I finally raced a subaru
03-04-2009, 10:38 AM
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Its a real shame that you guys have no idea what you're talking about - but yet still think you know something about it.....
You're trying to imagine it with your straw example, but you're using your straw example in the wrong manner... we don't measure the psi in the turbo (what you are calling the straw).. you measure it at the manifold (the end point).... once the manifold is pressurized to a certain psi (in our example 14psi), the wastegate opens and keeps the turbo from spinning any faster... so a 88mm tubro may only need to spin 10,000rpms to make 14psi on a certain engine at a certain RPM, but a 59mm turbo may need to spin 20,000rpms to make that same amount of airflow... even though the smaller turbo is working a lot harder to create that amount of airflow (and possibly out of efficiency), its still making the same 14psi that the larger turbo is (since the wastegate is keepign the larger turbo from spinning faster, and therefore creating more boost)....
I've already said it once... when thinking about it for an engine, your 1" straw is a stock ls1 intake, and your 2" straw is a 90mm fast intake. On the same boost level, you are going to make more power with the 2" straw(fast 90) because at that psi, more air is flowing ...
I know I was the valedictorean, but damn i can't see why its so hard to understand this concept... I can't think of any way to make it easier to understand... when speaking automotively the straw is NOT the turbo!!! the turbo is your LUNGS!! I'll see if i can dumb it down for you all... lets say you and a hooker off the street are going to both blow through a 1" straw... you (the smaller turbo) have to really blow hard to create 14psi of pressure in that 1" straw... hwoever, you were able to do it, and that 14psi of pressure was flowing 200cfm of air.... Now, the hooker (larger turbo) is an experienced "blower" with bigger lungs... therefore she doesn't have to work so hard to create 14psi inside that 1" straw... however, either way, there is still only 14psi of air in the straw, which no matter who's blowing air through it, its still only going to equal 200cfm of air going through the straw... the straw is your throttle body/intake manifold/heads - no matter how big the turbo is the air still needs to be reduced down to the size of your engine setup...
Want to think about it backward? maybe that'll maek it easier for you to understand... lets say each turbo is spinning 10,000rpms... the smaller turbo can only flow 150cfm of air at that rpm... the larger turbo can create 250cfm at that speed.... so yes, the larger turbo is flowing more air and can make more power... but when both flowing their respective amounts of air into the same engine, they're not going to have the same PSI!!!!!!!!!!!!!! the larger turbo will be creating more PSI, therfore more power.... if you want to keep it to a certain psi, you add a wastegate to slow the larger turbo down... then it slows down to 8000rpm, and is suddenly flowing 150cfm of air, just liek the smaller turbo... well now 150cfm of air is flowing out of each turbo toward the engine - and 150cfm of air is going to make a certain hp, no matter what source it comes from - small turbo, large turbo, or a hooker's lungs.... small changes in hp b/w turbos will be a result of the things i mentioned in an earlier post
holy ****... i'm out of comparisons
u guys can believe what you want, but when someone comes around that knows what they're talking about, and make you look like an idiot infront of your woman, just remember, "i told you so"
You're trying to imagine it with your straw example, but you're using your straw example in the wrong manner... we don't measure the psi in the turbo (what you are calling the straw).. you measure it at the manifold (the end point).... once the manifold is pressurized to a certain psi (in our example 14psi), the wastegate opens and keeps the turbo from spinning any faster... so a 88mm tubro may only need to spin 10,000rpms to make 14psi on a certain engine at a certain RPM, but a 59mm turbo may need to spin 20,000rpms to make that same amount of airflow... even though the smaller turbo is working a lot harder to create that amount of airflow (and possibly out of efficiency), its still making the same 14psi that the larger turbo is (since the wastegate is keepign the larger turbo from spinning faster, and therefore creating more boost)....
I've already said it once... when thinking about it for an engine, your 1" straw is a stock ls1 intake, and your 2" straw is a 90mm fast intake. On the same boost level, you are going to make more power with the 2" straw(fast 90) because at that psi, more air is flowing ...
I know I was the valedictorean, but damn i can't see why its so hard to understand this concept... I can't think of any way to make it easier to understand... when speaking automotively the straw is NOT the turbo!!! the turbo is your LUNGS!! I'll see if i can dumb it down for you all... lets say you and a hooker off the street are going to both blow through a 1" straw... you (the smaller turbo) have to really blow hard to create 14psi of pressure in that 1" straw... hwoever, you were able to do it, and that 14psi of pressure was flowing 200cfm of air.... Now, the hooker (larger turbo) is an experienced "blower" with bigger lungs... therefore she doesn't have to work so hard to create 14psi inside that 1" straw... however, either way, there is still only 14psi of air in the straw, which no matter who's blowing air through it, its still only going to equal 200cfm of air going through the straw... the straw is your throttle body/intake manifold/heads - no matter how big the turbo is the air still needs to be reduced down to the size of your engine setup...
Want to think about it backward? maybe that'll maek it easier for you to understand... lets say each turbo is spinning 10,000rpms... the smaller turbo can only flow 150cfm of air at that rpm... the larger turbo can create 250cfm at that speed.... so yes, the larger turbo is flowing more air and can make more power... but when both flowing their respective amounts of air into the same engine, they're not going to have the same PSI!!!!!!!!!!!!!! the larger turbo will be creating more PSI, therfore more power.... if you want to keep it to a certain psi, you add a wastegate to slow the larger turbo down... then it slows down to 8000rpm, and is suddenly flowing 150cfm of air, just liek the smaller turbo... well now 150cfm of air is flowing out of each turbo toward the engine - and 150cfm of air is going to make a certain hp, no matter what source it comes from - small turbo, large turbo, or a hooker's lungs.... small changes in hp b/w turbos will be a result of the things i mentioned in an earlier post
holy ****... i'm out of comparisons
u guys can believe what you want, but when someone comes around that knows what they're talking about, and make you look like an idiot infront of your woman, just remember, "i told you so" Last edited by SonofaBish; 03-04-2009 at 10:47 AM.
03-04-2009, 10:44 AM
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^ was waiting for that. and guys he does have a good point. PSI is measured at the very last point where the intake size is the same, thus the same CFM because the pressure is the same.
03-04-2009, 10:52 AM
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I'm open to debate, because i don't mean insult to anyone... but the FI world is a whole different world from what most LS1 cars are used to.... There's a lot of learning when it comes to FI, and most people in here will never be dumb enough to go turbo (like i was) so they won't ever have to learn it.... I spent 5-6 months researching and asking questions before i bought the turbos for my car - not to say turbo is a dumb move, I LOVED my car, but i should have thought about the fact that i wanted to build a house before i went and spent a ton on my car
03-04-2009, 10:55 AM
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Its a real shame that you guys have no idea what you're talking about - but yet still think you know something about it.....
You're trying to imagine it with your straw example, but you're using your straw example in the wrong manner... we don't measure the psi in the turbo (what you are calling the straw).. you measure it at the manifold (the end point).... once the manifold is pressurized to a certain psi (in our example 14psi), the wastegate opens and keeps the turbo from spinning any faster... so a 88mm tubro may only need to spin 10,000rpms to make 14psi on a certain engine at a certain RPM, but a 59mm turbo may need to spin 20,000rpms to make that same amount of airflow... even though the smaller turbo is working a lot harder to create that amount of airflow (and possibly out of efficiency), its still making the same 14psi that the larger turbo is (since the wastegate is keepign the larger turbo from spinning faster, and therefore creating more boost)....
I've already said it once... when thinking about it for an engine, your 1" straw is a stock ls1 intake, and your 2" straw is a 90mm fast intake. On the same boost level, you are going to make more power with the 2" straw(fast 90) because at that psi, more air is flowing ...
I know I was the valedictorean, but damn i can't see why its so hard to understand this concept... I can't think of any way to make it easier to understand... when speaking automotively the straw is NOT the turbo!!! the turbo is your LUNGS!! I'll see if i can dumb it down for you all... lets say you and a hooker off the street are going to both blow through a 1" straw... you (the smaller turbo) have to really blow hard to create 14psi of pressure in that 1" straw... hwoever, you were able to do it, and that 14psi of pressure was flowing 200cfm of air.... Now, the hooker (larger turbo) is an experienced "blower" with bigger lungs... therefore she doesn't have to work so hard to create 14psi inside that 1" straw... however, either way, there is still only 14psi of air in the straw, which no matter who's blowing air through it, its still only going to equal 200cfm of air going through the straw... the straw is your throttle body/intake manifold/heads - no matter how big the turbo is the air still needs to be reduced down to the size of your engine setup...
Want to think about it backward? maybe that'll maek it easier for you to understand... lets say each turbo is spinning 10,000rpms... the smaller turbo can only flow 150cfm of air at that rpm... the larger turbo can create 250cfm at that speed.... so yes, the larger turbo is flowing more air and can make more power... but when both flowing their respective amounts of air into the same engine, they're not going to have the same PSI!!!!!!!!!!!!!! the larger turbo will be creating more PSI, therfore more power.... if you want to keep it to a certain psi, you add a wastegate to slow the larger turbo down... then it slows down to 8000rpm, and is suddenly flowing 150cfm of air, just liek the smaller turbo... well now 150cfm of air is flowing out of each turbo toward the engine - and 150cfm of air is going to make a certain hp, no matter what source it comes from - small turbo, large turbo, or a hooker's lungs.... small changes in hp b/w turbos will be a result of the things i mentioned in an earlier post
holy ****... i'm out of comparisons u guys can believe what you want, but when someone comes around that knows what they're talking about, and make you look like an idiot infront of your woman, just remember, "i told you so"
You're trying to imagine it with your straw example, but you're using your straw example in the wrong manner... we don't measure the psi in the turbo (what you are calling the straw).. you measure it at the manifold (the end point).... once the manifold is pressurized to a certain psi (in our example 14psi), the wastegate opens and keeps the turbo from spinning any faster... so a 88mm tubro may only need to spin 10,000rpms to make 14psi on a certain engine at a certain RPM, but a 59mm turbo may need to spin 20,000rpms to make that same amount of airflow... even though the smaller turbo is working a lot harder to create that amount of airflow (and possibly out of efficiency), its still making the same 14psi that the larger turbo is (since the wastegate is keepign the larger turbo from spinning faster, and therefore creating more boost)....
I've already said it once... when thinking about it for an engine, your 1" straw is a stock ls1 intake, and your 2" straw is a 90mm fast intake. On the same boost level, you are going to make more power with the 2" straw(fast 90) because at that psi, more air is flowing ...
I know I was the valedictorean, but damn i can't see why its so hard to understand this concept... I can't think of any way to make it easier to understand... when speaking automotively the straw is NOT the turbo!!! the turbo is your LUNGS!! I'll see if i can dumb it down for you all... lets say you and a hooker off the street are going to both blow through a 1" straw... you (the smaller turbo) have to really blow hard to create 14psi of pressure in that 1" straw... hwoever, you were able to do it, and that 14psi of pressure was flowing 200cfm of air.... Now, the hooker (larger turbo) is an experienced "blower" with bigger lungs... therefore she doesn't have to work so hard to create 14psi inside that 1" straw... however, either way, there is still only 14psi of air in the straw, which no matter who's blowing air through it, its still only going to equal 200cfm of air going through the straw... the straw is your throttle body/intake manifold/heads - no matter how big the turbo is the air still needs to be reduced down to the size of your engine setup...
Want to think about it backward? maybe that'll maek it easier for you to understand... lets say each turbo is spinning 10,000rpms... the smaller turbo can only flow 150cfm of air at that rpm... the larger turbo can create 250cfm at that speed.... so yes, the larger turbo is flowing more air and can make more power... but when both flowing their respective amounts of air into the same engine, they're not going to have the same PSI!!!!!!!!!!!!!! the larger turbo will be creating more PSI, therfore more power.... if you want to keep it to a certain psi, you add a wastegate to slow the larger turbo down... then it slows down to 8000rpm, and is suddenly flowing 150cfm of air, just liek the smaller turbo... well now 150cfm of air is flowing out of each turbo toward the engine - and 150cfm of air is going to make a certain hp, no matter what source it comes from - small turbo, large turbo, or a hooker's lungs.... small changes in hp b/w turbos will be a result of the things i mentioned in an earlier post
holy ****... i'm out of comparisons
u guys can believe what you want, but when someone comes around that knows what they're talking about, and make you look like an idiot infront of your woman, just remember, "i told you so"
03-04-2009, 11:06 AM
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a certain CFM is going to create a specific PSI once restricted by a source.... there is no way for two different CFM flows to create the same PSI when both restricted by the same source....
03-04-2009, 11:17 AM
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I'm well aware of the difference....maybe i dont' describe it well...? but if you think i'm wrong, by all means, tell me what you think......
a certain CFM is going to create a specific PSI once restricted by a source.... there is no way for two different CFM flows to create the same PSI when both restricted by the same source....
a certain CFM is going to create a specific PSI once restricted by a source.... there is no way for two different CFM flows to create the same PSI when both restricted by the same source....
I think the difference in argument is that you're mentioning a motor can only take in so much air (straw reference) whereas Steiner and some of the others are talking about the amount of CFM a turbo can produce. If I'm correct, then you guys are arguing about two different things.
03-04-2009, 11:17 AM
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It would seem that being as boost pressure is measured INSIDE the intake,...and after the throttle body,..that 20 psi is 20 psi, is 20 psi. But it's not.
2 reasons why.
1. The smaller the turbo,..the harder it has to work to stuff the same amount of air through the compressor housing. The blades are turning faster to do the same job, and are cutting into the air more often,..and the air is being squeezed harder to get it through the small scroll of the compressor side. All this adds up to more heat in the intake air.
More heat in the intake air means the air will be less dense. And as we all know,...you can be at 600 feet above sea-level on a hot day and the car is sluggish,..and on the same piece of road on a cold day and the car is fast. Same air pressure,...but different air densities because of the temperature of the air.
Reason 2. What really counts is the total mass of the air being moved through the engine (see reason 1). If you had a really restrictive exhaust manifold,...not all of the combusted air could get out of the cylinders during the exhaust stroke. Some of it would be left in the cylinder.
Then durring the intake stroke,...the cylinder would injest less new air because some of the space would already be used up by exhaust gases left behind from the last combustion event. You can see what happens next. If the cylinder is injesting less air on the intake stroke,..it acts like a smaller engine,... and less new air delivered by the turbo will be required to maintain the 20 psi. As the engine gets bigger, (or the engine evacuates the exhaust more effectively) the more volume of air the turbo will be required to deliver to maintain boost pressure. Taken to the extreme,...picture a 1/2" diameter exhaust pipe. How much power would your car make? Even at 20 psi of boost, it would probably be under 80 whp.
I.e., ask yourself why headers make an engine more powerfull? Same intake air pressure. It's because they cause the cylinders to have more room for the next intake charge.
The hot-side of turbo acts as a restrictive header. The smaller the housing is,...or the more resistance the turbine wheel exibits (because it is working harder to turn the compressor wheel super fast), the more combustion gasses will be left in the cylinder. A big turbo that can easilly flow air through the compressor side will not require as much resistance to be put into the hot-side wheel (turbine) and the housing itself will also flow better. A double whammy in either direction.
Bottom line,...the bigger turbo will not restrict the exhaust as much,...it can flow air through the comprssor side easier and therefore inject less heat into the air,...and both of these will cause the turbo to have to flow more molecules of oxygen and nitrogen through the engine in order to get back up to the same boost pressure as the smaller turbo ran.
More volume of denser air means more molecules of air total,...and more air total means more stuff to expand,...and more oxygen and fuel to expand it with. All of which means more power.
So bottom line,...the 20 psi from the smaller turbo is 20 psi of hotter and less dense air,.....and the engine is taking smaller gulps of this less dense air at each cylinder filling.
What we really want to measure is the number of molecules of air we are flowing through the engine. We want to know the combined information of density, temperature and speed,...the combination of which gives us the amount of "mass" we are flowing (I.e. the number of air molecules). In fact this is so important,..there is a meter that does just that. It's called a "mass-flow meter". It's in the intake as you probably know. A constant amount of electricity heats a thin wire. Then a sensor measures the temperature of the wire. As air moves past the wire faster,...or is colder,..or is more dense,......more heat will be carried away from the wire, and the wire will get colder. We can read this in the output of the meter as voltage. This is why a tuner will pay far more attention to the mass flow meters' voltage output than he will to boost pressure.
And if that doesn't do it
PV=nRT
ideal gas law
p is the absolute pressure
v is the volume of the vessel
n is the amount of substance of gas
r is the ideal gas constant
t is the absolute temperature
2 reasons why.
1. The smaller the turbo,..the harder it has to work to stuff the same amount of air through the compressor housing. The blades are turning faster to do the same job, and are cutting into the air more often,..and the air is being squeezed harder to get it through the small scroll of the compressor side. All this adds up to more heat in the intake air.
More heat in the intake air means the air will be less dense. And as we all know,...you can be at 600 feet above sea-level on a hot day and the car is sluggish,..and on the same piece of road on a cold day and the car is fast. Same air pressure,...but different air densities because of the temperature of the air.
Reason 2. What really counts is the total mass of the air being moved through the engine (see reason 1). If you had a really restrictive exhaust manifold,...not all of the combusted air could get out of the cylinders during the exhaust stroke. Some of it would be left in the cylinder.
Then durring the intake stroke,...the cylinder would injest less new air because some of the space would already be used up by exhaust gases left behind from the last combustion event. You can see what happens next. If the cylinder is injesting less air on the intake stroke,..it acts like a smaller engine,... and less new air delivered by the turbo will be required to maintain the 20 psi. As the engine gets bigger, (or the engine evacuates the exhaust more effectively) the more volume of air the turbo will be required to deliver to maintain boost pressure. Taken to the extreme,...picture a 1/2" diameter exhaust pipe. How much power would your car make? Even at 20 psi of boost, it would probably be under 80 whp.
I.e., ask yourself why headers make an engine more powerfull? Same intake air pressure. It's because they cause the cylinders to have more room for the next intake charge.
The hot-side of turbo acts as a restrictive header. The smaller the housing is,...or the more resistance the turbine wheel exibits (because it is working harder to turn the compressor wheel super fast), the more combustion gasses will be left in the cylinder. A big turbo that can easilly flow air through the compressor side will not require as much resistance to be put into the hot-side wheel (turbine) and the housing itself will also flow better. A double whammy in either direction.
Bottom line,...the bigger turbo will not restrict the exhaust as much,...it can flow air through the comprssor side easier and therefore inject less heat into the air,...and both of these will cause the turbo to have to flow more molecules of oxygen and nitrogen through the engine in order to get back up to the same boost pressure as the smaller turbo ran.
More volume of denser air means more molecules of air total,...and more air total means more stuff to expand,...and more oxygen and fuel to expand it with. All of which means more power.
So bottom line,...the 20 psi from the smaller turbo is 20 psi of hotter and less dense air,.....and the engine is taking smaller gulps of this less dense air at each cylinder filling.
What we really want to measure is the number of molecules of air we are flowing through the engine. We want to know the combined information of density, temperature and speed,...the combination of which gives us the amount of "mass" we are flowing (I.e. the number of air molecules). In fact this is so important,..there is a meter that does just that. It's called a "mass-flow meter". It's in the intake as you probably know. A constant amount of electricity heats a thin wire. Then a sensor measures the temperature of the wire. As air moves past the wire faster,...or is colder,..or is more dense,......more heat will be carried away from the wire, and the wire will get colder. We can read this in the output of the meter as voltage. This is why a tuner will pay far more attention to the mass flow meters' voltage output than he will to boost pressure.
And if that doesn't do it
PV=nRT
ideal gas law
p is the absolute pressure
v is the volume of the vessel
n is the amount of substance of gas
r is the ideal gas constant
t is the absolute temperature
03-04-2009, 11:19 AM
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I'm well aware of the difference....maybe i dont' describe it well...? but if you think i'm wrong, by all means, tell me what you think......
a certain CFM is going to create a specific PSI once restricted by a source.... there is no way for two different CFM flows to create the same PSI when both restricted by the same source....
a certain CFM is going to create a specific PSI once restricted by a source.... there is no way for two different CFM flows to create the same PSI when both restricted by the same source....
03-04-2009, 11:24 AM
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well, i didnt read all of that ^^^ and just skimmed through it, but it sounds like you mentioned two things that i said earlier would affect the power - heat and difference in wheels, etc ...
either way, with an intercooler, i think the relative % change isn't going to be drastic ... 10hp on a 100hp motor is the same % change as 100hp on a 1000 hp motor... 100 sounds much worse than 10.....
but hey, i could be wrong - wouldnt be the first time... but until i see someone do a test back to back, i have my doubts the difference in hp would be DRASTIC, which is what i've been saying all along
either way, with an intercooler, i think the relative % change isn't going to be drastic ... 10hp on a 100hp motor is the same % change as 100hp on a 1000 hp motor... 100 sounds much worse than 10.....
but hey, i could be wrong - wouldnt be the first time... but until i see someone do a test back to back, i have my doubts the difference in hp would be DRASTIC, which is what i've been saying all along
03-04-2009, 11:27 AM
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It would seem that being as boost pressure is measured INSIDE the intake,...and after the throttle body,..that 20 psi is 20 psi, is 20 psi. But it's not.
2 reasons why.
1. The smaller the turbo,..the harder it has to work to stuff the same amount of air through the compressor housing. The blades are turning faster to do the same job, and are cutting into the air more often,..and the air is being squeezed harder to get it through the small scroll of the compressor side. All this adds up to more heat in the intake air.
More heat in the intake air means the air will be less dense. And as we all know,...you can be at 600 feet above sea-level on a hot day and the car is sluggish,..and on the same piece of road on a cold day and the car is fast. Same air pressure,...but different air densities because of the temperature of the air.
Reason 2. What really counts is the total mass of the air being moved through the engine (see reason 1). If you had a really restrictive exhaust manifold,...not all of the combusted air could get out of the cylinders during the exhaust stroke. Some of it would be left in the cylinder.
Then durring the intake stroke,...the cylinder would injest less new air because some of the space would already be used up by exhaust gases left behind from the last combustion event. You can see what happens next. If the cylinder is injesting less air on the intake stroke,..it acts like a smaller engine,... and less new air delivered by the turbo will be required to maintain the 20 psi. As the engine gets bigger, (or the engine evacuates the exhaust more effectively) the more volume of air the turbo will be required to deliver to maintain boost pressure. Taken to the extreme,...picture a 1/2" diameter exhaust pipe. How much power would your car make? Even at 20 psi of boost, it would probably be under 80 whp.
I.e., ask yourself why headers make an engine more powerfull? Same intake air pressure. It's because they cause the cylinders to have more room for the next intake charge.
The hot-side of turbo acts as a restrictive header. The smaller the housing is,...or the more resistance the turbine wheel exibits (because it is working harder to turn the compressor wheel super fast), the more combustion gasses will be left in the cylinder. A big turbo that can easilly flow air through the compressor side will not require as much resistance to be put into the hot-side wheel (turbine) and the housing itself will also flow better. A double whammy in either direction.
Bottom line,...the bigger turbo will not restrict the exhaust as much,...it can flow air through the comprssor side easier and therefore inject less heat into the air,...and both of these will cause the turbo to have to flow more molecules of oxygen and nitrogen through the engine in order to get back up to the same boost pressure as the smaller turbo ran.
More volume of denser air means more molecules of air total,...and more air total means more stuff to expand,...and more oxygen and fuel to expand it with. All of which means more power.
So bottom line,...the 20 psi from the smaller turbo is 20 psi of hotter and less dense air,.....and the engine is taking smaller gulps of this less dense air at each cylinder filling.
What we really want to measure is the number of molecules of air we are flowing through the engine. We want to know the combined information of density, temperature and speed,...the combination of which gives us the amount of "mass" we are flowing (I.e. the number of air molecules). In fact this is so important,..there is a meter that does just that. It's called a "mass-flow meter". It's in the intake as you probably know. A constant amount of electricity heats a thin wire. Then a sensor measures the temperature of the wire. As air moves past the wire faster,...or is colder,..or is more dense,......more heat will be carried away from the wire, and the wire will get colder. We can read this in the output of the meter as voltage. This is why a tuner will pay far more attention to the mass flow meters' voltage output than he will to boost pressure.
And if that doesn't do it
PV=nRT
ideal gas law
p is the absolute pressure
v is the volume of the vessel
n is the amount of substance of gas
r is the ideal gas constant
t is the absolute temperature
2 reasons why.
1. The smaller the turbo,..the harder it has to work to stuff the same amount of air through the compressor housing. The blades are turning faster to do the same job, and are cutting into the air more often,..and the air is being squeezed harder to get it through the small scroll of the compressor side. All this adds up to more heat in the intake air.
More heat in the intake air means the air will be less dense. And as we all know,...you can be at 600 feet above sea-level on a hot day and the car is sluggish,..and on the same piece of road on a cold day and the car is fast. Same air pressure,...but different air densities because of the temperature of the air.
Reason 2. What really counts is the total mass of the air being moved through the engine (see reason 1). If you had a really restrictive exhaust manifold,...not all of the combusted air could get out of the cylinders during the exhaust stroke. Some of it would be left in the cylinder.
Then durring the intake stroke,...the cylinder would injest less new air because some of the space would already be used up by exhaust gases left behind from the last combustion event. You can see what happens next. If the cylinder is injesting less air on the intake stroke,..it acts like a smaller engine,... and less new air delivered by the turbo will be required to maintain the 20 psi. As the engine gets bigger, (or the engine evacuates the exhaust more effectively) the more volume of air the turbo will be required to deliver to maintain boost pressure. Taken to the extreme,...picture a 1/2" diameter exhaust pipe. How much power would your car make? Even at 20 psi of boost, it would probably be under 80 whp.
I.e., ask yourself why headers make an engine more powerfull? Same intake air pressure. It's because they cause the cylinders to have more room for the next intake charge.
The hot-side of turbo acts as a restrictive header. The smaller the housing is,...or the more resistance the turbine wheel exibits (because it is working harder to turn the compressor wheel super fast), the more combustion gasses will be left in the cylinder. A big turbo that can easilly flow air through the compressor side will not require as much resistance to be put into the hot-side wheel (turbine) and the housing itself will also flow better. A double whammy in either direction.
Bottom line,...the bigger turbo will not restrict the exhaust as much,...it can flow air through the comprssor side easier and therefore inject less heat into the air,...and both of these will cause the turbo to have to flow more molecules of oxygen and nitrogen through the engine in order to get back up to the same boost pressure as the smaller turbo ran.
More volume of denser air means more molecules of air total,...and more air total means more stuff to expand,...and more oxygen and fuel to expand it with. All of which means more power.
So bottom line,...the 20 psi from the smaller turbo is 20 psi of hotter and less dense air,.....and the engine is taking smaller gulps of this less dense air at each cylinder filling.
What we really want to measure is the number of molecules of air we are flowing through the engine. We want to know the combined information of density, temperature and speed,...the combination of which gives us the amount of "mass" we are flowing (I.e. the number of air molecules). In fact this is so important,..there is a meter that does just that. It's called a "mass-flow meter". It's in the intake as you probably know. A constant amount of electricity heats a thin wire. Then a sensor measures the temperature of the wire. As air moves past the wire faster,...or is colder,..or is more dense,......more heat will be carried away from the wire, and the wire will get colder. We can read this in the output of the meter as voltage. This is why a tuner will pay far more attention to the mass flow meters' voltage output than he will to boost pressure.
And if that doesn't do it
PV=nRT
ideal gas law
p is the absolute pressure
v is the volume of the vessel
n is the amount of substance of gas
r is the ideal gas constant
t is the absolute temperature
03-04-2009, 11:43 AM
#55
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what i think he is trying to say is the area you are filling with air IE the intake plenum, at 14 psi will always have the same CFM because thats the limiting factor, sure if you port your intake manifold, then with the same boost you can achieve more CFM, but regaurdless of what size the turbo is pushing air in, the intake plenum can only hold a certain level of CFM at a certain boost level.
I've already said it once... when thinking about it for an engine, your 1" straw is a stock ls1 intake, and your 2" straw is a 90mm fast intake. On the same boost level, you are going to make more power with the 2" straw(fast 90) because at that psi, more air is flowing ...
What it comes down to is where your boost source is setup. For instance you say that you measure PSI at the intake manifold. On the Evo's, the stock boost solenoid is connected to the outlet pipe on the turbo then back to the wastegate. So if I run the line off of my outlet pipe to a boost gauge, then change out my intake manifold, my gauge will still read the same PSI and I won't gain any more power because the pressure dropped inside the actual manifold but remained the same coming out of the turbo. If my boost gauge is setup on the intake manifold, then the pressure should drop because it's less restrictive. In order to keep the pressure the same inside the manifold, I must make the turbo work a little harder and spin it a little faster thus creating more CFM. It's all relative to where you measure your PSI.
I'm also open for debate, if I'm wrong then I welcome corrections/criticism. Always up for learning new things.
03-04-2009, 12:05 PM
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It means a bigger turbo is going to move just a little more air, make a colder charge and be less restrictive in the exhaust which all causes just a little more power with the combination.
The CFM vs PSI debate has been proven and that was the equation I wrote. PSI is based off of resistance and not volume. Take a car and run it at 3000' and one at sea level, which car makes more power? The car at sea level because it sees more volume of air even though cars run at abolute pressure. Same thing with a turbo car on 10PSI the bigger turbo is going to be able to condense the air better at the same PSI to make it move more air at the same PSI but it will be very small difference. The reason it will make the air more condensed is because the wheels are in a more efficient range and will heat the air less and have a bigger blade to condense the air.
03-04-2009, 12:34 PM
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Yes, I believe we are on the same page. I'll use this quote as an example:
If you go from a 1" straw (stock ls1 intake) to a 2" straw (90mm fast intake) and change NOTHING else, meaning you don't manually up the boost, your boost will drop. Maybe a few PSI, maybe half a PSI, but any time you lessen the restriction you lessen the pressure.
What it comes down to is where your boost source is setup. For instance you say that you measure PSI at the intake manifold. On the Evo's, the stock boost solenoid is connected to the outlet pipe on the turbo then back to the wastegate. So if I run the line off of my outlet pipe to a boost gauge, then change out my intake manifold, my gauge will still read the same PSI and I won't gain any more power because the pressure dropped inside the actual manifold but remained the same coming out of the turbo. If my boost gauge is setup on the intake manifold, then the pressure should drop because it's less restrictive. In order to keep the pressure the same inside the manifold, I must make the turbo work a little harder and spin it a little faster thus creating more CFM. It's all relative to where you measure your PSI.
I'm also open for debate, if I'm wrong then I welcome corrections/criticism. Always up for learning new things.
If you go from a 1" straw (stock ls1 intake) to a 2" straw (90mm fast intake) and change NOTHING else, meaning you don't manually up the boost, your boost will drop. Maybe a few PSI, maybe half a PSI, but any time you lessen the restriction you lessen the pressure.
What it comes down to is where your boost source is setup. For instance you say that you measure PSI at the intake manifold. On the Evo's, the stock boost solenoid is connected to the outlet pipe on the turbo then back to the wastegate. So if I run the line off of my outlet pipe to a boost gauge, then change out my intake manifold, my gauge will still read the same PSI and I won't gain any more power because the pressure dropped inside the actual manifold but remained the same coming out of the turbo. If my boost gauge is setup on the intake manifold, then the pressure should drop because it's less restrictive. In order to keep the pressure the same inside the manifold, I must make the turbo work a little harder and spin it a little faster thus creating more CFM. It's all relative to where you measure your PSI.
I'm also open for debate, if I'm wrong then I welcome corrections/criticism. Always up for learning new things.
I guess that's the difference in setups... my boost reference was on the intake manifold, along with the MAP reference for the 3bar SD tune
03-04-2009, 01:23 PM
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It means a bigger turbo is going to move just a little more air, make a colder charge and be less restrictive in the exhaust which all causes just a little more power with the combination.
The CFM vs PSI debate has been proven and that was the equation I wrote. PSI is based off of resistance and not volume. Take a car and run it at 3000' and one at sea level, which car makes more power? The car at sea level because it sees more volume of air even though cars run at abolute pressure. Same thing with a turbo car on 10PSI the bigger turbo is going to be able to condense the air better at the same PSI to make it move more air at the same PSI but it will be very small difference. The reason it will make the air more condensed is because the wheels are in a more efficient range and will heat the air less and have a bigger blade to condense the air.
The CFM vs PSI debate has been proven and that was the equation I wrote. PSI is based off of resistance and not volume. Take a car and run it at 3000' and one at sea level, which car makes more power? The car at sea level because it sees more volume of air even though cars run at abolute pressure. Same thing with a turbo car on 10PSI the bigger turbo is going to be able to condense the air better at the same PSI to make it move more air at the same PSI but it will be very small difference. The reason it will make the air more condensed is because the wheels are in a more efficient range and will heat the air less and have a bigger blade to condense the air.
03-04-2009, 01:51 PM
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Just my 02. I think alot of the confusion here is turbo vs supercharger. A larger supercharger flowing the same PSI uses less HP to drive and therefor produces more power to the rear wheels. While heat and resistance limit the HP a turbo motor makes it is not as dramatic as a supercharger.