exhaust question... for the tech gurus...
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From: Tipp City, Ohio
exhaust question... for the tech gurus...
So, I have a 2.5 inch catback, and a header with a 2.25 inch collector coming. I also want to buy a high0flow cat, but I don't know whether I should buy a 2.25 inch cat, or a 2.5 inch cat... help please!! I was thinking that since the cat is going to restrict the flow a little bit, i should go with the 2.5. that way, the exhaust gas leaving the engine doesnt lose as much momentum going into the cat.... but i dont know all that much about the physics of air, I am just making a kind of educated guess.
any input (with an explanation) would be great!! Thanks in advance.
any input (with an explanation) would be great!! Thanks in advance.
Senior Member
Joined: 05-07-07
Posts: 1,847
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From: Rochester, NY
im pretty sure the general consensus is that high flow cats barely restrict airflow, its just the stock ones from the factory that are horrid.
plus 4 cylinders need some restriction so u get a little back pressure.
its kinda like if u take the muffler off of an old lawn mower it will barely work b/c no back pressure...but if you take a header off a v8 it just makes it stronger, the larger engines dont need as much pressure.
soooo basically my vote would be 2.25 will probably be fine. but im not a mechanic so hold off for Adam or someone to chime in
plus 4 cylinders need some restriction so u get a little back pressure.
its kinda like if u take the muffler off of an old lawn mower it will barely work b/c no back pressure...but if you take a header off a v8 it just makes it stronger, the larger engines dont need as much pressure.
soooo basically my vote would be 2.25 will probably be fine. but im not a mechanic so hold off for Adam or someone to chime in
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Joined: 05-07-07
Posts: 1,847
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From: Rochester, NY
wow this sounds so sweet:
http://www.magnaflow.com/02product/s...e=Sport&id=982
click on the 3rd Sound Clip
how much is the price difference? if it's more then $20 i'd say get the 2.5, i think 2.5" is the perfect fit for 2.0-2.4's as far as neccessary back pressure goes...
http://www.magnaflow.com/02product/s...e=Sport&id=982
click on the 3rd Sound Clip
how much is the price difference? if it's more then $20 i'd say get the 2.5, i think 2.5" is the perfect fit for 2.0-2.4's as far as neccessary back pressure goes...
waaaaay too much misinformation in here
1) all back pressure is bad....you want the exhaust leaving your engine rapidly...at lower RPMs, a smaller exhaust will do this fine, but at higher RPMs, there wont be enough room for all the gasses, so you'd want a larger exhaust. this is the reason you dont want a 3" catback on a 2.2 the gasses would be moving so slowly that youd see a loss in HP (you want the exhaust moving fast to help with scavenging, which is a whole nother essay)
2) high flow cats have been stock on all cars since the early 90's atleast....car manufacturers dont put restrictive parts on the car when theres less restrictive parts that cost the exact same...
ironically chris seems to understand it the most, as he's the only one who mentioned exhaust velocity
also, removing a header is VERY rarely going to gain you HP....most V8s looking for more power will run open header....meaning there is indeed a header, but nothing coming after it....the header ensures the gasses leaving the chamber are moving at a fast enough velocity to increase scavenging, and the lack of any additional piping removes ALL RESTRICTIONS, including cats, mufflers, bends in the piping, and resonators
i think adam has a write up on scavenging and exhaust sizes
to reiterate in a different way...each exhaust has a HP range that is ideally fit to that exhaust.....say you buy an exhaust that ideally flows 220hp worth of exhaust for your ss s/c, however, at say, 3000RPMs, your only making something like 80hp because your not at your peak HP at 6200RPM and your not under full boost. so for the lower portion of your powerband, you have waay to big on an exhaust that is actually hurting your performance.
the same applies to n/a cars. assuming you have a perfectly flat torque curve (you dont, but this is just to prove a point), your HP peaks at 6.3kRPMs @ 145, which means at 3kRPMs, your only making about 70hp, and at 4kRPMs your only making 100hp
this is why auto manufacturers make exhausts aimed for a 170hp car and put them on a 210hp car...that way, you dont suffer from too great a loss of power at low RPMs, and the exhaust isnt too restrictive as to rob HP in the high RPMs also
i hope this all makes sense to you
1) all back pressure is bad....you want the exhaust leaving your engine rapidly...at lower RPMs, a smaller exhaust will do this fine, but at higher RPMs, there wont be enough room for all the gasses, so you'd want a larger exhaust. this is the reason you dont want a 3" catback on a 2.2 the gasses would be moving so slowly that youd see a loss in HP (you want the exhaust moving fast to help with scavenging, which is a whole nother essay)
2) high flow cats have been stock on all cars since the early 90's atleast....car manufacturers dont put restrictive parts on the car when theres less restrictive parts that cost the exact same...
ironically chris seems to understand it the most, as he's the only one who mentioned exhaust velocity
also, removing a header is VERY rarely going to gain you HP....most V8s looking for more power will run open header....meaning there is indeed a header, but nothing coming after it....the header ensures the gasses leaving the chamber are moving at a fast enough velocity to increase scavenging, and the lack of any additional piping removes ALL RESTRICTIONS, including cats, mufflers, bends in the piping, and resonators
i think adam has a write up on scavenging and exhaust sizes
to reiterate in a different way...each exhaust has a HP range that is ideally fit to that exhaust.....say you buy an exhaust that ideally flows 220hp worth of exhaust for your ss s/c, however, at say, 3000RPMs, your only making something like 80hp because your not at your peak HP at 6200RPM and your not under full boost. so for the lower portion of your powerband, you have waay to big on an exhaust that is actually hurting your performance.
the same applies to n/a cars. assuming you have a perfectly flat torque curve (you dont, but this is just to prove a point), your HP peaks at 6.3kRPMs @ 145, which means at 3kRPMs, your only making about 70hp, and at 4kRPMs your only making 100hp
this is why auto manufacturers make exhausts aimed for a 170hp car and put them on a 210hp car...that way, you dont suffer from too great a loss of power at low RPMs, and the exhaust isnt too restrictive as to rob HP in the high RPMs also
i hope this all makes sense to you
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Joined: 09-08-07
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From: Palmyra, NY
Its right that high flow cats have been used since the 90's.
At one point, I'll say it again, cats used pellets in them to do there thing. These packed pellets were a restriction from the get go and as a cat did it thing, cleaning the exhaust up, the gaps between the pellets began to clog. Catsin the past would all most 100% clog and you'd have a cherry red cat after about 1/2 hour running on a bad one. Todays cats are honey combed, not much different in generating restriction than a glass pack muffler. Only the size now becomes the difference in flow, which takes us to the next points.
Now were not talking straight out race engines that get shut down in the traps that require every fraction of a horsepower, were talking street engines with cams and other requirements that NEED backpressure in order to operate correctly. The amount of backpressure needed is .75-1.5 PSI as a minimum standard. This keeps the computers, 02 sensors and the likes from throwing cells at you and going into limp modes.
Exhaust 101 will teach you that in order to satisfy minimum requirements, you want 1.8CFM for a mild motor to 2.2 CFM for a wild motor, thats the mount of exhaust CFM per horsepower of flow. Every cubic inch of exhaust pipe size flows 115 CFM. If you want to do the math, go ahead, but I'll tell you for simplicity, 1 inch in diameter per 100 h.p.
Then velocity in a simply put ideal. Pipes should always stay to a minimum, the same size from beginning to end. Slowly going smaller some place back in the exhaust can yield sometimes better results since as gas leaves the motor, it cools, CONTRACTION, this all keep velocity close to being constant. Within the system if you go larger, you'll create and EXPANSION point, which allows the gases to expand, which in turn SLOWS velocity.
How do you correctly size the exhaust???Starts at the collector, same size as the collector, or smaller!!!!!
At one point, I'll say it again, cats used pellets in them to do there thing. These packed pellets were a restriction from the get go and as a cat did it thing, cleaning the exhaust up, the gaps between the pellets began to clog. Catsin the past would all most 100% clog and you'd have a cherry red cat after about 1/2 hour running on a bad one. Todays cats are honey combed, not much different in generating restriction than a glass pack muffler. Only the size now becomes the difference in flow, which takes us to the next points.
Now were not talking straight out race engines that get shut down in the traps that require every fraction of a horsepower, were talking street engines with cams and other requirements that NEED backpressure in order to operate correctly. The amount of backpressure needed is .75-1.5 PSI as a minimum standard. This keeps the computers, 02 sensors and the likes from throwing cells at you and going into limp modes.
Exhaust 101 will teach you that in order to satisfy minimum requirements, you want 1.8CFM for a mild motor to 2.2 CFM for a wild motor, thats the mount of exhaust CFM per horsepower of flow. Every cubic inch of exhaust pipe size flows 115 CFM. If you want to do the math, go ahead, but I'll tell you for simplicity, 1 inch in diameter per 100 h.p.
Then velocity in a simply put ideal. Pipes should always stay to a minimum, the same size from beginning to end. Slowly going smaller some place back in the exhaust can yield sometimes better results since as gas leaves the motor, it cools, CONTRACTION, this all keep velocity close to being constant. Within the system if you go larger, you'll create and EXPANSION point, which allows the gases to expand, which in turn SLOWS velocity.
How do you correctly size the exhaust???Starts at the collector, same size as the collector, or smaller!!!!!
Thread Starter
Senior Member
Joined: 06-23-06
Posts: 1,967
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From: Tipp City, Ohio
Its right that high flow cats have been used since the 90's.
At one point, I'll say it again, cats used pellets in them to do there thing. These packed pellets were a restriction from the get go and as a cat did it thing, cleaning the exhaust up, the gaps between the pellets began to clog. Catsin the past would all most 100% clog and you'd have a cherry red cat after about 1/2 hour running on a bad one. Todays cats are honey combed, not much different in generating restriction than a glass pack muffler. Only the size now becomes the difference in flow, which takes us to the next points.
Now were not talking straight out race engines that get shut down in the traps that require every fraction of a horsepower, were talking street engines with cams and other requirements that NEED backpressure in order to operate correctly. The amount of backpressure needed is .75-1.5 PSI as a minimum standard. This keeps the computers, 02 sensors and the likes from throwing cells at you and going into limp modes.
Exhaust 101 will teach you that in order to satisfy minimum requirements, you want 1.8CFM for a mild motor to 2.2 CFM for a wild motor, thats the mount of exhaust CFM per horsepower of flow. Every cubic inch of exhaust pipe size flows 115 CFM. If you want to do the math, go ahead, but I'll tell you for simplicity, 1 inch in diameter per 100 h.p.
Then velocity in a simply put ideal. Pipes should always stay to a minimum, the same size from beginning to end. Slowly going smaller some place back in the exhaust can yield sometimes better results since as gas leaves the motor, it cools, CONTRACTION, this all keep velocity close to being constant. Within the system if you go larger, you'll create and EXPANSION point, which allows the gases to expand, which in turn SLOWS velocity.
How do you correctly size the exhaust???Starts at the collector, same size as the collector, or smaller!!!!!
At one point, I'll say it again, cats used pellets in them to do there thing. These packed pellets were a restriction from the get go and as a cat did it thing, cleaning the exhaust up, the gaps between the pellets began to clog. Catsin the past would all most 100% clog and you'd have a cherry red cat after about 1/2 hour running on a bad one. Todays cats are honey combed, not much different in generating restriction than a glass pack muffler. Only the size now becomes the difference in flow, which takes us to the next points.
Now were not talking straight out race engines that get shut down in the traps that require every fraction of a horsepower, were talking street engines with cams and other requirements that NEED backpressure in order to operate correctly. The amount of backpressure needed is .75-1.5 PSI as a minimum standard. This keeps the computers, 02 sensors and the likes from throwing cells at you and going into limp modes.
Exhaust 101 will teach you that in order to satisfy minimum requirements, you want 1.8CFM for a mild motor to 2.2 CFM for a wild motor, thats the mount of exhaust CFM per horsepower of flow. Every cubic inch of exhaust pipe size flows 115 CFM. If you want to do the math, go ahead, but I'll tell you for simplicity, 1 inch in diameter per 100 h.p.
Then velocity in a simply put ideal. Pipes should always stay to a minimum, the same size from beginning to end. Slowly going smaller some place back in the exhaust can yield sometimes better results since as gas leaves the motor, it cools, CONTRACTION, this all keep velocity close to being constant. Within the system if you go larger, you'll create and EXPANSION point, which allows the gases to expand, which in turn SLOWS velocity.
How do you correctly size the exhaust???Starts at the collector, same size as the collector, or smaller!!!!!
Senior Member
Joined: 05-07-07
Posts: 1,847
Likes: 0
From: Rochester, NY
waaaaay too much misinformation in here
1) all back pressure is bad....you want the exhaust leaving your engine rapidly...at lower RPMs, a smaller exhaust will do this fine, but at higher RPMs, there wont be enough room for all the gasses, so you'd want a larger exhaust. this is the reason you dont want a 3" catback on a 2.2 the gasses would be moving so slowly that youd see a loss in HP (you want the exhaust moving fast to help with scavenging, which is a whole nother essay)
2) high flow cats have been stock on all cars since the early 90's atleast....car manufacturers dont put restrictive parts on the car when theres less restrictive parts that cost the exact same...
ironically chris seems to understand it the most, as he's the only one who mentioned exhaust velocity
also, removing a header is VERY rarely going to gain you HP....most V8s looking for more power will run open header....meaning there is indeed a header, but nothing coming after it....the header ensures the gasses leaving the chamber are moving at a fast enough velocity to increase scavenging, and the lack of any additional piping removes ALL RESTRICTIONS, including cats, mufflers, bends in the piping, and resonators
i think adam has a write up on scavenging and exhaust sizes
to reiterate in a different way...each exhaust has a HP range that is ideally fit to that exhaust.....say you buy an exhaust that ideally flows 220hp worth of exhaust for your ss s/c, however, at say, 3000RPMs, your only making something like 80hp because your not at your peak HP at 6200RPM and your not under full boost. so for the lower portion of your powerband, you have waay to big on an exhaust that is actually hurting your performance.
the same applies to n/a cars. assuming you have a perfectly flat torque curve (you dont, but this is just to prove a point), your HP peaks at 6.3kRPMs @ 145, which means at 3kRPMs, your only making about 70hp, and at 4kRPMs your only making 100hp
this is why auto manufacturers make exhausts aimed for a 170hp car and put them on a 210hp car...that way, you dont suffer from too great a loss of power at low RPMs, and the exhaust isnt too restrictive as to rob HP in the high RPMs also
i hope this all makes sense to you
1) all back pressure is bad....you want the exhaust leaving your engine rapidly...at lower RPMs, a smaller exhaust will do this fine, but at higher RPMs, there wont be enough room for all the gasses, so you'd want a larger exhaust. this is the reason you dont want a 3" catback on a 2.2 the gasses would be moving so slowly that youd see a loss in HP (you want the exhaust moving fast to help with scavenging, which is a whole nother essay)
2) high flow cats have been stock on all cars since the early 90's atleast....car manufacturers dont put restrictive parts on the car when theres less restrictive parts that cost the exact same...
ironically chris seems to understand it the most, as he's the only one who mentioned exhaust velocity
also, removing a header is VERY rarely going to gain you HP....most V8s looking for more power will run open header....meaning there is indeed a header, but nothing coming after it....the header ensures the gasses leaving the chamber are moving at a fast enough velocity to increase scavenging, and the lack of any additional piping removes ALL RESTRICTIONS, including cats, mufflers, bends in the piping, and resonators
i think adam has a write up on scavenging and exhaust sizes
to reiterate in a different way...each exhaust has a HP range that is ideally fit to that exhaust.....say you buy an exhaust that ideally flows 220hp worth of exhaust for your ss s/c, however, at say, 3000RPMs, your only making something like 80hp because your not at your peak HP at 6200RPM and your not under full boost. so for the lower portion of your powerband, you have waay to big on an exhaust that is actually hurting your performance.
the same applies to n/a cars. assuming you have a perfectly flat torque curve (you dont, but this is just to prove a point), your HP peaks at 6.3kRPMs @ 145, which means at 3kRPMs, your only making about 70hp, and at 4kRPMs your only making 100hp
this is why auto manufacturers make exhausts aimed for a 170hp car and put them on a 210hp car...that way, you dont suffer from too great a loss of power at low RPMs, and the exhaust isnt too restrictive as to rob HP in the high RPMs also
i hope this all makes sense to you
better yet, take your muffler off your lawn mower and try to cut the grass...it aint gonna work too well man
how do you think manufacturers built exhaust systems according to hp...the lower the hp the more backpressure is needed so the smaller the exhaust size.
your right about the open headers, thanks for proving my point...v8 and some larger v6's dont even need an exhaust because the engine creates enough backpressure to maintain their powerband without any extra restriction.
its kinda like a vaccum, it needs some backpressure to create suction, if you ran a "Free flowing" exhaust, or open headers on a tiny 4 cyl, the engine wont run very well...
just research it online
"at lower revs you need a little backpressure to stop the air/fuel mixture from 'falling' out of the combustion chamber, and at higher revs you need zero or negative backpressure, for best power."
http://www2b.abc.net.au/science/k2/s...pic202160.shtm
so what you want is enough backpressure to help your low end hp and torque, but not too much to hurt your high band performance.
i think people are confused...restriction creates back pressure..we all understand this right?
please correct me if I am wrong, but I do believe that air expands when it cools, thats why you want an intake to bing in cooler air, because the air is more dense, therefore more cfm of air brought in. Like I said, I may be wrond, but that is the way I have always thought it was supposed to work.
let me ask this: why would an engine need back pressure? why would you ever want resistance against the flow of exhaust? you want the exhaust traveling rapidly at all times, which dictates smaller piping for low RPMs and larger piping for high RPMs, but restrictions are still unwanted
also, to the guy who said you want 1" per 100hp....that doesnt even make sense mathematically. a 3" diameter exhaust flows 9.4X more gasses than a 1" diameter exhaust
these are circular exhausts, not squares...
Pi(1)˛=3.14in˛
Pi(2)˛=6.28in˛
Pi(3)˛=9.45in˛
furthermore, your saying a Z06 needs a 5in exhaust?
Pi(5)˛=78.53in˛
the Zo6 only makes 2.5 times more exhaust than a ss s/c, but according to your theory it needs an exhaust that will fow 12.5x more gasses
btw...i have taken my exhaust off my car and it ran the exact same, albeit a lot louder...and i dont have a lawnmower to test this theory upon
as for your quote shaun....the conditions required for something like what you quoited would require some wild racing cams with a lot of overlap, which would inherently be designed to generate lower HP numbers at low RPMs and higher HP numbers at higher RPMs, so you would want a restrictionless high flowing exhaust for this setup
you would want n exhaust aimed at hgh RPM horsepower to compliment your cams that also dictate high RPM horsepower
i wish adam would come in here...
here...i found one of adams stickies:
2. Backpressure is something you need
First understand that your engine is a big air pump. It ingests oxygen (or maybe even is forced fed oxygen) and expels wastes. The most important part to also understand is that velocity of air traveling plays effect in both parts. If you don't understand what velocity is:
Quote: Originally Posted by
In physics, velocity is defined as the rate of change of displacement or the rate of displacement. It is a vector physical quantity, both speed and direction are required to define it.
When talking about your exhaust system, increasing the diameter of your exhaust system or even increasing the primaries on your manifold/header, it effects the velocity of the exhaust leaving your engine. If it doesn't make much sense to you, think of it like this:
Take a straw that is 3 centimeters in diameter and blow into it. The air from your lungs is now traveling through your mouth and into a closed area at a certain rate of speed. When it reaches into the straw, what determines the speed is the force of the air travel in this inclosed space. Now take a straw that is 6 centimeters in diameter and the same length. Lets also assume that the air leaving your lungs and traveling through your mouth is going at the same speed/velocity. With having a larger diameter straw, this is going to change how fast air will travel through the straw...hence change the velocity of the air.
Using the example above, your lungs is the engine, your mouth is the ports on the cylinder head(s) and the straw is your exhaust system.
Now that you understand what velocity is, understand what backpressure is.
Backpressure is exactly that, air (or in this case exhaust waste) traveling back torwards the engine. Now most of us understand that it's about how fast the exhaust gets out of the motor and the exhaust system. If there is a force of air traveling out of engine into one direction, backpressure would be a force of air attempting to travel in the opposite direction.
Backpressure is caused by restriction, whether it be a crimp in the exhaust system, for example, a crush bent pipe:
or it be because of a very small diametered exhaust system (including primaries on the manifold/header).
The most important thing to understand in the cases of having an engine that is naturally aspirated or supercharged (turbocharged engines don't run off this method post-turbocharger) is that when you increase the diameter, you are getting rid of backpressure...yes but you are at the same time effecting the velocity of air traveling out of your exhaust system. It's basically taking the good with the bad.
Why is exhaust velocity important? Exhaust velocity does have a direct relation to your powerband and where it is located which is why it's very important for you to pick the proper size exhaust system in the first place. The wrong size can effect your powerband in the negative way (well, negative to you as the user of the vehicle). The slower the velocity, the later in the RPM Band your powerband will be moved to. If off extremely, it will make the performance of your engine very peaky or at worst, make you lose power.
Overall, everything should be considered and it is important to think about exhaust velocity...not backpressure. I would also suggest reading up on scavenging effects.
First understand that your engine is a big air pump. It ingests oxygen (or maybe even is forced fed oxygen) and expels wastes. The most important part to also understand is that velocity of air traveling plays effect in both parts. If you don't understand what velocity is:
Quote: Originally Posted by
In physics, velocity is defined as the rate of change of displacement or the rate of displacement. It is a vector physical quantity, both speed and direction are required to define it.
When talking about your exhaust system, increasing the diameter of your exhaust system or even increasing the primaries on your manifold/header, it effects the velocity of the exhaust leaving your engine. If it doesn't make much sense to you, think of it like this:
Take a straw that is 3 centimeters in diameter and blow into it. The air from your lungs is now traveling through your mouth and into a closed area at a certain rate of speed. When it reaches into the straw, what determines the speed is the force of the air travel in this inclosed space. Now take a straw that is 6 centimeters in diameter and the same length. Lets also assume that the air leaving your lungs and traveling through your mouth is going at the same speed/velocity. With having a larger diameter straw, this is going to change how fast air will travel through the straw...hence change the velocity of the air.
Using the example above, your lungs is the engine, your mouth is the ports on the cylinder head(s) and the straw is your exhaust system.
Now that you understand what velocity is, understand what backpressure is.
Backpressure is exactly that, air (or in this case exhaust waste) traveling back torwards the engine. Now most of us understand that it's about how fast the exhaust gets out of the motor and the exhaust system. If there is a force of air traveling out of engine into one direction, backpressure would be a force of air attempting to travel in the opposite direction.
Backpressure is caused by restriction, whether it be a crimp in the exhaust system, for example, a crush bent pipe:
or it be because of a very small diametered exhaust system (including primaries on the manifold/header).
The most important thing to understand in the cases of having an engine that is naturally aspirated or supercharged (turbocharged engines don't run off this method post-turbocharger) is that when you increase the diameter, you are getting rid of backpressure...yes but you are at the same time effecting the velocity of air traveling out of your exhaust system. It's basically taking the good with the bad.
Why is exhaust velocity important? Exhaust velocity does have a direct relation to your powerband and where it is located which is why it's very important for you to pick the proper size exhaust system in the first place. The wrong size can effect your powerband in the negative way (well, negative to you as the user of the vehicle). The slower the velocity, the later in the RPM Band your powerband will be moved to. If off extremely, it will make the performance of your engine very peaky or at worst, make you lose power.
Overall, everything should be considered and it is important to think about exhaust velocity...not backpressure. I would also suggest reading up on scavenging effects.
Last edited by HunterKiller89; Sep 15, 2007 at 05:22 PM. Reason: Automerged Doublepost
Senior Member
Joined: 05-07-07
Posts: 1,847
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From: Rochester, NY
colder air contracts....this is how engines work. combustion in the cylinder causes the gasses to heat up and expand so rapidly that they push the piston downward
let me ask this: why would an engine need back pressure? why would you ever want resistance against the flow of exhaust? you want the exhaust traveling rapidly at all times, which dictates smaller piping for low RPMs and larger piping for high RPMs, but restrictions are still unwanted
also, to the guy who said you want 1" per 100hp....that doesnt even make sense mathematically. a 3" diameter exhaust flows 9.4X more gasses than a 1" diameter exhaust
these are circular exhausts, not squares...
Pi(1)˛=3.14in˛
Pi(2)˛=6.28in˛
Pi(3)˛=9.45in˛
furthermore, your saying a Z06 needs a 5in exhaust?
Pi(5)˛=78.53in˛
the Zo6 only makes 2.5 times more exhaust than a ss s/c, but according to your theory it needs an exhaust that will fow 12.5x more gasses
btw...i have taken my exhaust off my car and it ran the exact same, albeit a lot louder...and i dont have a lawnmower to test this theory upon
as for your quote shaun....the conditions required for something like what you quoited would require some wild racing cams with a lot of overlap, which would inherently be designed to generate lower HP numbers at low RPMs and higher HP numbers at higher RPMs, so you would want a restrictionless high flowing exhaust for this setup
you would want n exhaust aimed at hgh RPM horsepower to compliment your cams that also dictate high RPM horsepower
i wish adam would come in here...
here...i found one of adams stickies:
me and adam read all the same books and though hes ahead of me in understanding cars...im not far behind...im atleast far enough to fully understand everything about the exhaust and intake
let me ask this: why would an engine need back pressure? why would you ever want resistance against the flow of exhaust? you want the exhaust traveling rapidly at all times, which dictates smaller piping for low RPMs and larger piping for high RPMs, but restrictions are still unwanted
also, to the guy who said you want 1" per 100hp....that doesnt even make sense mathematically. a 3" diameter exhaust flows 9.4X more gasses than a 1" diameter exhaust
these are circular exhausts, not squares...
Pi(1)˛=3.14in˛
Pi(2)˛=6.28in˛
Pi(3)˛=9.45in˛
furthermore, your saying a Z06 needs a 5in exhaust?
Pi(5)˛=78.53in˛
the Zo6 only makes 2.5 times more exhaust than a ss s/c, but according to your theory it needs an exhaust that will fow 12.5x more gasses
btw...i have taken my exhaust off my car and it ran the exact same, albeit a lot louder...and i dont have a lawnmower to test this theory upon
as for your quote shaun....the conditions required for something like what you quoited would require some wild racing cams with a lot of overlap, which would inherently be designed to generate lower HP numbers at low RPMs and higher HP numbers at higher RPMs, so you would want a restrictionless high flowing exhaust for this setup
you would want n exhaust aimed at hgh RPM horsepower to compliment your cams that also dictate high RPM horsepower
i wish adam would come in here...
here...i found one of adams stickies:
me and adam read all the same books and though hes ahead of me in understanding cars...im not far behind...im atleast far enough to fully understand everything about the exhaust and intake
you sound like you know your stuff, so i'll leave it to Adam or someone else with some "authority" to settle this.
idealy it would be cool if you could have a changing exhaust, pipe that got smaller in diameter at low rpm's and larger at high! future racing technology..
New Member
Joined: 09-08-07
Posts: 80
Likes: 0
From: Palmyra, NY
Adam and myself both agreed on the basic fundamentals of exhaust.
I said 1 inch per 100 h.p. as a base starting point which is fairly close.
A Z06 does actually follow that rule along with hosts of other cars.
See a Z06 has twin 2 1/2-3 inch exhaust pipes, doesnt it??
Most all V-8's use dual exhausts, dont they??
A bank of 4 cylinders, 100 h.p. per inch and it adds up, doesnt it?
If your gonna run a single exhaust pipe on more than 4 cylinders, then the other part of 1.8-2.2CFM , per h.p. , per one inch of area then comes into play.
Pipes are a restriction, just being a pipe.
Exhaust gases are restricted in expansion to the size of the pipe.
You taking the exhaust off the car and running it and saying it ran the same is not a truth. It can not be since the 02 arent seeing anything, so what does the computer do?
But in the same breath, do that again and nail it to the wall and you'll see why at the end of a drag strip open headered cars shut down. Makes a mess out of the pushrods, valves or any other super heated part when you take your foot off, it sucks in very cool air compared to the combustion process and wham, bent ****.
No lets just forget racing, no one here does it for a living with there vehicles, so these principles of back pressure of some amount etc are needed.
Shaun, it isnt that far off before you may see something that adjust, exhaust wise per RPM.
But then again you see it every day if you pay attention.
You think those flaps on the top of the stacks on semi's are there why??? Same purpose!!!!
They actually have those flaps, spring loaded for street cars, but people dont think for what there worth, only the look aspect of having them, that turns them off.
I said 1 inch per 100 h.p. as a base starting point which is fairly close.
A Z06 does actually follow that rule along with hosts of other cars.
See a Z06 has twin 2 1/2-3 inch exhaust pipes, doesnt it??
Most all V-8's use dual exhausts, dont they??
A bank of 4 cylinders, 100 h.p. per inch and it adds up, doesnt it?
If your gonna run a single exhaust pipe on more than 4 cylinders, then the other part of 1.8-2.2CFM , per h.p. , per one inch of area then comes into play.
Pipes are a restriction, just being a pipe.
Exhaust gases are restricted in expansion to the size of the pipe.
You taking the exhaust off the car and running it and saying it ran the same is not a truth. It can not be since the 02 arent seeing anything, so what does the computer do?
But in the same breath, do that again and nail it to the wall and you'll see why at the end of a drag strip open headered cars shut down. Makes a mess out of the pushrods, valves or any other super heated part when you take your foot off, it sucks in very cool air compared to the combustion process and wham, bent ****.
No lets just forget racing, no one here does it for a living with there vehicles, so these principles of back pressure of some amount etc are needed.
Shaun, it isnt that far off before you may see something that adjust, exhaust wise per RPM.
But then again you see it every day if you pay attention.
You think those flaps on the top of the stacks on semi's are there why??? Same purpose!!!!
They actually have those flaps, spring loaded for street cars, but people dont think for what there worth, only the look aspect of having them, that turns them off.
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From: Canada
keep in mind...that while the word "backpressure" may not be exactly correct....in a way it is.
these engines benefit from a balance of low restriction, with high exhaust velocity, which is achieved by giving it a free flowing exhaust system, while having enough restriction in the system to keep the velocitys high.
Conceptually the easy way to think of it is breathing into a garden hose, and feeling the air come out the other end....youll feel it.
whereas if you breath down a 6 inch diameter water pipe....you arent going to feel ****.
except maybe lightheaded and out of breath....exactly how your engine will feel
so technically, you need a certain amount of restriction, which technically DOES cause backpressure....to allow the volume of exhaust gas to completely fill the exhaust chamber, and allow the flow to keep high velocity.
this is not the levels of timed tuned backpressure of a 2-stroke or anything....
but in truth our small engine does require a certain amount.
these engines benefit from a balance of low restriction, with high exhaust velocity, which is achieved by giving it a free flowing exhaust system, while having enough restriction in the system to keep the velocitys high.
Conceptually the easy way to think of it is breathing into a garden hose, and feeling the air come out the other end....youll feel it.
whereas if you breath down a 6 inch diameter water pipe....you arent going to feel ****.
except maybe lightheaded and out of breath....exactly how your engine will feel
so technically, you need a certain amount of restriction, which technically DOES cause backpressure....to allow the volume of exhaust gas to completely fill the exhaust chamber, and allow the flow to keep high velocity.
this is not the levels of timed tuned backpressure of a 2-stroke or anything....
but in truth our small engine does require a certain amount.
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Joined: 01-05-06
Posts: 10,877
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From: East Brunswick, NJ
LOL
I love how I'm mentioned in every post on this thread
Your engine would run like crap more so because of lack of scavenging effects. On modern day cars, your car would run like crap without a exhaust system because of your O2 sensor not reading any exhaust fuel mixture, so it will think there is a problem and might go into limp mode or dump fuel.
It's about the velocity of exhaust flow that helps balance power output.
Also, manufacturers use smaller exhaust piping for sound purposes as well. Think of it like this, if sound has room to travel, it will be louder and echo louder. Yell in a small room and then yell in a tunnel. It's the same thing as far as engine sound and why some exhaust systems are quieter than some or vice versa.
I think the confusion is more so that people use the word "restriction" too much in the sense that restriction = bad. Fact is, no matter what exhaust system you have, there is going to be some form of restriction, it's just the amount of how much. Matter of fact, lets throw that word out the window. Replace it with velocity. Velocity is of the importance.
I don't really want to go into it too much but read the stickies I wrote (or even the one that Hunter copied from as a example) and you'd understand what I mean.
I love how I'm mentioned in every post on this thread
Originally Posted by Shawn672
Um no...take off your exhaust and you're car wont run for crap...smaller engines NEED backpressure, otherwise everyone would run open headers..
how do you think manufacturers built exhaust systems according to hp...the lower the hp the more backpressure is needed so the smaller the exhaust size.
Also, manufacturers use smaller exhaust piping for sound purposes as well. Think of it like this, if sound has room to travel, it will be louder and echo louder. Yell in a small room and then yell in a tunnel. It's the same thing as far as engine sound and why some exhaust systems are quieter than some or vice versa.
i think people are confused...restriction creates back pressure..we all understand this right?
I don't really want to go into it too much but read the stickies I wrote (or even the one that Hunter copied from as a example) and you'd understand what I mean.
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From: Tipp City, Ohio
so, my setup is (will be when it gets here), an Tsudo header with a 2.25 inch collector, a 2.25 inch Magnaflow high-flow cat, then the 2/5 inch exhaust I have sitting in front of me. Is that fact that the piping for the catback is larger then all of the rest going to hinder me in anyway?
Last edited by ChrisAult2004; Sep 15, 2007 at 09:57 PM.
so, my setup is (will be when it gets here), an Tsudo header with a 2.25 inch collector, a 2.25 inch Magnaflow high-flow cat, then the 2/5 inch exhaust I have sitting in front of me. Is that fact that the piping for the catback is larger then all of the rest going to hinder me in anyway?
You taking the exhaust off the car and running it and saying it ran the same is not a truth. It can not be since the 02 arent seeing anything, so what does the computer do?
But in the same breath, do that again and nail it to the wall and you'll see why at the end of a drag strip open headered cars shut down. Makes a mess out of the pushrods, valves or any other super heated part when you take your foot off, it sucks in very cool air compared to the combustion process and wham, bent ****.
But in the same breath, do that again and nail it to the wall and you'll see why at the end of a drag strip open headered cars shut down. Makes a mess out of the pushrods, valves or any other super heated part when you take your foot off, it sucks in very cool air compared to the combustion process and wham, bent ****.
it was my mistake...i assumed you understood i meant without a catback exhaust
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