big turbo vrs little turbo
Originally Posted by Jeff_Jeske
When thinking about flow, try not to focus solely on the compressor side. As was stated, the power gained from a big turbo compressor is from improved efficiency and to feed the engine at higher rpm. But most of the time when a big turbo is installed it usually has a larger turbine wheel and housing. Exhaust flows out with less resistance. That's where you get more "flow" at the same psi. For example, lets say we put in a restrictor plate in the exhaust collector ( or anywhere in the exhaust) that has 5 cm area. (less than stock at 6 cm) No matter which compressor wheel you put on it won't "flow" any better, because of the bottleneck in the collector. 15 psi = 15 psi.
The flow is limited to how fast air can get through this restriction. Naturally when you open it up with something big like an 8 cm housing, it will flow much more. 15 psi @ 5cm< 15 psi@8 cm.
The flow is limited to how fast air can get through this restriction. Naturally when you open it up with something big like an 8 cm housing, it will flow much more. 15 psi @ 5cm< 15 psi@8 cm.
Interesting and valuable sites on turbos:
http://www.gnttype.org/techarea/pict...de/turbos.html
http://www.honeywell.com/sites/ts/tt...s_howworks.htm
http://www.gnttype.org/techarea/pict...de/turbos.html
http://www.honeywell.com/sites/ts/tt...s_howworks.htm
ok look a turbo like the 16g makes on average on 93 (with mods, tuning, cams...) 330-350awhp
well you take that same car and throw a 35r on it and tune it and you are looking at 430 awhp at the same boost levels.
well you take that same car and throw a 35r on it and tune it and you are looking at 430 awhp at the same boost levels.
Originally Posted by jroller
I disagree my friend, that restrictor plate you speak of does exist, its called the OEM exhaust/cat sys and people DO slap bigger compressors on and do make more power. Also its worth noting that people who have driven cars with big turbos vs OEM's find it easier to "pre spool" the big turbo, especially from a roll. Roadracing is another story witch I have little or no expierence with.
The increase in power comes from the bigger turbo's ability to increase exhaust flow. Once the exhaust flow is opened up the motor can move much more air at the same PSI.
As for the stock exhaust and cat being a restrictor, well its not high flow but its not a restrictor in the same sense. Basically the bigger turbo will still be able to cram more exhaust through those items than a smaller turbo will. Keeping it real..... very few people would try to bolt on a bigger turbo to the stock exhaust and cat so that is kind of a silly comparison.
Evolved Member
Joined: Nov 2003
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From: Bangkok
I still don't see how you can get more inlet air flow for the same psi in the intake mani.
The fact that the exhaust can flow more freely or whatever is to do with efficiency of the exhaust side of the turbine. This works in combination with the intake side flow curves. These two together with some thermodynamics will give perhaps some kind of picture of the working range of the system. The way I see it, it is just a matter of a bigger turbo being more suitable for increased boost levels...
The fact that the exhaust can flow more freely or whatever is to do with efficiency of the exhaust side of the turbine. This works in combination with the intake side flow curves. These two together with some thermodynamics will give perhaps some kind of picture of the working range of the system. The way I see it, it is just a matter of a bigger turbo being more suitable for increased boost levels...
because the bigger turbo has a better ability to compress the air(takes up less space) and thus, able to push more air into the combustion chamber, add tuning to kick up the spark timing, dump more fuel and you have yourself more explosions=more power
Originally Posted by x838nwy
I'm no expert at these things but this is what I can conclude from a point of view of an engineer. I'm pretty certain it is correct, but please let me know if I have got it wrong.
If we're talking about a maniold pressure, then the engine sees a pressure and not a turbo.
At a particular engine speed, the engine will draw the air at a particular (fixed) volumetric flow rate - certain number of cubic inches per minute say. This depends on the engine geometry. The mass of air that actually gets to go into the engine is dependent on the pressure in your intake mani. The higher the pressure at the same volume = greater *** of air that goes into your engine = more power. Therefore, if the two turbos show the same boost pressure (in the intake mani) at the same rpm, the same mass of air enters the engine and therefore the same power is made.
If you have a gas header (big pipe with a few smaller ones coming out of it, like a manifold) connected to a supply (like a compressor) and all the outlets of the header are fully open then the pressure in the header will be at a certain value. If you want to flow more, the pressure in the header will have to increase. In other words, at maximum demand conditions, you can't flow more at the same pressure. Therefore, given same conditions (amount of air drawn by the engine) the two turbos supply the same amount of air (mass and volume).
Where the larger turbo wins, I think, is efficiency. Basically, as you increase the speed of a turbine, you reach a point of peak efficiency (most work done for the work input) before and after which the efficiency drops. The larger turbos are likely to be working more in their high efficiency range than the smaller turbo. The stock turbo is only designed to operate within a certain range (boost, flow rate, etc.). Out of that range it is less effective.
At/near max rpm, the flow rate which the turbo must provide is probably about the maximum the stocker can do so while remaining acceptably efficient. If you increase the boost from stock, then you're asking it to work outside it's efficient region and it will probably not be able to provide the boost you're asking of it at the higher rpms.
Another thing, I suppose, is that while manifold pressure is measured and we assume the flow to the engine is constant, it isn't. So the manifold pressure fluctuates in pulses and perhaps a larger turbo (as it is not being pushed towards the edges of its capabilities) can provide a more stable flow to the engine.
If we're talking about a maniold pressure, then the engine sees a pressure and not a turbo.
At a particular engine speed, the engine will draw the air at a particular (fixed) volumetric flow rate - certain number of cubic inches per minute say. This depends on the engine geometry. The mass of air that actually gets to go into the engine is dependent on the pressure in your intake mani. The higher the pressure at the same volume = greater *** of air that goes into your engine = more power. Therefore, if the two turbos show the same boost pressure (in the intake mani) at the same rpm, the same mass of air enters the engine and therefore the same power is made.
If you have a gas header (big pipe with a few smaller ones coming out of it, like a manifold) connected to a supply (like a compressor) and all the outlets of the header are fully open then the pressure in the header will be at a certain value. If you want to flow more, the pressure in the header will have to increase. In other words, at maximum demand conditions, you can't flow more at the same pressure. Therefore, given same conditions (amount of air drawn by the engine) the two turbos supply the same amount of air (mass and volume).
Where the larger turbo wins, I think, is efficiency. Basically, as you increase the speed of a turbine, you reach a point of peak efficiency (most work done for the work input) before and after which the efficiency drops. The larger turbos are likely to be working more in their high efficiency range than the smaller turbo. The stock turbo is only designed to operate within a certain range (boost, flow rate, etc.). Out of that range it is less effective.
At/near max rpm, the flow rate which the turbo must provide is probably about the maximum the stocker can do so while remaining acceptably efficient. If you increase the boost from stock, then you're asking it to work outside it's efficient region and it will probably not be able to provide the boost you're asking of it at the higher rpms.
Another thing, I suppose, is that while manifold pressure is measured and we assume the flow to the engine is constant, it isn't. So the manifold pressure fluctuates in pulses and perhaps a larger turbo (as it is not being pushed towards the edges of its capabilities) can provide a more stable flow to the engine.
Further more, i think the stock turbo is working out of it's efficiency range in stock form already, and this is most evident by tapering PSI; therefore not keeping the manifold pressure constant at the desired psi. Sure this may be by design of
engineers; but my personal opinion is it is overworked stock.A lager turbo is going to hold that manifold pressure contant at 22 psi all the way to the moon, and that is where power is made; while the stock turbo is struggling along at 19 psi and dropping quickly (not to mention the hot air it's blowing in).
There's so much misinformation in this thread, it's ridiculous.
Evillution has it completely correct. Go back and read all of his posts to understand. A few more people have added additionally insight into real-world example, like Jeff Jeske, in which he talks about the volumetric efficiency increasing.
I have seen a lot of this in this thread:
"A bigger turbo flows more at a given boost pressure".
That is COMPLETELY wrong. A bigger turbo is RATED for more flow at a given boost pressure. Turbos are usually rated at 2 bar absolute pressure, or about 15 psi to you and me. The compressor maps will sometimes have the max flow marked at this point. All this means is that the turbo CAN flow that much air. But, to see how much air it will flow on a 2.0 L engine, you have to plot the points on a compressor map.
The flow rate from 15 psi on a two liter engine, keeping temperature, RPM, and volumetric efficiency constant, will be the same no matter what turbo you use.
Now, in the real world, those variables do change....the biggest change from a smaller turbo to a bigger turbo is the temperature and also the volumetric efficiency, which is what Jeff Jeske was talking about.
It is this temperature and volumetric efficiency change that will give you different mass airflows and HP numbers at the same PSI for different turbos.
The reason why people upgrade their turbos is because they plan on running much higher boost levels than the stock turbo can efficiently support. The stock turbo would be so far off of it's map and at it's choke speed, that (1) it couldn't produce any more pressure and (2) the temperature would be very hot. A bigger turbo at this same temperature, would be in it's efficiency range, spinning at an efficient RPM, too, so the charge would be cool and the turbo would be able to hold the boost to redline.
This may help people who have never seen a compressor map or how to calculate flows through an engine easier. I found this compressor map calcuator on the web and I added the Evo8 16G (our turbo) to the spreadsheet. Go ahead and change the boost pressures around, volumetric efficiencies, etc, and see how the points plot onto the different compressor maps. It will be very clear why some turbos perform so well at certain boost level. For example, look at the 50 trim at like 22-24 psi. Then look at a much bigger turbo at like 30 psi and compare how the points are on the different maps.
Chris has hosted the file for me in post #4 in this thread.
Click here for a direct download.
Eric
Evillution has it completely correct. Go back and read all of his posts to understand. A few more people have added additionally insight into real-world example, like Jeff Jeske, in which he talks about the volumetric efficiency increasing.
I have seen a lot of this in this thread:
"A bigger turbo flows more at a given boost pressure".
That is COMPLETELY wrong. A bigger turbo is RATED for more flow at a given boost pressure. Turbos are usually rated at 2 bar absolute pressure, or about 15 psi to you and me. The compressor maps will sometimes have the max flow marked at this point. All this means is that the turbo CAN flow that much air. But, to see how much air it will flow on a 2.0 L engine, you have to plot the points on a compressor map.
The flow rate from 15 psi on a two liter engine, keeping temperature, RPM, and volumetric efficiency constant, will be the same no matter what turbo you use.
Now, in the real world, those variables do change....the biggest change from a smaller turbo to a bigger turbo is the temperature and also the volumetric efficiency, which is what Jeff Jeske was talking about.
It is this temperature and volumetric efficiency change that will give you different mass airflows and HP numbers at the same PSI for different turbos.
The reason why people upgrade their turbos is because they plan on running much higher boost levels than the stock turbo can efficiently support. The stock turbo would be so far off of it's map and at it's choke speed, that (1) it couldn't produce any more pressure and (2) the temperature would be very hot. A bigger turbo at this same temperature, would be in it's efficiency range, spinning at an efficient RPM, too, so the charge would be cool and the turbo would be able to hold the boost to redline.
This may help people who have never seen a compressor map or how to calculate flows through an engine easier. I found this compressor map calcuator on the web and I added the Evo8 16G (our turbo) to the spreadsheet. Go ahead and change the boost pressures around, volumetric efficiencies, etc, and see how the points plot onto the different compressor maps. It will be very clear why some turbos perform so well at certain boost level. For example, look at the 50 trim at like 22-24 psi. Then look at a much bigger turbo at like 30 psi and compare how the points are on the different maps.
Chris has hosted the file for me in post #4 in this thread.
Click here for a direct download.
Eric
Last edited by l2r99gst; Nov 15, 2006 at 01:59 PM.
Admin Emeritus
Joined: Jan 2003
Posts: 2,239
Likes: 101
From: NR Reading PA
And here is a link to a good explanation of how to read a compressor flow map. Hopefully it will help.
http://www.automotivearticles.com/Turbo_Selection.shtml
http://www.automotivearticles.com/Turbo_Selection.shtml
Newbie
Joined: Aug 2006
Posts: 8
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From: Barbados
don't look at how much air the turbo pushes out, look at how much air goes into the engine at a set manifold pressure and rpm, if a small turbo pushing 22psi at 8000rpm and it is in the maximum efficiency range, putting on a big turbo and run it at 22psi at 8000 rpm , the amount of air going into the engine would be basically the same, but the big turbo would has less back
pressure at the turbine and that is where it would gain the power, ie the piston uses less hp to push out the exhaust gases
pressure at the turbine and that is where it would gain the power, ie the piston uses less hp to push out the exhaust gases
Why is it my small Campbell Housfeld air compressor takes 10 minutes to get to 40psi and the bigger shop compressor does it in 3 min? Its still 40 psi. C.H. was fresh out of compressore maps.
Evolving Member
Joined: Dec 2004
Posts: 113
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From: Portland, OR
Originally Posted by ExViTermini
ok look a turbo like the 16g makes on average on 93 (with mods, tuning, cams...) 330-350awhp
well you take that same car and throw a 35r on it and tune it and you are looking at 430 awhp at the same boost levels.
well you take that same car and throw a 35r on it and tune it and you are looking at 430 awhp at the same boost levels.
Well that is because horsepower equals torque over time. The increased power is because you can hold that PSI to redline thus creating more peak horsepower. You can see the torque difference is minimal, only difference is where the torque is located. This is an unrelated topic for a different day though. Sorry for going OT.
Newbie
Joined: Nov 2006
Posts: 1
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From: BRISTOL TENN
There has been a few good pionts on the subject. But there is more to than this.
One thing Boost or PSI is a measurment of resistance not flow. When two different size turbos reach 10psi of boost it does not mean they are flowing the same amount. It is how much reasistance is being applied.
A turbo is a air pump in a way. The bigger the pump the more air it will flow. The resistance come from the Valves closing (mostly). If you could take a head,intake and throttle body setup(not bolted on a engine) and run a turbo at its max and then open all four intake vavles you would not get any boost reading at all
( unless it was some major hugh turbo). Then if you took a reading of the flow coming out of each valve/intake port the bigger the turbo used the more air would be flowing. Also as meantioned before the cooler charge from the bigger turbo has the air more dense because of the cooler charge. Then the exhaust side being bigger on the bigger turbo also helps because of more exhaust flow.
Most of you agree that the exhaust being bigger helped the engine flow more exhaust out, so way wouldnt it be the same on the intake side. There is some resistance from the intake & head ports, but very little. Most of it comes from the valve size & closing of the valve. Although the intake & head ports do effect performance it is more of a velocity thing (which does effect flow though).
Now when the engine is running you only have one intake valve open at a time ( basicly, there is some overlap). So this creates resistance( or boost) which makes the waste gate regulate the turbo to what ever boost you have set your system to be at. But when the valve open, it starts free flowing and the bigger turbo will pump more air threw it than a small one. This is because the port,valves and intake is not enough of a restriction to hold the air flow back that the biggere turbo can provide. Now if you could or was running a turbo big enough that the ports and valves became a big enough restriction to create boost then going to a even bigger turbo would not give anymore air. But this would take such a big turbo that it would not fit under the hood of your car.
So when you go to a bigger turbo it will pump more air into the engine at the same boost. When the valve open it gets more air in because the bigger turbo is moving more denser air. So the air moves faster threw the ports becuase there is more available from the bigger pump(turbo) & not enough restriction from the ports,valves & intake to slow it down to the piont of no more air. If you ever took a turbo engine and just remove the head and install bigger valves and port & polish then you noticed that it made more power with less boost. It is because the restriction was relieved and the air is allowed to move threw faster wich gives it more air. Then the turbo doesnt have to work as hard because there is less restriction, even though it is getting more air threw it doesnt have the pressure build up against the Compressor wheel from the boost. You can even max out a turbo that normally maxed out at say 32 psi on your engine and it will max out at say 22 psi instead(for example) because it can only flow so much air for its size no matter what the Boost/restriction level is.
I hope this helps you to understand & I am sure it can get more in detail & involved than this. But this should help you enough to know what a bigger turbo is doing & how it gives more air.
Jess
One thing Boost or PSI is a measurment of resistance not flow. When two different size turbos reach 10psi of boost it does not mean they are flowing the same amount. It is how much reasistance is being applied.
A turbo is a air pump in a way. The bigger the pump the more air it will flow. The resistance come from the Valves closing (mostly). If you could take a head,intake and throttle body setup(not bolted on a engine) and run a turbo at its max and then open all four intake vavles you would not get any boost reading at all
( unless it was some major hugh turbo). Then if you took a reading of the flow coming out of each valve/intake port the bigger the turbo used the more air would be flowing. Also as meantioned before the cooler charge from the bigger turbo has the air more dense because of the cooler charge. Then the exhaust side being bigger on the bigger turbo also helps because of more exhaust flow.
Most of you agree that the exhaust being bigger helped the engine flow more exhaust out, so way wouldnt it be the same on the intake side. There is some resistance from the intake & head ports, but very little. Most of it comes from the valve size & closing of the valve. Although the intake & head ports do effect performance it is more of a velocity thing (which does effect flow though).
Now when the engine is running you only have one intake valve open at a time ( basicly, there is some overlap). So this creates resistance( or boost) which makes the waste gate regulate the turbo to what ever boost you have set your system to be at. But when the valve open, it starts free flowing and the bigger turbo will pump more air threw it than a small one. This is because the port,valves and intake is not enough of a restriction to hold the air flow back that the biggere turbo can provide. Now if you could or was running a turbo big enough that the ports and valves became a big enough restriction to create boost then going to a even bigger turbo would not give anymore air. But this would take such a big turbo that it would not fit under the hood of your car.
So when you go to a bigger turbo it will pump more air into the engine at the same boost. When the valve open it gets more air in because the bigger turbo is moving more denser air. So the air moves faster threw the ports becuase there is more available from the bigger pump(turbo) & not enough restriction from the ports,valves & intake to slow it down to the piont of no more air. If you ever took a turbo engine and just remove the head and install bigger valves and port & polish then you noticed that it made more power with less boost. It is because the restriction was relieved and the air is allowed to move threw faster wich gives it more air. Then the turbo doesnt have to work as hard because there is less restriction, even though it is getting more air threw it doesnt have the pressure build up against the Compressor wheel from the boost. You can even max out a turbo that normally maxed out at say 32 psi on your engine and it will max out at say 22 psi instead(for example) because it can only flow so much air for its size no matter what the Boost/restriction level is.
I hope this helps you to understand & I am sure it can get more in detail & involved than this. But this should help you enough to know what a bigger turbo is doing & how it gives more air.
Jess
You're partially right.
This thread was never about whether you can flow more air at a given boost with less restriction. Of course you can. That's a given.
We're talking about the difference between a big turbo and a small turbo on the same exact setup at the same exact psi. The only reason the bigger turbo will move more air through the engine at the same boost level is because of reduced temperatures, due to higher compressor efficiency, or increased VE, due to the larger hot side.
There are only 4 factors involved in the equation for airflow through (not threw) an engine:
1. Pressure ratio (Boost)
2. Displacement
3. RPM
4. Volumetric efficiency
The other factor is temperature, which comes into play when the efficiencies of the compressor wheel are different between the two turbos.
There are only two things chaning when going to a bigger turbo:
1. VE
2. Temperature (if the psi that you are testing lands at different efficiencies on the compressor map)
If you hypothetically kept the hotside the same size and just increase the compressor side, the VE is basically staying the same. Then its's just temperature. You can look at the compressor map calculator that I posted for this to see the efficiencies of different turbos at different boost levels and RPMs.
Actually, a smaller turbo will actually flow more air at a lower psi where the smaller turbo is more efficient than a monster turbo.
Just for your reference, airflow through an engine is:
Aiflow = (Cid*RPM*VE)/(1728*2)
Then you multiply the pressure ratio to this, based on the boost you are running:
PR= (boost+14.7)/14.7
When you add a turbo with different efficiencies into the equation, you can find the different outlet temperatures with these equations:
T2 = T1 x (P2/P1)^0.286
where P1 is atmospheric pressure
P2 is absolute pressure coming out of the compressor (absolute boost pressure)
T1 is intake temperature
T2 is temperature leaving compressor
But, since the compressor isn't 100% efficient, we have to add in the compressor efficiency, so the final outlet temperature of the turbo compressor can be found by this equation:
T3 = T1 + (T2-T1)/CE
where CE is the compressor efficieny
Eric
This thread was never about whether you can flow more air at a given boost with less restriction. Of course you can. That's a given.
We're talking about the difference between a big turbo and a small turbo on the same exact setup at the same exact psi. The only reason the bigger turbo will move more air through the engine at the same boost level is because of reduced temperatures, due to higher compressor efficiency, or increased VE, due to the larger hot side.
There are only 4 factors involved in the equation for airflow through (not threw) an engine:
1. Pressure ratio (Boost)
2. Displacement
3. RPM
4. Volumetric efficiency
The other factor is temperature, which comes into play when the efficiencies of the compressor wheel are different between the two turbos.
There are only two things chaning when going to a bigger turbo:
1. VE
2. Temperature (if the psi that you are testing lands at different efficiencies on the compressor map)
If you hypothetically kept the hotside the same size and just increase the compressor side, the VE is basically staying the same. Then its's just temperature. You can look at the compressor map calculator that I posted for this to see the efficiencies of different turbos at different boost levels and RPMs.
Actually, a smaller turbo will actually flow more air at a lower psi where the smaller turbo is more efficient than a monster turbo.
Just for your reference, airflow through an engine is:
Aiflow = (Cid*RPM*VE)/(1728*2)
Then you multiply the pressure ratio to this, based on the boost you are running:
PR= (boost+14.7)/14.7
When you add a turbo with different efficiencies into the equation, you can find the different outlet temperatures with these equations:
T2 = T1 x (P2/P1)^0.286
where P1 is atmospheric pressure
P2 is absolute pressure coming out of the compressor (absolute boost pressure)
T1 is intake temperature
T2 is temperature leaving compressor
But, since the compressor isn't 100% efficient, we have to add in the compressor efficiency, so the final outlet temperature of the turbo compressor can be found by this equation:
T3 = T1 + (T2-T1)/CE
where CE is the compressor efficieny
Eric