12-09-2001, 01:46 PM
good post....now when people tell me I'm stupid for telling them that zero backpressure is a good thing I can just direct them to this post instead of typing it out every time:lol2:
can you believe that one guy even told me that his car would not move if he had zero exhaust backpressure:lol2:
can you believe that one guy even told me that his car would not move if he had zero exhaust backpressure:lol2:
fritz_269
12-10-2001, 01:52 PM
Originally posted by madtownhonda
can you believe that one guy even told me that his car would not move if he had zero exhaust backpressure:lol2:
Did he have a jet car? ;)
can you believe that one guy even told me that his car would not move if he had zero exhaust backpressure:lol2:
Did he have a jet car? ;)
82civicwagon
12-14-2001, 06:38 PM
an excelent post. a little drawn out and not enough emphasis on cylinder scavaging and its effects, but over all ony of the best post's i've read:flash:
toilet_ninja
01-02-2002, 09:21 PM
82civic wagon, about your sig.......it's spelled punani, and it's pu in sai....just clearing things. peace.
ricodemus
01-30-2002, 01:09 PM
hehe..I'm a mechanical engineer :) . Actually, I'm kind of a rocket scientist too, since the company I work for designs/makes rocket engines. Anyway, I've always been trying to apply what I've learned in school to cars, since I dont really know much about cars. But everything you posted about pressure and everything is correct by fluid dynamics. Its actually called bernoulli's equation if anyone wants to look it up. It covers flow from one point to another and pressure and velocity are proportional at both ends (pressure to velocity^2). So any increase in one variable is a decrease in another. keep in mind there are a lot of head losses due to pressure drops,etc, bernoulli's is an ideal case. Thanks for the info, tho, you guys rock.
rybred
02-06-2002, 10:24 PM
hey! so by what you explained there, do you think it's only wise to go with 3 inch exhaust piping only if my engine was fully blown pushin about 10psi boost or more??
I'm going to get some custom piping done and i was thinkin to go with 2.5 inch tubing, with my 92 lude si, and i don't have a turbo in yet, but would it be wise to go with a smaller 2" or so until i have a turbo in? Also do you know what an effect a 3 angle valve job would have on backpressure if any? I got that done recently all in part of the prep for the turbo install! Any feed back would be greatly appriciated! Thanks!
I'm going to get some custom piping done and i was thinkin to go with 2.5 inch tubing, with my 92 lude si, and i don't have a turbo in yet, but would it be wise to go with a smaller 2" or so until i have a turbo in? Also do you know what an effect a 3 angle valve job would have on backpressure if any? I got that done recently all in part of the prep for the turbo install! Any feed back would be greatly appriciated! Thanks!
ricodemus
02-07-2002, 12:40 AM
there are a lot of numbers to crunch right there, and obviously automobile exhaust is not ideal (friction, etc), I was just confirming what others said. I guess I would look at what aftermarket companies have on their specs, because they are the ones who run the tests and hopefully design for optimum hp/torque gains and not just being loud. I'm sure mimicking there pipe diameter and choosing a smooth shape for your piping will yield you good results. Like I said, just cuz I know a few equations doesnt mean I know the best thing for your car or any car. But I definitely wouldnt want my exhaust hindering my overall output by not having enough flow. Remember that turbo uses exhaust gases, so I'd figure that it might not change the total mass flow, if anything it would decrease it. Hmm...shit dude, I dunno...:) . Good luck - hopefully someone else can give you a better answer.
texan
02-07-2002, 12:52 AM
Originally posted by rybred
hey! so by what you explained there, do you think it's only wise to go with 3 inch exhaust piping only if my engine was fully blown pushin about 10psi boost or more??
I'm going to get some custom piping done and i was thinkin to go with 2.5 inch tubing, with my 92 lude si, and i don't have a turbo in yet, but would it be wise to go with a smaller 2" or so until i have a turbo in? Also do you know what an effect a 3 angle valve job would have on backpressure if any? I got that done recently all in part of the prep for the turbo install! Any feed back would be greatly appriciated! Thanks!
When it comes to a turbocharged motor, it's virtually impossible to go too big on the exhaust diameter. 3" is definitely a great size, and anything larger would be very difficult to fit under the car without serious scraping problems.
A 3 angle valve job only helps to efficiently flow air into and out-of the motor, it has no direct effect on exhaust backpressure. I don't know about the H23A, but the 5th gen Prelude H22A4 comes from the factory with a 3 angle valve job.
hey! so by what you explained there, do you think it's only wise to go with 3 inch exhaust piping only if my engine was fully blown pushin about 10psi boost or more??
I'm going to get some custom piping done and i was thinkin to go with 2.5 inch tubing, with my 92 lude si, and i don't have a turbo in yet, but would it be wise to go with a smaller 2" or so until i have a turbo in? Also do you know what an effect a 3 angle valve job would have on backpressure if any? I got that done recently all in part of the prep for the turbo install! Any feed back would be greatly appriciated! Thanks!
When it comes to a turbocharged motor, it's virtually impossible to go too big on the exhaust diameter. 3" is definitely a great size, and anything larger would be very difficult to fit under the car without serious scraping problems.
A 3 angle valve job only helps to efficiently flow air into and out-of the motor, it has no direct effect on exhaust backpressure. I don't know about the H23A, but the 5th gen Prelude H22A4 comes from the factory with a 3 angle valve job.
Someguy
02-07-2002, 01:33 AM
As Texan said, its possible to go too big with the exhaust with a turbo (especially the down pipe), but not easily. Too large of a pipe is effectly like not having a pipe at all, and as a result you can cause some turbulance and reversion issues, but that's not going to be a problem here. 3 inch is fine for the turbo'd motor.
It is big for the motor in NA though. If you are planning to add a turbo why not save yourself some time and money and just do it all at once?
It is big for the motor in NA though. If you are planning to add a turbo why not save yourself some time and money and just do it all at once?
Prelussion
03-04-2002, 03:56 PM
Originally posted by Someguy
As Texan said, its possible to go too big with the exhaust with a turbo (especially the down pipe), but not easily. Too large of a pipe is effectly like not having a pipe at all, and as a result you can cause some turbulance and reversion issues, but that's not going to be a problem here. 3 inch is fine for the turbo'd motor.
It is big for the motor in NA though. If you are planning to add a turbo why not save yourself some time and money and just do it all at once?
W/ a large down pipe you will likely lose your A/C and if you have a short shifter and a 3" exhaust i bet it'll rub. plus your exhaust will rub other places as well.
As Texan said, its possible to go too big with the exhaust with a turbo (especially the down pipe), but not easily. Too large of a pipe is effectly like not having a pipe at all, and as a result you can cause some turbulance and reversion issues, but that's not going to be a problem here. 3 inch is fine for the turbo'd motor.
It is big for the motor in NA though. If you are planning to add a turbo why not save yourself some time and money and just do it all at once?
W/ a large down pipe you will likely lose your A/C and if you have a short shifter and a 3" exhaust i bet it'll rub. plus your exhaust will rub other places as well.
Marasmus
03-06-2002, 06:27 PM
Originally posted by Someguy
As Texan said, its possible to go too big with the exhaust with a turbo (especially the down pipe), but not easily. Too large of a pipe is effectly like not having a pipe at all, and as a result you can cause some turbulance and reversion issues, but that's not going to be a problem here. 3 inch is fine for the turbo'd motor.
It is big for the motor in NA though. If you are planning to add a turbo why not save yourself some time and money and just do it all at once?
Correct me if i'm wrong here - i'm totally speculating, as I've never personally worked with turbos - just superchargers.
A turbo functions by allowing exhaust gases to flow by a spool, which gathers rotational velocity from the exhaust gases. The higher the rotational velocity (and the geometry of the spool), the higher the boost.
Preludes come with 2 or 2.25" piping if I remember correctly. If you stick a fat 3" pipe on something like a prelude si, you're significantly slowing down the velocity of the air which turns your spool. What this will do is raise the bottom-end RPM at which your turbo is useful, and lower the PSI boost of the turbo at most RPMs (excluding high RPM, at which you can reach a terminal rotational velocity due to the geometry of the spool). If I'm thinking about this correctly, this can seriously weaken the power of your turbo.
I did some number-crunching on my car (2000 integra GS-R), taking displacement (1800cc), top RPM (8000 rpm) and pipe diameter(2.25in) (and thus cross-sectional area) into account. I came up with a maximum velocity of 219 ft/s from the car, stock. At top RPM, it's still below the 240 ft/s "magic number". In other words, I'd have to do some max RPM changes or forced-induction before my exhaust velocity reached an alarming level.
Adding a turbo to my car, pushing 8 psi (beyond that would probably trigger detonation due to the 10.0:1 compression ratio), would not increase my exhaust pressure by 8 psi because turbo sucks gas from the exhaust to feed the intake. Let's take a random guess, for the sake of example, that 50% (4 psi) of that psi boost gets recycled to the intake. That means that the default ambient pressure, 14.696 psi (1 atmosphere at sea level) would be increased to 18.696 psi. That's a 27.2% pressure increase. In order to keep the overall maximum exhaust pressure the same, I would need to increase the cross-sectional area of my exhaust piping by 27.2%.
The math:
exhaust dia^2 * 1.272 = ideal dia^2
2.25in ^2 * 1.272 = ideal dia^2
6.4395 sq. inches = ideal dia^2
2.538 in = ideal diameter
So.. if I bolted on an 8psi turbo, my ideal piping diameter, to keep my exact max velocity (which is a little bit on the low side, actually), would be 2.5 inches. I could just scam Type-R headers and cat, and grab some 2.5" aftermarket cat-back exhaust. However, a 3" pipe would completely drop my exhaust velocity, slow down the turbo, and really hurt my low-to-mid range.
Unless you're planning something like a 15psi boost on a 'lude, I don't think there's ever a need to go to 3" exhaust on a street car.
If anyone sees anything I screwed up here, please shout! I'm just making some guesses here based on my very rusty math. :)
As Texan said, its possible to go too big with the exhaust with a turbo (especially the down pipe), but not easily. Too large of a pipe is effectly like not having a pipe at all, and as a result you can cause some turbulance and reversion issues, but that's not going to be a problem here. 3 inch is fine for the turbo'd motor.
It is big for the motor in NA though. If you are planning to add a turbo why not save yourself some time and money and just do it all at once?
Correct me if i'm wrong here - i'm totally speculating, as I've never personally worked with turbos - just superchargers.
A turbo functions by allowing exhaust gases to flow by a spool, which gathers rotational velocity from the exhaust gases. The higher the rotational velocity (and the geometry of the spool), the higher the boost.
Preludes come with 2 or 2.25" piping if I remember correctly. If you stick a fat 3" pipe on something like a prelude si, you're significantly slowing down the velocity of the air which turns your spool. What this will do is raise the bottom-end RPM at which your turbo is useful, and lower the PSI boost of the turbo at most RPMs (excluding high RPM, at which you can reach a terminal rotational velocity due to the geometry of the spool). If I'm thinking about this correctly, this can seriously weaken the power of your turbo.
I did some number-crunching on my car (2000 integra GS-R), taking displacement (1800cc), top RPM (8000 rpm) and pipe diameter(2.25in) (and thus cross-sectional area) into account. I came up with a maximum velocity of 219 ft/s from the car, stock. At top RPM, it's still below the 240 ft/s "magic number". In other words, I'd have to do some max RPM changes or forced-induction before my exhaust velocity reached an alarming level.
Adding a turbo to my car, pushing 8 psi (beyond that would probably trigger detonation due to the 10.0:1 compression ratio), would not increase my exhaust pressure by 8 psi because turbo sucks gas from the exhaust to feed the intake. Let's take a random guess, for the sake of example, that 50% (4 psi) of that psi boost gets recycled to the intake. That means that the default ambient pressure, 14.696 psi (1 atmosphere at sea level) would be increased to 18.696 psi. That's a 27.2% pressure increase. In order to keep the overall maximum exhaust pressure the same, I would need to increase the cross-sectional area of my exhaust piping by 27.2%.
The math:
exhaust dia^2 * 1.272 = ideal dia^2
2.25in ^2 * 1.272 = ideal dia^2
6.4395 sq. inches = ideal dia^2
2.538 in = ideal diameter
So.. if I bolted on an 8psi turbo, my ideal piping diameter, to keep my exact max velocity (which is a little bit on the low side, actually), would be 2.5 inches. I could just scam Type-R headers and cat, and grab some 2.5" aftermarket cat-back exhaust. However, a 3" pipe would completely drop my exhaust velocity, slow down the turbo, and really hurt my low-to-mid range.
Unless you're planning something like a 15psi boost on a 'lude, I don't think there's ever a need to go to 3" exhaust on a street car.
If anyone sees anything I screwed up here, please shout! I'm just making some guesses here based on my very rusty math. :)
texan
03-06-2002, 07:00 PM
That all makes perfect sense to me... if pressure concerns at low airflow levels are the most important constiuent to turbocharger exhaust design AFTER the turbine section. Which to my knowledge is not the case, and here's my reasoning:
The turbine section of any turbocharger represents the primary restriction to flow in the exhaust path, and is always responsible for a great deal of pressure gain in the manifold itself. Basically any portion of the exhaust system which is ahead of the turbine inlet has high pressure under non-idle and cruise engine running, which means everywhere we are concerned about performance. Additionally, what makes a turbo spool up quickly is, as exhaust system design from one side of the turbine to the other is concerned, is pressure differential. At lower flow levels the large (sometimes ridiculously large) piping aft of the turbine presents the largest possible difference in pressure from one side to the other, hence also aids in spooling the turbo better than any other setup. To make the piping smaller will have no positive effect on lowering gas pressure, since the dynamics of exhaust flow change so extremely after flowing through a turbine to begin with. Think of the turbine as the blender, the exhaust gasses as the veggies, and I think you'll get the general idea. What it will have an effect on is overall flow ability vs. pressure, and that effect will be negative concerning any remotely workable piping size (surely if you went to 7" wide you might find problems, but you couldn't fit such a large pipe under there in the first place). So in essence, a turbo exhaust is a completely different beast than NA or supercharged engine setups and therefore requires a different approach to maximizing performance through piping size choice.
Basically, what we're after is a piping size that will not present any significant pressure gains at the airflow levels we will be seeing, especially at higher RPM and boost levels. The low RPM spool caracteristics will take care of themselves well with this line of thinking, and ensure that not only are we allowing spool to commence unimpeded by pressure gain aft of the turbine, but also that overal flow will not be restricted at high RPM's and high boost.
The turbine section of any turbocharger represents the primary restriction to flow in the exhaust path, and is always responsible for a great deal of pressure gain in the manifold itself. Basically any portion of the exhaust system which is ahead of the turbine inlet has high pressure under non-idle and cruise engine running, which means everywhere we are concerned about performance. Additionally, what makes a turbo spool up quickly is, as exhaust system design from one side of the turbine to the other is concerned, is pressure differential. At lower flow levels the large (sometimes ridiculously large) piping aft of the turbine presents the largest possible difference in pressure from one side to the other, hence also aids in spooling the turbo better than any other setup. To make the piping smaller will have no positive effect on lowering gas pressure, since the dynamics of exhaust flow change so extremely after flowing through a turbine to begin with. Think of the turbine as the blender, the exhaust gasses as the veggies, and I think you'll get the general idea. What it will have an effect on is overall flow ability vs. pressure, and that effect will be negative concerning any remotely workable piping size (surely if you went to 7" wide you might find problems, but you couldn't fit such a large pipe under there in the first place). So in essence, a turbo exhaust is a completely different beast than NA or supercharged engine setups and therefore requires a different approach to maximizing performance through piping size choice.
Basically, what we're after is a piping size that will not present any significant pressure gains at the airflow levels we will be seeing, especially at higher RPM and boost levels. The low RPM spool caracteristics will take care of themselves well with this line of thinking, and ensure that not only are we allowing spool to commence unimpeded by pressure gain aft of the turbine, but also that overal flow will not be restricted at high RPM's and high boost.
Someguy
03-06-2002, 11:56 PM
My reasoning is along the lines of Texan's. The intake side of a turbo is very similiar to that of an oddly placed centrifical supercharger. The exhaust side is completely different then that of a supercharged or NA engine though.
On a NA engine the exhaulst gasses act as a series of pulses. In that situation exhaulst tuning becomes very critical since the response of a linear system to a pulse is typically a series of oscilations. To make the best effiency in your system you have to hit the lows and miss the highs of that response. That's pretty tricky. A similiar situation occurs with a supercharger, besides you have possitive intake manifold pressure, and is therefore easier to overdo it (a large portion of your intake charge goes right back out your exhaulst ports during valve overlap).
With a turbo the input of the system is much more steady. This is because, as Texan mentioned, the turbo acts as a major restriction. The exhaulst manifolds for turbos are much different then for NA applications. A good one won't have much of anything in the way of tubes. Basically its where pressure builds. The turbo does "blend" the flow also. A good demonstration of that is if you take the muffler(s) off a turbo'd motor and off a NA one. The NA will be loud as hell, while the turbo'd motor will be much quieter. Its the difference between gasses "pouring" out and "punching" (for lack of better terms) out.
Well, time for bed. :)
On a NA engine the exhaulst gasses act as a series of pulses. In that situation exhaulst tuning becomes very critical since the response of a linear system to a pulse is typically a series of oscilations. To make the best effiency in your system you have to hit the lows and miss the highs of that response. That's pretty tricky. A similiar situation occurs with a supercharger, besides you have possitive intake manifold pressure, and is therefore easier to overdo it (a large portion of your intake charge goes right back out your exhaulst ports during valve overlap).
With a turbo the input of the system is much more steady. This is because, as Texan mentioned, the turbo acts as a major restriction. The exhaulst manifolds for turbos are much different then for NA applications. A good one won't have much of anything in the way of tubes. Basically its where pressure builds. The turbo does "blend" the flow also. A good demonstration of that is if you take the muffler(s) off a turbo'd motor and off a NA one. The NA will be loud as hell, while the turbo'd motor will be much quieter. Its the difference between gasses "pouring" out and "punching" (for lack of better terms) out.
Well, time for bed. :)
Someguy
03-07-2002, 12:01 AM
Originally posted by Prelussion
W/ a large down pipe you will likely lose your A/C and if you have a short shifter and a 3" exhaust i bet it'll rub. plus your exhaust will rub other places as well.
Finding the balance between ideal and practical is the difference between science and engineering; IE art. :cool:
W/ a large down pipe you will likely lose your A/C and if you have a short shifter and a 3" exhaust i bet it'll rub. plus your exhaust will rub other places as well.
Finding the balance between ideal and practical is the difference between science and engineering; IE art. :cool:
rybred
03-07-2002, 06:20 PM
thanks everyone! that pretty much cleared everything up in terms of backpressure...at least i have a much better understanding of the difference between superchargers and turbos!
HACO_ID_85
03-26-2002, 10:33 AM
I couldn't read the whole thing i got bored so i'm just going to ask. I went and baught a muffler then i went to a muffler shop and got 2.5" piping running from the cat back. I was talking to the guy and he said that if i were to gut or replace my cat and add headers i would lose my low end torque. is this correct. or can i add headers and gut the cat .I drive a 90 accord
93vtecer
11-22-2002, 12:57 PM
Yup he sure is right you need a certain amount of back pressure on low end. Ask anyone who has had an exhaust leak, your car may even idol funny. You will see the gain at higher rpm's, You can just look at your tach and you will see the rpm's raise quicker as you approach red line.
zippeay
04-28-2003, 02:00 AM
Ok I just put a 2 1/2 inch exhaust on my 92 civic "with a b16a" full header back exhaust with a high flow cat. Its so DAMN LOUD!!! I don't like it, plus I was racing my friend "he has a 2001 VW golf 1.8T" and I was doing fine until I hit 100 then he passed me by like I was standing still. Is this because my exhaust is too big? Should I put a stock cat back in? The only thing else I have done right now is cold air intake. Please help
eckoman_pdx
06-02-2003, 12:12 AM
I have a 92 Civic with a LS in it, also, no header, only a Cold Airi intake It's currently an LS auto). When I bought a universal muffler and had someone build me a catback system, he said that anything above 2 1/4 and I'd lose pressure. It would hurt performance. Since I have an LS, he told me to go with a 2 if its an AT, since the AT needs more backpresuure at the low end. He said that a 2 1/4 would give me a little more top end, not quite as much low (whihc isnt as much a problem for MT). Anyways, he said dont go above 2 1/4, so I didnt, and thats where I sit. I think 2 1/2 would be too much for a b16a. I could be wrong, but thats what I heard. Also, I put a resonator on my exhaust system, really quited it down. Thats what DC sports uses, a 15" or 18" resonator, I think. Hope this helps. If I'm wrong on any of this, feel free to correct me:)
Hatchie Boy
06-22-2003, 06:29 AM
you guys are smart
ooooh ahhhh smart
ooooh ahhhh smart
97gsr
07-11-2003, 11:32 PM
NICE WRITEUP....THANKS!! :bigthumb:
here... i'll throw up my setup...
and let me know what you guys think...
'97 GS-R...
I have AEM intake...
just bought the rs-r ex mag exhaust(2.36 inch)
and will mate it with the jdm oem 4-1 header(2.5 inch)
modified free flow cat(not stock cat)
after that, i might do cams, and wrap it up with fuel pressure regulator, apexi v-afc and some tuning! :bigthumb:
thanks guys!!
/hijack
here... i'll throw up my setup...
and let me know what you guys think...
'97 GS-R...
I have AEM intake...
just bought the rs-r ex mag exhaust(2.36 inch)
and will mate it with the jdm oem 4-1 header(2.5 inch)
modified free flow cat(not stock cat)
after that, i might do cams, and wrap it up with fuel pressure regulator, apexi v-afc and some tuning! :bigthumb:
thanks guys!!
/hijack
Ricochet
07-14-2003, 03:00 PM
Wow, spend $600+ on a great exhaust, $400+ on a tuned header, $200+ on an intake, and you gain a whopping 15-20 horsepower. Thanks for all the tech info, but it's such a large headache to think about with minimal payback in real life.
blue4g2def
07-19-2003, 06:24 PM
That EXUP thing is an amazing idea...but they only made it for bikes?
were there any made for car applications?
were there any made for car applications?
ShwedishFish
08-10-2003, 12:14 AM
Im a senior in mechanical engineering and i get most of what you are saying but im having trouble applying to what i should do for my ride.
Looking at my stock exhuast i was wondering why it goes from about 2" to 1.5" near the muffler, but now i see thats their to maintain the exhaust gas velocity.
but what im trying to figure out is what my best option for modifying my exhaust to get better performance. I bought some "DC style" headers and a high flow cat, and that seemed to decrease low end torque a lil and maybe help high end a lil.
now im trying to figure out what to do about a catback, cost is a big concern for me being in school and all, so i could get a 2.25" catback for under $300, but im wondering if this will even benefit my performance much? what should i do?
Looking at my stock exhuast i was wondering why it goes from about 2" to 1.5" near the muffler, but now i see thats their to maintain the exhaust gas velocity.
but what im trying to figure out is what my best option for modifying my exhaust to get better performance. I bought some "DC style" headers and a high flow cat, and that seemed to decrease low end torque a lil and maybe help high end a lil.
now im trying to figure out what to do about a catback, cost is a big concern for me being in school and all, so i could get a 2.25" catback for under $300, but im wondering if this will even benefit my performance much? what should i do?
eckoman_pdx
08-11-2003, 04:05 AM
Originally posted by ShwedishFish
Im a senior in mechanical engineering and i get most of what you are saying but im having trouble applying to what i should do for my ride.
Looking at my stock exhuast i was wondering why it goes from about 2" to 1.5" near the muffler, but now i see thats their to maintain the exhaust gas velocity.
but what im trying to figure out is what my best option for modifying my exhaust to get better performance. I bought some "DC style" headers and a high flow cat, and that seemed to decrease low end torque a lil and maybe help high end a lil.
now im trying to figure out what to do about a catback, cost is a big concern for me being in school and all, so i could get a 2.25" catback for under $300, but im wondering if this will even benefit my performance much? what should i do?
For exhuast, don't go over a 2 1/4" piping for your cat-back system. As for the header, was it 4-1 or a tri-y header (4-2-1)? The 4-1 gives more to the top-end at the expenisve of the low-end, the 4-2-1 is better it you want to mantain your low end. If you put in a 4-1 header, thats a big part of your reason there.
Im a senior in mechanical engineering and i get most of what you are saying but im having trouble applying to what i should do for my ride.
Looking at my stock exhuast i was wondering why it goes from about 2" to 1.5" near the muffler, but now i see thats their to maintain the exhaust gas velocity.
but what im trying to figure out is what my best option for modifying my exhaust to get better performance. I bought some "DC style" headers and a high flow cat, and that seemed to decrease low end torque a lil and maybe help high end a lil.
now im trying to figure out what to do about a catback, cost is a big concern for me being in school and all, so i could get a 2.25" catback for under $300, but im wondering if this will even benefit my performance much? what should i do?
For exhuast, don't go over a 2 1/4" piping for your cat-back system. As for the header, was it 4-1 or a tri-y header (4-2-1)? The 4-1 gives more to the top-end at the expenisve of the low-end, the 4-2-1 is better it you want to mantain your low end. If you put in a 4-1 header, thats a big part of your reason there.
Ricochet
08-11-2003, 10:33 AM
You can order a high-flow cat from JEG's for $100. I have one on my car and haven't had a check engine light or exhaust leak since. You will have to weld it into the piping yourself though, or have somebody do it, because it doesn't come with bolt holes or anything.
ShwedishFish
08-11-2003, 03:09 PM
the headers i got are 4-2-1, to be honest im not 100% sure about top end vs low end perfomance gains losses i have seen, they are minimal either way i guess
i got a high flow cat off ebay, with bolts, gaskets, correct dimensions for $40, it seems to help a lil but since i put that and headers on i get an annoing rattle/whistle at mid range rpms sometimes, i ran it for a day with no O2 i hope the rattle isnt cause i burned it out doing that or something like that
thanks for the advice and knowledge
i got a high flow cat off ebay, with bolts, gaskets, correct dimensions for $40, it seems to help a lil but since i put that and headers on i get an annoing rattle/whistle at mid range rpms sometimes, i ran it for a day with no O2 i hope the rattle isnt cause i burned it out doing that or something like that
thanks for the advice and knowledge
eckoman_pdx
08-12-2003, 05:44 AM
Originally posted by ShwedishFish
the headers i got are 4-2-1, to be honest im not 100% sure about top end vs low end perfomance gains losses i have seen, they are minimal either way i guess
i got a high flow cat off ebay, with bolts, gaskets, correct dimensions for $40, it seems to help a lil but since i put that and headers on i get an annoing rattle/whistle at mid range rpms sometimes, i ran it for a day with no O2 i hope the rattle isnt cause i burned it out doing that or something like that
thanks for the advice and knowledge
You are welcome, I'm always glad to help.:bigthumb:
the headers i got are 4-2-1, to be honest im not 100% sure about top end vs low end perfomance gains losses i have seen, they are minimal either way i guess
i got a high flow cat off ebay, with bolts, gaskets, correct dimensions for $40, it seems to help a lil but since i put that and headers on i get an annoing rattle/whistle at mid range rpms sometimes, i ran it for a day with no O2 i hope the rattle isnt cause i burned it out doing that or something like that
thanks for the advice and knowledge
You are welcome, I'm always glad to help.:bigthumb:
cowtnadv
10-10-2003, 06:50 AM
It depends on your HP rating - go to Flowmastermufflers.com - they have a chart as to what pipe size to use and check out their Delta Flow technology! Read through info at the top of left hand menu bar - it covers a lot of what the top pf the forum states!
A tech there told me they are coming out with a new Delta Flow technology around Dec with a muffler called Super 40 that will out do their existing lines!
I always say new techo is better than old!
Peace man . . . or noise ha ha!
Ron
A tech there told me they are coming out with a new Delta Flow technology around Dec with a muffler called Super 40 that will out do their existing lines!
I always say new techo is better than old!
Peace man . . . or noise ha ha!
Ron
parsloe3x
10-22-2003, 10:57 PM
Ls motor in crx..which mounts can anyone recomend me ?
Ricochet
10-22-2003, 11:56 PM
Ls motor in crx..which mounts can anyone recomend me ?
uh..
I think you posted in the wrong area dude.
uh..
I think you posted in the wrong area dude.
whtteg
10-23-2003, 12:05 AM
Yea talk about off topic lol:screwy:
Ricochet
10-23-2003, 12:12 AM
I'll give change to the bum anyways.
Here man, www.hasport.com
Go to the crx forum next time, er 88-91 Civic.
Here man, www.hasport.com
Go to the crx forum next time, er 88-91 Civic.
VeNoM_Racing_101
12-06-2003, 08:49 PM
Ok, so what can i do to increase my delta p?? you can always take the cat off to decrease back pressure, but is the only thing i can do to increase delta p is exaust piping and headers?
Doubletap
12-09-2003, 12:17 PM
Alright just want to run this by the resident geniuses, mechanical engineers and experts in fluid dynamics. For a turbo application could another plus to haveing a large downpipe be the pressure drop created when air moves from a small pipe o a larger one. I believe they use this on carburaters to speed up the velocity of the fuel air mixture by runnng it from a large opening through a smaller restriction then back to a large one. When the air exits the smaller restriction it expands to fill the larger diameter space lowering the pressure and pulling the air behind it in faster. Could this apply as well when the (relativly) small amount of air exits the turbo and expands into the large downpipe lowering the ambient pressure of the air and effectivly pulling more air through the turbo to try to equalize. Wow or maybe the fact that the turbo is shoving air out into the downpipe evens things out and this is way off bt if anyone has a though let me kow I am curious about this one.
Thanks,
DoubleTap
Thanks,
DoubleTap
mreff
12-19-2003, 08:41 PM
Car won't run wo/ back pressure? well, unless he was driving in space. regardless of exhaust you will have a minimum of 14.7 psi at sea level
bbyzfr6
12-31-2003, 10:42 AM
Rybred:
Not being an a$$, but in your search engine type exhaust pipe diameter vs. h.p. or horsepower. Several websites have charts that agree with one another.
You can go to flow masters website and look at there pipe dia. vs hp and ci. www.flowmastermufflers.com and click systems on the left side of the page.
I have not calculated to see if any of these numbers make since, but they look reasonable and the people posting seem to know what they are doing. Plus they are not making any money selling you bigger pipe.
Not being an a$$, but in your search engine type exhaust pipe diameter vs. h.p. or horsepower. Several websites have charts that agree with one another.
You can go to flow masters website and look at there pipe dia. vs hp and ci. www.flowmastermufflers.com and click systems on the left side of the page.
I have not calculated to see if any of these numbers make since, but they look reasonable and the people posting seem to know what they are doing. Plus they are not making any money selling you bigger pipe.
riceizyummy
01-02-2004, 11:26 AM
so do you lose HP with backpressure?
DeleriousZ
04-02-2004, 12:27 AM
Alright just want to run this by the resident geniuses, mechanical engineers and experts in fluid dynamics. For a turbo application could another plus to haveing a large downpipe be the pressure drop created when air moves from a small pipe o a larger one. I believe they use this on carburaters to speed up the velocity of the fuel air mixture by runnng it from a large opening through a smaller restriction then back to a large one. When the air exits the smaller restriction it expands to fill the larger diameter space lowering the pressure and pulling the air behind it in faster. Could this apply as well when the (relativly) small amount of air exits the turbo and expands into the large downpipe lowering the ambient pressure of the air and effectivly pulling more air through the turbo to try to equalize. Wow or maybe the fact that the turbo is shoving air out into the downpipe evens things out and this is way off bt if anyone has a though let me kow I am curious about this one.
Thanks,
DoubleTap
Ok i know this post/thread is really old, but i just thought i'd reply for the fun of it. yes, they do have this for turbo's.. it's not EXACTLY the same thing, but it's pretty darn close... all the high end tubular manifolds have what is called a merge collector which works on the same principles. it takes the exhaust from the cylinders and forces it through a smaller diameter (say 1.75 inches) then opens up to the outlet size of the manifold. it is commonly seen on the larger engines (v8's) and normally on n/a vehicles. just my :2cents:
Thanks,
DoubleTap
Ok i know this post/thread is really old, but i just thought i'd reply for the fun of it. yes, they do have this for turbo's.. it's not EXACTLY the same thing, but it's pretty darn close... all the high end tubular manifolds have what is called a merge collector which works on the same principles. it takes the exhaust from the cylinders and forces it through a smaller diameter (say 1.75 inches) then opens up to the outlet size of the manifold. it is commonly seen on the larger engines (v8's) and normally on n/a vehicles. just my :2cents:
Esky
09-02-2004, 02:04 AM
Me thinks the confusion comes from guys converting to low backpressure exhaust system and lost the low-end torque... and therefore saying backpressure is good (for low-end torque at least).
Read from somewhere that this is due to the combustion chamber being able to expel the spent gases easier with the lowered backpressure, but then at the same time, at low rpm the intake charge isn't exactly charging into the chamber and filling it up well. So the net result is less air/fuel mixture (including exhaust gas) to compress, and therefore worse low-end torque.
Not sure whether this is the correct scenario but it makes sense to me.
Read from somewhere that this is due to the combustion chamber being able to expel the spent gases easier with the lowered backpressure, but then at the same time, at low rpm the intake charge isn't exactly charging into the chamber and filling it up well. So the net result is less air/fuel mixture (including exhaust gas) to compress, and therefore worse low-end torque.
Not sure whether this is the correct scenario but it makes sense to me.
SaabJohan
09-16-2004, 06:52 PM
The scavenge effect of the exhaust manifold needs certian speeds to work. If you fit the largest pipes you can fit you will lose power, but it's not caused by exhaust backpressure but because of the loss of the scavenge effect.
sracing
12-24-2004, 12:14 PM
SaabJohan is correct.
The lower the backpressure the better in every case. The problem comes in is determining the lowest back pressure at what RPM. No pipes at all would give the closest to absolute zero back pressure. However, with the pipes tuned correctly you can achieve scavange effects that in theory make the back pressure less than 0 at certain RPM/loads. Thus, cylinder efficiencies are gained.
That's why they are called "tuned" headers. The size, length and shape provide these gains only at certain RPM ranges.
We dyno test headers for a company here. Typical applications we test are 4:2:1 systems. Four cylinders are tuned with the primary 4 into 2 pipe for a particular RPM. Then the 2 into one pipe is tuned for a slightly different peak RPM. This broadens the range that the headers are most effective. Depending upon the application, these 2 RPM ranges vary. For a race engine that may operate at the same RPM most of its use, these points are the same. For one than runs from 4800 to 6600 the sections are tuned farther apart.
Jim
SR Racing
The lower the backpressure the better in every case. The problem comes in is determining the lowest back pressure at what RPM. No pipes at all would give the closest to absolute zero back pressure. However, with the pipes tuned correctly you can achieve scavange effects that in theory make the back pressure less than 0 at certain RPM/loads. Thus, cylinder efficiencies are gained.
That's why they are called "tuned" headers. The size, length and shape provide these gains only at certain RPM ranges.
We dyno test headers for a company here. Typical applications we test are 4:2:1 systems. Four cylinders are tuned with the primary 4 into 2 pipe for a particular RPM. Then the 2 into one pipe is tuned for a slightly different peak RPM. This broadens the range that the headers are most effective. Depending upon the application, these 2 RPM ranges vary. For a race engine that may operate at the same RPM most of its use, these points are the same. For one than runs from 4800 to 6600 the sections are tuned farther apart.
Jim
SR Racing
Esky
12-26-2004, 06:58 PM
Interesting...
Speaking of header systems, do you hv any experience with 4-1 shortie headers? Some claimed that they found this to be the best compromise between 4-2-1 & normal long 4-1 headers.
Speaking of header systems, do you hv any experience with 4-1 shortie headers? Some claimed that they found this to be the best compromise between 4-2-1 & normal long 4-1 headers.
sracing
12-27-2004, 08:49 AM
Interesting...
Speaking of header systems, do you hv any experience with 4-1 shortie headers? Some claimed that they found this to be the best compromise between 4-2-1 & normal long 4-1 headers.
Yes, but it's not quite so easy to make that generalization. All things being equal, the 4:2:1 are going to provide a wider HP band. Long pipes more HP at lower RPM's and short ones at higher RPM. But the size of the pipe and the merge point are going to determine the total performance.
BTW, I am not a header expert. My expertise is only in doing tests for header companies.
This much I have learned... In general, headers for street cars are typically of the worst quality product we come across. From ALL companies. They are typically poorly welded, ill designed and a wild tuning compromise. Part of this is due to the design of the cars they must fit into and the attmepts to make them installable. But for the most part the only headers that we have seen that fit well, are built well AND provide appreciable gains are those that are designed for full out race cars. All work to some degree, but are not optimum.
Jim
SR Racing
Speaking of header systems, do you hv any experience with 4-1 shortie headers? Some claimed that they found this to be the best compromise between 4-2-1 & normal long 4-1 headers.
Yes, but it's not quite so easy to make that generalization. All things being equal, the 4:2:1 are going to provide a wider HP band. Long pipes more HP at lower RPM's and short ones at higher RPM. But the size of the pipe and the merge point are going to determine the total performance.
BTW, I am not a header expert. My expertise is only in doing tests for header companies.
This much I have learned... In general, headers for street cars are typically of the worst quality product we come across. From ALL companies. They are typically poorly welded, ill designed and a wild tuning compromise. Part of this is due to the design of the cars they must fit into and the attmepts to make them installable. But for the most part the only headers that we have seen that fit well, are built well AND provide appreciable gains are those that are designed for full out race cars. All work to some degree, but are not optimum.
Jim
SR Racing
Esky
12-27-2004, 06:58 PM
Yes, but it's not quite so easy to make that generalization. All things being equal, the 4:2:1 are going to provide a wider HP band. Long pipes more HP at lower RPM's and short ones at higher RPM. But the size of the pipe and the merge point are going to determine the total performance.
BTW, I am not a header expert. My expertise is only in doing tests for header companies.
I thot it's the other way round, i.e. longer pipe for more top-end horses (as featured in most 4-1 systems, and virtually most, if not all, aftermarket 4-1 headers for Hondas)?
BTW, I am not a header expert. My expertise is only in doing tests for header companies.
I thot it's the other way round, i.e. longer pipe for more top-end horses (as featured in most 4-1 systems, and virtually most, if not all, aftermarket 4-1 headers for Hondas)?
sracing
12-28-2004, 10:44 AM
I thot it's the other way round, i.e. longer pipe for more top-end horses (as featured in most 4-1 systems, and virtually most, if not all, aftermarket 4-1 headers for Hondas)?
Nope. Everything else equal... It's longer for LOW RPM and shorter for HIGH RPM. THe same is true for induction runner length.
Do a search on the web. There are dozens of formula's for computing the peak HP range vs header length.
Jim
SR Racing
Nope. Everything else equal... It's longer for LOW RPM and shorter for HIGH RPM. THe same is true for induction runner length.
Do a search on the web. There are dozens of formula's for computing the peak HP range vs header length.
Jim
SR Racing
Esky
12-28-2004, 07:15 PM
Ok, so it's like intake pipe design- short & fat for good hp and long & slim for good torque.
So why aren't the VTECs using shortie 4-1, since most of the time the aim is for max bhp?
So why aren't the VTECs using shortie 4-1, since most of the time the aim is for max bhp?
sracing
12-29-2004, 08:58 AM
Ok, so it's like intake pipe design- short & fat for good hp and long & slim for good torque.
So why aren't the VTECs using shortie 4-1, since most of the time the aim is for max bhp?
Sort of.. You have to understand Torque and HP better than what your statement seems to indicate...
HP=Torque x RPM / 5252
Hp IS Torque across the element of time. Your statement should really say. " short & fat for PEAK hp at high RPM and long & slim for PEAK HP at low RPM."
You normally attempt to tune so that your engine produces it's peak HP at the RPM range you are going to operate at. NOT neccessarilly just peak HP.
IE. One could design the induction system so that the peak occurs at 8500 RPM and your HP would be very high. But if your engine can't live there or your drive train ratio aren't practical at that point it would make no sense. Plus at low RPM, the engines limited torque would make it less driviable.
If however, 6000 is where it is tuned for you would be able to operate the car in it most effective range. Your HP peak would be lower but the area under the curve that is utilized would contain more HP.
I don't know about the V-TEC shortie, but again remember, the aim is not for max HP, but for max HP under the curve in the area you will be operating the engine in normally.
Jim
SR Racing
So why aren't the VTECs using shortie 4-1, since most of the time the aim is for max bhp?
Sort of.. You have to understand Torque and HP better than what your statement seems to indicate...
HP=Torque x RPM / 5252
Hp IS Torque across the element of time. Your statement should really say. " short & fat for PEAK hp at high RPM and long & slim for PEAK HP at low RPM."
You normally attempt to tune so that your engine produces it's peak HP at the RPM range you are going to operate at. NOT neccessarilly just peak HP.
IE. One could design the induction system so that the peak occurs at 8500 RPM and your HP would be very high. But if your engine can't live there or your drive train ratio aren't practical at that point it would make no sense. Plus at low RPM, the engines limited torque would make it less driviable.
If however, 6000 is where it is tuned for you would be able to operate the car in it most effective range. Your HP peak would be lower but the area under the curve that is utilized would contain more HP.
I don't know about the V-TEC shortie, but again remember, the aim is not for max HP, but for max HP under the curve in the area you will be operating the engine in normally.
Jim
SR Racing
BrunoRiviera
01-16-2005, 05:47 AM
Adding Creatine in your fuel tank will boost your engine thanks to it's extra oxydation power... Just try it!
Note : Make sure the powder size do not exceed your thin fuel filter mesh size!
Note : Make sure the powder size do not exceed your thin fuel filter mesh size!
stockprelude
02-06-2006, 12:09 PM
here is some input from another mechanical engineer.
im don know if im just reiterating previous statement but, it needs to be explained in general terms for people to understand. at low RPMs , backpressure needs to be built for there to be any movement of the system. backpressure is built up until your pistons begin to move due to these pressure forces. if you have too large of an exhaust diameter and not enough air flow in, there will be no pressure build up at low RPM's resulting in less power in this range. a larger exhaust is only necessary when you are increasing the air flow into the head. for NA this would be adding a new intake manifold air intake cams. for turbo your exhaust size definately needs to be increased for max efficiency depending on the size of the turbo. and of course the other force that adds to cylinder pressure is air/fuel mixture. most often you need to change the air fuel mix in a car with exhuast and headers because combustion is not at peak efficiency, any more. backpressure is mainly a sign of how well your system is preforming with all the other components combined.
simply stated, if you are wondering what a backpressure exhaust is doing, it is allowing the exhaust to move more easily through the system without sacrificing the all important BACKPRESSURE.
im don know if im just reiterating previous statement but, it needs to be explained in general terms for people to understand. at low RPMs , backpressure needs to be built for there to be any movement of the system. backpressure is built up until your pistons begin to move due to these pressure forces. if you have too large of an exhaust diameter and not enough air flow in, there will be no pressure build up at low RPM's resulting in less power in this range. a larger exhaust is only necessary when you are increasing the air flow into the head. for NA this would be adding a new intake manifold air intake cams. for turbo your exhaust size definately needs to be increased for max efficiency depending on the size of the turbo. and of course the other force that adds to cylinder pressure is air/fuel mixture. most often you need to change the air fuel mix in a car with exhuast and headers because combustion is not at peak efficiency, any more. backpressure is mainly a sign of how well your system is preforming with all the other components combined.
simply stated, if you are wondering what a backpressure exhaust is doing, it is allowing the exhaust to move more easily through the system without sacrificing the all important BACKPRESSURE.
curtis73
02-06-2006, 12:21 PM
Backpressure: (BAK' preshur) noun; what it feels like to have my foot in your butt for reviving a thread from 2001 :naughty:
Its a good thread, but let's not revive the oldies.
Its a good thread, but let's not revive the oldies.
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