Basic turbo question
What KevinD said was very detailed and instructive, but couple things he mentioned needed to be corrected.
sharkm87, small and big turbo do NOT move same amount of air.
They move same volume of air because of limitations of actual 3d volume the cylinders occupy, but they move same volume of air in different velocity and different DENSITY which is mass/volume. Mass is greater for the big turbo.
We all know which is bigger by looking at 30lb/min wheel vs 50lb/min wheel right?
sharkm87, small and big turbo do NOT move same amount of air.
They move same volume of air because of limitations of actual 3d volume the cylinders occupy, but they move same volume of air in different velocity and different DENSITY which is mass/volume. Mass is greater for the big turbo.
We all know which is bigger by looking at 30lb/min wheel vs 50lb/min wheel right?
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Argh!
Yeah this thread I think kinda boiled down to some really great info combined with people missing the distinction between volumetric flow and mass flow. That's why they call that thing stuck to your air filter a MAF and not a VAF.
EDIT: BTW as funny as the dead-horse emoticon is, it kinda makes me cross my legs after watching it for a while.
Yeah this thread I think kinda boiled down to some really great info combined with people missing the distinction between volumetric flow and mass flow. That's why they call that thing stuck to your air filter a MAF and not a VAF.
EDIT: BTW as funny as the dead-horse emoticon is, it kinda makes me cross my legs after watching it for a while.
Last edited by Joehunk; Aug 15, 2007 at 11:18 AM.
yup 
you've basically repeated what i've said 
however, i will agree with you that CFM is a volumetric flow rate (which is repeating what i said), however, the CFM going into the motor (not right after the compressor), is entirely dictated by the motor, not the turbo. again, the volume is fixed. its the volume of the motor. the density changes... and the turbo and intercooler are what effect that, thus thats why i pointed out the mass flow rate, which indicates that a bigger turbo does have a higher mass flow rate. the mass flow rate going into the motor is different from one turbo to the next, but the volumetric flow rate is not different.
the velocity of the air is dictated by the CFM, which is dictated by the engine displacement. so the speed of the air going into the motor is dependant on how fast the motor is going.
at higher RPMs when the stock turbo "tapers" boost, its because the mass flow rate of the turbo is insufficient to maintain that pressure at that rpm. the CFM of the motor is still increasing as the rpms increase, but the mass flow rate relative to RPM drops off.
volume rate increases (i.e. rpms go up) -> mass flow rate stays the say (small turbo limited) -> efficiency drops off (temperatures increase) -> same mass, more volume higher temperatures = less pressure, lower density
bigger turbo:
volume rate increases (i.e. rpms go up) -> mass flow rate also increases (big turbo not limited) -> efficiency is also better (temperatures are lower) -> volume rate increases, mass rate increases, temperatures remain cooler = pressure remains high, density also remains high = much more air.
one thing to consider in all this is where the big turbos make the power. everyone knows a 35r on the 2L with 93 oct and 23psi doesn't make power until deep in the RPM range, say peak power at 7-8k rpm. this is where the stock turbo runs out of steam a 35R with 23psi at 7k rpm is far more psi then the stock turbo is going to give (what, the stock turbo is lucky to give 21psi pushing as hard as it can at 7000rpm).
if you analyze the dyno graphs you will see that a stock turbo will actually makeas much or more torque then a 35R on the same motor with same gas/boost level. thats because the stock turbo makes the power earlier, and this is where horse power has a smaller multiplication from the torque because of the lower RPM. at higher rpm, if the stock turbo could maintain the 300+ftlb of torque, it would have awsome power numbers just like the 35r. 300 ftlb of torque at 7000rpm is 400 whp. 300ftlb at 3500rpm is only 200hp. you can see why a 35r has higher HP numbers all things being equal. the torque numbers generally are in the same ball park, thats because the VOLUMETRIC flow rate is the relatively the same for both motors. but the higher efficiency of the 35r (i.e. lower temperatures when the air is compressed), and the ability to maintain the mass flow rate as the volumetric flow rate increases is where you see the torque numbers hold strong at higher RPM, and thus much better HP numbers.
you've basically repeated what i've said however, i will agree with you that CFM is a volumetric flow rate (which is repeating what i said), however, the CFM going into the motor (not right after the compressor), is entirely dictated by the motor, not the turbo. again, the volume is fixed. its the volume of the motor. the density changes... and the turbo and intercooler are what effect that, thus thats why i pointed out the mass flow rate, which indicates that a bigger turbo does have a higher mass flow rate. the mass flow rate going into the motor is different from one turbo to the next, but the volumetric flow rate is not different.
the velocity of the air is dictated by the CFM, which is dictated by the engine displacement. so the speed of the air going into the motor is dependant on how fast the motor is going.
at higher RPMs when the stock turbo "tapers" boost, its because the mass flow rate of the turbo is insufficient to maintain that pressure at that rpm. the CFM of the motor is still increasing as the rpms increase, but the mass flow rate relative to RPM drops off.
volume rate increases (i.e. rpms go up) -> mass flow rate stays the say (small turbo limited) -> efficiency drops off (temperatures increase) -> same mass, more volume higher temperatures = less pressure, lower density
bigger turbo:
volume rate increases (i.e. rpms go up) -> mass flow rate also increases (big turbo not limited) -> efficiency is also better (temperatures are lower) -> volume rate increases, mass rate increases, temperatures remain cooler = pressure remains high, density also remains high = much more air.
one thing to consider in all this is where the big turbos make the power. everyone knows a 35r on the 2L with 93 oct and 23psi doesn't make power until deep in the RPM range, say peak power at 7-8k rpm. this is where the stock turbo runs out of steam a 35R with 23psi at 7k rpm is far more psi then the stock turbo is going to give (what, the stock turbo is lucky to give 21psi pushing as hard as it can at 7000rpm).
if you analyze the dyno graphs you will see that a stock turbo will actually makeas much or more torque then a 35R on the same motor with same gas/boost level. thats because the stock turbo makes the power earlier, and this is where horse power has a smaller multiplication from the torque because of the lower RPM. at higher rpm, if the stock turbo could maintain the 300+ftlb of torque, it would have awsome power numbers just like the 35r. 300 ftlb of torque at 7000rpm is 400 whp. 300ftlb at 3500rpm is only 200hp. you can see why a 35r has higher HP numbers all things being equal. the torque numbers generally are in the same ball park, thats because the VOLUMETRIC flow rate is the relatively the same for both motors. but the higher efficiency of the 35r (i.e. lower temperatures when the air is compressed), and the ability to maintain the mass flow rate as the volumetric flow rate increases is where you see the torque numbers hold strong at higher RPM, and thus much better HP numbers.
Cubic feet per minute is a non-SI unit of measurement of air-flow (or gas) that indicates how much cubit feet of air (or gas) pass by a stationary point in one minute.
At a given rpm (not rising or declining because we're not talking about compressor efficiency), CFM is dictated by the TURBO, NOT THE ENGINE because
which device is actually moving the air???? Compressor wheel (inducer, exducer) are the ones that are actually moving the air.
I don't have a better way of saying this but, Bigger the turbo, bigger the CFM.
What you and me are arguing is like egg or chicken first?
I'm starting the flow of air from the turbo whereas I THINK you are starting the flow of air from the engine.....
[QUOTE=KevinD;4647666]
however, the CFM going into the motor (not right after the compressor), is entirely dictated by the motor, not the turbo.
This is where our agreements diverge.
CFM of the engine changes when the rpm or the displacement changes.
Since the displacement of the engine is the same whether small turbo, big turbo,
the change in rpm will indeed change the CFM going INTO the engine and hence the efficiency of the compressor wheels kick in.
But I was looking at this with the two same engines (2.0L) with different sized turbo that are at the SAME rpm so that we can compare only 1 variable without making other variables such as effiency kick in by changing the rpm.
We all know the bottom line is big turbo moves more air than the small turbo.
however, the CFM going into the motor (not right after the compressor), is entirely dictated by the motor, not the turbo.
This is where our agreements diverge.
CFM of the engine changes when the rpm or the displacement changes.
Since the displacement of the engine is the same whether small turbo, big turbo,
the change in rpm will indeed change the CFM going INTO the engine and hence the efficiency of the compressor wheels kick in.
But I was looking at this with the two same engines (2.0L) with different sized turbo that are at the SAME rpm so that we can compare only 1 variable without making other variables such as effiency kick in by changing the rpm.
We all know the bottom line is big turbo moves more air than the small turbo.
I don't think we're on the same boat here.
Cubic feet per minute is a non-SI unit of measurement of air-flow (or gas) that indicates how much cubit feet of air (or gas) pass by a stationary point in one minute.
At a given rpm (not rising or declining because we're not talking about compressor efficiency), CFM is dictated by the TURBO, NOT THE ENGINE because
which device is actually moving the air???? Compressor wheel (inducer, exducer) are the ones that are actually moving the air.
I don't have a better way of saying this but, Bigger the turbo, bigger the CFM.
What you and me are arguing is like egg or chicken first?
I'm starting the flow of air from the turbo whereas I THINK you are starting the flow of air from the engine.....
Cubic feet per minute is a non-SI unit of measurement of air-flow (or gas) that indicates how much cubit feet of air (or gas) pass by a stationary point in one minute.
At a given rpm (not rising or declining because we're not talking about compressor efficiency), CFM is dictated by the TURBO, NOT THE ENGINE because
which device is actually moving the air???? Compressor wheel (inducer, exducer) are the ones that are actually moving the air.
I don't have a better way of saying this but, Bigger the turbo, bigger the CFM.
What you and me are arguing is like egg or chicken first?
I'm starting the flow of air from the turbo whereas I THINK you are starting the flow of air from the engine.....
At X RPM, the engine will only consume Y ft.³ of air per time interval. You could put a tiny tiny turbo in the system, or a GT42R, and the motor will still only use Y CFM. The larger turbo *CAN* flow a higher CFM, but that only matters when you're past the efficiency of a small turbo and you can no longer pressurize the intake tract to the desired PSI anymore.
The gain in power comes from the decreased intake charge temperature of the larger turbo allowing the density of the oxygen in the charge to increase.
Without getting into some incredibly over-complex discussion, the velocity of the air in the intake tract will be essentially the same, since it is still limited to a given CFM at a given pressure. Especially since it's not a straight path and any gain there might be from the compressor is most likely *****owned by the turbulence created from the intercooler/bends in tubing/etc.
Argh!
Yeah this thread I think kinda boiled down to some really great info combined with people missing the distinction between volumetric flow and mass flow. That's why they call that thing stuck to your air filter a MAF and not a VAF.
EDIT: BTW as funny as the dead-horse emoticon is, it kinda makes me cross my legs after watching it for a while.
Yeah this thread I think kinda boiled down to some really great info combined with people missing the distinction between volumetric flow and mass flow. That's why they call that thing stuck to your air filter a MAF and not a VAF.
EDIT: BTW as funny as the dead-horse emoticon is, it kinda makes me cross my legs after watching it for a while.
Wow, a self-pronounced engineer discovered a concept of mass. And dissed his own thread. Way to go.
The system is limited by the volume of the cylinders.
At X RPM, the engine will only consume Y ft.³ of air per time interval. You could put a tiny tiny turbo in the system, or a GT42R, and the motor will still only use Y CFM. The larger turbo *CAN* flow a higher CFM, but that only matters when you're past the efficiency of a small turbo and you can no longer pressurize the intake tract to the desired PSI anymore.
The gain in power comes from the decreased intake charge temperature of the larger turbo allowing the density of the oxygen in the charge to increase.
Without getting into some incredibly over-complex discussion, the velocity of the air in the intake tract will be essentially the same, since it is still limited to a given CFM at a given pressure. Especially since it's not a straight path and any gain there might be from the compressor is most likely *****owned by the turbulence created from the intercooler/bends in tubing/etc.
At X RPM, the engine will only consume Y ft.³ of air per time interval. You could put a tiny tiny turbo in the system, or a GT42R, and the motor will still only use Y CFM. The larger turbo *CAN* flow a higher CFM, but that only matters when you're past the efficiency of a small turbo and you can no longer pressurize the intake tract to the desired PSI anymore.
The gain in power comes from the decreased intake charge temperature of the larger turbo allowing the density of the oxygen in the charge to increase.
Without getting into some incredibly over-complex discussion, the velocity of the air in the intake tract will be essentially the same, since it is still limited to a given CFM at a given pressure. Especially since it's not a straight path and any gain there might be from the compressor is most likely *****owned by the turbulence created from the intercooler/bends in tubing/etc.
I dont' know where your claim is coming from but the large turbo will always create greater CFM than a smaller turbo at a given pressure.
And let's take this literally. CFM is cubic feet of air passing through one point in one minute. If one's CFM is greater when the volume is constant (intake manifold being the same, engine size being the same), of course the velocity has to be greater. It doesn't take a rocket scientist to understand how the CFM works.
oh well..w/e i dont' care anymore
I dont' know where your claim is coming from but the large turbo will always create greater CFM than a smaller turbo at a given pressure.
And let's take this literally. CFM is cubic feet of air passing through one point in one minute. If one's CFM is greater when the volume is constant (intake manifold being the same, engine size being the same), of course the velocity has to be greater. It doesn't take a rocket scientist to understand how the CFM works.
oh well..w/e i dont' care anymore
And let's take this literally. CFM is cubic feet of air passing through one point in one minute. If one's CFM is greater when the volume is constant (intake manifold being the same, engine size being the same), of course the velocity has to be greater. It doesn't take a rocket scientist to understand how the CFM works.
oh well..w/e i dont' care anymore
This isn't an open-ended tube with a fixed volume, where increasing the velocity of the air will increase the CFM. It's a gated "airlock" type system that has a fixed exit rate.
I dont' know where your claim is coming from but the large turbo will always create greater CFM than a smaller turbo at a given pressure.
And let's take this literally. CFM is cubic feet of air passing through one point in one minute. If one's CFM is greater when the volume is constant (intake manifold being the same, engine size being the same), of course the velocity has to be greater. It doesn't take a rocket scientist to understand how the CFM works.
oh well..w/e i dont' care anymore
And let's take this literally. CFM is cubic feet of air passing through one point in one minute. If one's CFM is greater when the volume is constant (intake manifold being the same, engine size being the same), of course the velocity has to be greater. It doesn't take a rocket scientist to understand how the CFM works.
oh well..w/e i dont' care anymore
this made me laugh, if you have a fixed volume rate (i.e. holding rpm constant) how can you have an increase in CFM? cause as you said yourself, CFM is a VOLUME rate. and no, the turbo is not what is moving the air, the MOTOR is what moves the air. if you take away the turbo, a motor will still flow air at a given CFM rate depending on RPM (as pessimistick has pointed out). however, without a motor, a turbo will not flow anything 
also, sense i just so happen to be a rocket scientist, i think it may take a rocket scientist to understand this stuff
i do see what your getting at... at the outlet of the turbo the CFM rate will be different for a given turbo, however that CFM rate depends on intercooler effciency and turbo efficiency because the amount of change in the density before and after the intercooler is what will dictate the CFM delta right after the turbo and going into the engine. again, the engine strickly controls the CFM going into the engine NOT THE TURBO. if you take away the intercooler and all the piping between the turbo and engine, the CFM of the turbo will have to be exactly the same as the engine, no more. my point is shown by if the turbo is right on the engine, and the engine is not spinning but the turbo is, what is the CFM of the turbo? 0 would be the correct answer. if you understand the concept of mass flow rate, you would see that it is what you are thinking about. the mass flow rate of a gt42r is massive compared to the 16g. but the cfm will be relatively close at a given rpm/boost level.
You guys seriousely need to stop talking about CFM.
Engine's VE is only good for a certain CFM. And if your engine is good for 500, it does not matter wether your turbo is good for 1000, you still will only fit 500 in there.
Also OP's question is tricky anyway. He runs 23-ish at 4k rpm, which drops off to probably 19 by red line, because it's INEFFICIENT when it has to spin this hard.
For a big turbo it's a piece of cake to create 23 psi of resistance at high rpm's.
Torque numbers in low rpm's will be similar, but a big turbo is able to flow same mass of air waaay past a small one has begun reducing the flow. And that's where you are gonna make power.
Engine's VE is only good for a certain CFM. And if your engine is good for 500, it does not matter wether your turbo is good for 1000, you still will only fit 500 in there.
Also OP's question is tricky anyway. He runs 23-ish at 4k rpm, which drops off to probably 19 by red line, because it's INEFFICIENT when it has to spin this hard.
For a big turbo it's a piece of cake to create 23 psi of resistance at high rpm's.
Torque numbers in low rpm's will be similar, but a big turbo is able to flow same mass of air waaay past a small one has begun reducing the flow. And that's where you are gonna make power.
There is a very simple answer to this question: The air is more dense at the same volume of air, because the turbo is more efficient.
This big hose/small hose stuff is nonsense. It's not because it is "flowing more" either. You are measuring the pressure after the turbo in the same size hose, regardless of how big the turbo is. In other words, with a more efficient turbo with the wastegate set to actuate at the same pressure AFTER the turbo, you will get the exact same VOLUME of air, but it will be cooler and therefore have more MASS, and more OXYGEN, so there is more potential for more BURNING OF FUEL, and less of a risk of PRE-IGNITION (causing knock), so timing may be advanced over the less efficient turbo. Any increase in VOLUME over the stock turbo going into the same plumbing will INCREASE THE PRESSURE which is not what the OP was asking about.
This big hose/small hose stuff is nonsense. It's not because it is "flowing more" either. You are measuring the pressure after the turbo in the same size hose, regardless of how big the turbo is. In other words, with a more efficient turbo with the wastegate set to actuate at the same pressure AFTER the turbo, you will get the exact same VOLUME of air, but it will be cooler and therefore have more MASS, and more OXYGEN, so there is more potential for more BURNING OF FUEL, and less of a risk of PRE-IGNITION (causing knock), so timing may be advanced over the less efficient turbo. Any increase in VOLUME over the stock turbo going into the same plumbing will INCREASE THE PRESSURE which is not what the OP was asking about.
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From: wexford,pa
we still confused and on this topic, did you not read my post?
here ok.
Because the displacement of the engine is a physical dimension, the discharge flow volume (cfm) of the turbocharger (except when Intercooled) is equal to the displaced volume of the engine at speed as corrected by volumetric efficiency of the engine. The inlet cfm through the air cleaner is something more than the engine displacement. This has led some people to claim that turbochargers increase the cfm of the engine. In actuality, the cfm of the engine hasn’t changed, but the DENSITY of the air has.
This is an important principle. Turbocharging increases the DENSITY of the air going to the engine. It doesn’t change engine displacement. Since turbocharging changes density, the cfm flowrate from the inlet to the compressor to the cylinder head of the engine is no longer constant. Attempting to talk about the cfm flow of a turbocharged engine provokes all kinds of inconsistent concepts. The one thing that is consistent about turbocharging is the MASS FLOWRATE of the air as measured in pounds per minute. This is why turbocharger maps show lb/min versus pressure ratio rather than cfm versus psi boost. Trying to work in either cfm or psi boost on a compressor map generates a map that is only correct at one place and at one time. A mass flow – pressure ratio map can be applied anywhere on earth at any time.
here ok.
Because the displacement of the engine is a physical dimension, the discharge flow volume (cfm) of the turbocharger (except when Intercooled) is equal to the displaced volume of the engine at speed as corrected by volumetric efficiency of the engine. The inlet cfm through the air cleaner is something more than the engine displacement. This has led some people to claim that turbochargers increase the cfm of the engine. In actuality, the cfm of the engine hasn’t changed, but the DENSITY of the air has.
This is an important principle. Turbocharging increases the DENSITY of the air going to the engine. It doesn’t change engine displacement. Since turbocharging changes density, the cfm flowrate from the inlet to the compressor to the cylinder head of the engine is no longer constant. Attempting to talk about the cfm flow of a turbocharged engine provokes all kinds of inconsistent concepts. The one thing that is consistent about turbocharging is the MASS FLOWRATE of the air as measured in pounds per minute. This is why turbocharger maps show lb/min versus pressure ratio rather than cfm versus psi boost. Trying to work in either cfm or psi boost on a compressor map generates a map that is only correct at one place and at one time. A mass flow – pressure ratio map can be applied anywhere on earth at any time.