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    Thread: The Official 1.8T forum turbo theory topic.

    1. Global Moderator iThread's Avatar
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      08-27-2009 03:42 PM #1
      This topic is going to be specifically for debate about turbos, how they work, and the theory involved.

      Hopefully hashing it all out here will keep the other topics free of the clutter this debate causes.

      The rules will be strictly enforced here, especially the rules about personal attacks.


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      08-27-2009 03:43 PM #2
      Do bigger turbos flow more volume of air through the engine? No.

      Volumetric flow in an engine is controlled by engine displacement and RPM. The gains from a larger turbocharger come from its ability to flow air more efficiently.

      So, at the same RPM on the same engine, a k03 sport and GT-4088 are flowing the same VOLUME of air out of the compressor.

      Are they flowing the same mass of air? NO.

      Does volume flow have anything to do with performance by itself? NO!

      Performance comes from mass flow of air, because it gives you the ability to burn more mass fuel and release more heat energy, which creates pressure, which creates force, which when acting over a distance creates torque, which at a speed creates power, etc etc etc.

      Reading compressor maps for volume using pressure is not how its supposed to work. You are supposed to get your volume from the VOLUME FLOW of the engine, then you can use your boost pressure at that volume flow, combined with the compressor efficiency to calculate MASS flow.

      Modified by thom337 at 3:45 PM 8-27-2009


      Modified by thom337 at 3:48 PM 8-27-2009

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      08-27-2009 03:44 PM #3
      aaahahahahahahahahahahahahaha
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      08-27-2009 03:49 PM #4
      I <3 Boost.

      So... I'm confused as to what exactly is to be debated here?

      Are we trying to explain that compressor wheels are rated in lb/min (easily converted to CFM) or... what?


    5. 08-27-2009 03:50 PM #5
      A long time ago a man invented intercoolers to fix the density problems
      I had no problems keeping my intake temperature down with K03S with very good intercooler so temperature is the same as GT28RS I have right now.

      Density is a function of pressure and temperature, both are the same for both turbos meaning flowing in 100g/s of air with k03s and flowing 100g/s with GT28RS ...same **** ..it's the same air volume. Now with

      GT28RS you will flow more air mass, meaning you are flowing more volume as well


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      08-27-2009 03:52 PM #6
      Quote, originally posted by 04VDubGLI »
      Are we trying to explain that compressor wheels are rated in lb/min (easily converted to CFM) or... what?

      Anything and everything relating to turbos and how they function.

      It's primarily so that people who want to debate about it can do it in here rather than in other topics.

      Hopefully it works.


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      08-27-2009 03:53 PM #7
      Quote, originally posted by 04VDubGLI »
      I <3 Boost.

      So... I'm confused as to what exactly is to be debated here?

      Are we trying to explain that compressor wheels are rated in lb/min (easily converted to CFM) or... what?

      People were basically trying to say that bigger turbos push more volume flow into the engine, which is incorrect. The do suck in a greater volume at the compressor inlet, but at the compressor outlet the volume flow is dictated by the engine volumetric flowrate which is dictated by the engine speed and displacement.

      Compressor maps are meant to be used at the compressor outlet...so the volume axes of the graph is dealing with how much volume the engine is flowing. Then, by using the working pressure ratio and efficiency, you can calculate mass flow which gives you a true idea of performance. CFM tells you very little about how much power the turbo will make without any density data.

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    8. 08-27-2009 03:56 PM #8
      Let me get this going!

      On a 1.8T motor, a K03S and a GT35R both set to 15 psi are flowing the same volume of air into the engine. (Not taking into account the increase in Volumetric Efficiency created by the larger hot side).

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      08-27-2009 03:58 PM #9
      Quote, originally posted by mescaline »

      Density is a function of pressure and temperature, both are the same for both turbos meaning flowing in 100g/s of air with k03s and flowing 100g/s with GT28RS ...same **** ..it's the same air volume.

      How is temperature the same for both turbos? The k03 charge will likely be less efficient, and therefore hotter. The GT28rs charge will be more efficient, and therefor colder, and denser. It is the same volume of air, with a different density, which results in a different mass flow, which results in different power output.

      Quote, originally posted by mescaline »

      Now with GT28RS you will flow more air mass, meaning you are flowing more volume as well

      No...you cannot flow more volume than the engine has empty displacement space. If you are talking about compressor inlets, yes there is a great volume coming INTO the compressor (suction tube), but when its pumping at a pressure it is pumping the same volume INTO the engine as it would with any turbocharger.

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    10. 08-27-2009 03:59 PM #10
      Quote, originally posted by thom337 »

      People were basically trying to say that bigger turbos push more volume flow into the engine, which is incorrect. The do suck in a greater volume at the compressor inlet, but at the compressor outlet the volume flow is dictated by the engine volumetric flowrate which is dictated by the engine speed and displacement.

      Compressor maps are meant to be used at the compressor outlet...so the volume axes of the graph is dealing with how much volume the engine is flowing. Then, by using the working pressure ratio and efficiency, you can calculate mass flow which gives you a true idea of performance. CFM tells you very little about how much power the turbo will make without any density data.

      Density of air changes all the time (mountains, sea level), this is why VW is using MAF and not...VAF ...lol, engine doesn't know what kind of IC you have either.

      Will bigger turbo flow more dense air? Probably, not necessarily...but is it making power on more dense air only? Not a chance, more dense air is responsible for 2% of extra power you get. More air inside the engine (and I mean volume) is responsible for everything else (AND APR OF COURSE) (:


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      08-27-2009 04:02 PM #11
      this is so funny stuff.

      MASS airflow sensor.

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    12. 08-27-2009 04:03 PM #12
      Quote, originally posted by mescaline »
      More air inside the engine (and I mean volume) is responsible for everything else (AND APR OF COURSE) (:

      The part you are forgetting is that you cannot increase volume without increasing pressure, or decreasing restriction.

      There are 2 reasons why larger turbos make more power :

      1) The larger compressor is more efficient.
      2) The turbine is less restrictive.

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      08-27-2009 04:05 PM #13

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      08-27-2009 04:06 PM #14
      Quote, originally posted by mescaline »

      Density of air changes all the time (mountains, sea level), this is why VW is using MAF and not...VAF ...lol, engine doesn't know what kind of IC you have either.

      Quote, originally posted by mescaline »

      Will bigger turbo flow more dense air? Probably, not necessarily...but is it making power on more dense air only?

      If it is the same volume, colder, and at the same pressure must it not also be more dense? YES.


      Quote, originally posted by mescaline »

      .but is it making power on more dense air only? Not a chance, more dense air is responsible for 2% of extra power you get.

      Power output is DIRECTLY proportional to charge density. If density is doubled, power output must double. Again, basic thermodynamics and physics.


      Quote, originally posted by mescaline »

      More air inside the engine (and I mean volume) is responsible for everything else (AND APR OF COURSE) (:

      There is not more volume of air in the engine with a larger compressor. The engine can only be filled until it is completely full, which is its volumetric capacity. Gains come from decreasing temperature and increasing pressure.

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      08-27-2009 04:08 PM #15
      Quote, originally posted by thom337 »

      There is not more volume of air in the engine with a larger compressor. The engine can only be filled until it is completely full, which is its volumetric capacity. Gains come from decreasing temperature and increasing pressure.

      Volumetric Efficiency.

      Density = Mass/Volume... Just something to keep in mind for everyone?


    16. 08-27-2009 04:09 PM #16
      Quote, originally posted by thom337 »

      If it is the same volume, colder, and at the same pressure must it not also be more dense? YES.

      Why would it be colder? I said at the begining that I had 50C intake temperature with both turbos. Pressure is also the same.


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      08-27-2009 04:14 PM #17
      Quote, originally posted by mescaline »

      Why would it be colder? .

      Compressor efficiency. Are we talking about turbos or intercoolers?

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    18. 08-27-2009 04:14 PM #18
      Quote, originally posted by 04VDubGLI »
      http://www.turbobygarrett.com/....html

      THank you, this proves the point rather well :

      Quote, originally posted by TurboByGarrett.com »
      EXAMPLE:

      I have an engine that I would like to use to make 400Hp, I want to choose an air/fuel ratio of 12 and use a BSFC of 0.55. Plugging these numbers into the formula from above:

      of air.

      Thus, a compressor map that has the capability of at least 44 pounds per minute of airflow capacity is a good starting point.

      Note that nowhere in this calculation did we enter any engine displacement or RPM numbers. This means that for any engine, in order to make 400 Hp, it needs to flow about 44 lb/min (this assumes that BSFC remains constant across all engine types).

      Naturally, a smaller displacement engine will require more boost or higher engine speed to meet this target than a larger engine will. So how much boost pressure would be required?

      The only way to get 44 lb/min into the motor is by increasing the boost.

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    19. 08-27-2009 04:19 PM #19
      Quote, originally posted by thom337 »

      Compressor efficiency. Are we talking about turbos or intercoolers?

      Turbo or intercoolers? Are you saying if I add a good intercooler that will cool down K03S air to 40C and GT28RS is at 40C they will make the same power? That would be cheap heh.

      I am telling you that intake temperature of both turbos were the same, 50C...this is logged...I have a log here somewhere, also both turbos were operating at 18 psi @ 4000 rpm. Now why would MAF report ~100g/s more for GT28RS if all parameters are the same?


    20. 08-27-2009 04:21 PM #20
      Quote, originally posted by Agtronic »

      The only way to get 44 lb/min into the motor is by increasing the boost.

      Wrong again, bigger turbo will get 44lb/min into engine at the same boost level as smaller turbo. 44lb/min which you can easily convert to CFM.


      Modified by mescaline at 1:22 PM 8-27-2009


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      08-27-2009 04:25 PM #21
      Quote, originally posted by mescaline »

      Turbo or intercoolers? Are you saying if I add a good intercooler that will cool down K03S air to 40C and GT28RS is at 40C they will make the same power? That would be cheap heh.

      That's exactly right. If you have a GT28RS making the same pressure and temperature at the same RPM as a K03, they will make exactly the same power. Now, cooling down a K03 that much and operating it reliably would be quite the task, but yes if you were able to do that they would make the same power and be flowing the same mass flow of air.


      Quote, originally posted by mescaline »

      I am telling you that intake temperature of both turbos were the same, 50C...this is logged...I have a log here somewhere, also both turbos were operating at 18 psi @ 4000 rpm. Now why would MAF report ~100g/s more for GT28RS if all parameters are the same?

      It wouldn't. The situation you are describing is only possible if the IAT sensor is taking too long to register a temperature change. Try going steady state on a dyno at that same rpm and situation and then tell me the results. At the same manifold temperature, pressure, and RPM, the mass flowrates must be the same.


      Modified by thom337 at 4:30 PM 8-27-2009

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      08-27-2009 04:27 PM #22
      Quote, originally posted by mescaline »

      Wrong again, bigger turbo will get 44lb/min into engine at the same boost level as smaller turbo.

      Nope, it can do it at a lower boost level because the compressor exit temperatures won't be as high.

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      08-27-2009 04:31 PM #23
      Let's interupt this ummm discussion to welcome our new moderator.. Welcome to the forum ithread And no.. I'm not being sarcastic..

    24. 08-27-2009 04:34 PM #24
      Quote, originally posted by thom337 »

      That's exactly right. If you have a GT28RS making the same pressure and temperature at the same RPM as a K03, they will make exactly the same power. Now, cooling down a K03 that much and operating it reliably would be quite the task, but yes if you were able to do that they would make the same power and be flowing the same mass flow of air.

      This is just...dumb.
      I told you that I had 50C intake temperatures with K03S and with GT28RS, 18 psi @ 4000rpm. This is not faulty IAT sensor, ask around...lots of people have 50C intake temperatures with K03S...this is nothing unusual.
      Bigger turbo can't double the air density man.

      Anyway am going for a run, lots of dense air outside... i will burn off this fat i gained over vacation tonight ha ha (:


      Modified by iThread at 1:37 PM 8-27-2009


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      08-27-2009 04:36 PM #25
      Quote, originally posted by thom337 »

      There is not more volume of air in the engine with a larger compressor. The engine can only be filled until it is completely full, which is its volumetric capacity. Gains come from decreasing temperature and increasing pressure.

      the way a turbo works is by effectively cramming air into an engine, increasing its volumetric effeciency. this is why a gt28 at the same psi as a k03 will undoubtely make more power, because it is a) lowering charge temps and b) forcing more air into the engine. by your logic a bigger turbo just decreases charge temps and increases density. this makes sense to a point. but as you get into bigger turbos the temps arent going to change much more but cfm's are going to go up. its all about efficient volume

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      08-27-2009 04:39 PM #26
      Quote, originally posted by mescaline »

      This is just...dumb.
      I told you that I had 50C intake temperatures with K03S and with GT28RS, 18 psi @ 4000rpm. This is not faulty IAT sensor, ask around...lots of people have 50C intake temperatures with K03S...this is nothing unusual.
      Bigger turbo can't double the air density man, what the hell is wrong with you... two working brain cells would figure this out.

      Anyway am going for a run, lots of dense air outside... i will burn off this fat i gained over vacation tonight ha ha (:

      I did not say the IAT sensor was "faulty". However, they are not meant to respond to rapid changes in temperature. You need a nice data logging setup and a good thermocouple. That is why I suggested doing a steady state pull at the same boost on a dyno, then you will see the temperature changes between the two turbos. Unless you have a holy intercooler core from the pope, your temperatures in the manifold for a K03s and a GT28rs at the same pressure and RPM will NOT be the same...and if they were, your mass flow would be the same.

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    27. 08-27-2009 04:40 PM #27
      Quote, originally posted by dubchuck117 »

      the way a turbo works is by effectively cramming air into an engine, increasing its volumetric effeciency. this is why a gt28 at the same psi as a k03 will undoubtely make more power, because it is a) lowering charge temps and b) forcing more air into the engine. by your logic a bigger turbo just decreases charge temps and increases density. this makes sense to a point. but as you get into bigger turbos the temps arent going to change much more but cfm's are going to go up. its all about efficient volume

      x2


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      08-27-2009 04:43 PM #28
      I think the confusion is one is ignoring the turbocharger and the other the engine. A larger turbo in the same engine will be much closer to the surge line then a smaller turbo on the map at a given boost. If left, unrestricted, it will flow more air, volume, mass whatever per revolution. PSI is whats built up in the charge piping as this is FORCED induction. If you have a free flowing event, nothing is forced. Both parties are correct to a certain degree. But the efficiency of a 35R at 15psi is much more so then the k03. Its taking less revolutions to do so and consuming less energy...
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    29. 08-27-2009 04:43 PM #29
      Quote, originally posted by thom337 »

      I did not say the IAT sensor was "faulty". However, they are not meant to respond to rapid changes in temperature. You need a nice data logging setup and a good thermocouple. That is why I suggested doing a steady state pull at the same boost on a dyno, then you will see the temperature changes between the two turbos. Unless you have a holy intercooler core from the pope, your temperatures in the manifold for a K03s and a GT28rs at the same pressure and RPM will NOT be the same...and if they were, your mass flow would be the same.

      Even there is 40C or 100C difference in temperature, it won't affect density so much to double the power. With k03s I had stock IC before and I was pushing some insane boost levels, like 1.5bar or so...I logged IAT to like 90C, then I got bigger IC, it got temperature down to 50C ...guess what, power of engine didn't double...I gained maybe 5whp ...if that, but I had cooler engine. Your theory about air density is certainly there, but as said before...it's responsible for making 2% of extra power you get with bigger turbo.


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      08-27-2009 04:43 PM #30
      Quote, originally posted by dubchuck117 »

      the way a turbo works is by effectively cramming air into an engine, increasing its volumetric effeciency. this is why a gt28 at the same psi as a k03 will undoubtely make more power, because it is a) lowering charge temps and b) forcing more air into the engine. by your logic a bigger turbo just decreases charge temps and increases density. this makes sense to a point. but as you get into bigger turbos the temps arent going to change much more but cfm's are going to go up. its all about efficient volume

      How can you fill something more than 100%? A turbo doesn't not keep filling something that is full, it causes a pressure rise which increases the DENSITY of the air in the engine. Turbos are all about CHARGE DENSITY.

      In regard to volumetric efficiency, I have seen some people regard turbos as simply increasing the volumetric efficiency beyond 100%. However, it then becomes somewhat of a mis-nomer as it doesn't make sense to fill something beyond 100%. Its better to describe it as 100% full, but with a higher density. The only time I see +100% VE used is when calculations are being made using atmospheric density in which case this makes sense.

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      08-27-2009 04:46 PM #31
      Quote, originally posted by thom337 »

      How can you fill something more than 100%? A turbo doesn't not keep filling something that is full, it causes a pressure rise which increases the DENSITY of the air in the engine. Turbos are all about CHARGE DENSITY.

      In regard to volumetric efficiency, I have seen some people regard turbos as simply increasing the volumetric efficiency beyond 100%. However, it then becomes somewhat of a mis-nomer as it doesn't make sense to fill something beyond 100%. Its better to describe it as 100% full, but with a higher density. The only time I see +100% VE used is when calculations are being made using atmospheric density in which case this makes sense.

      You can, its called surge as its pushing past the turbo..

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    32. 08-27-2009 04:47 PM #32
      Quote, originally posted by AL@PagParts.com »
      I think the confusion is one is ignoring the turbocharger and the other the engine. A larger turbo in the same engine will be much closer to the surge line then a smaller turbo on the map at a given boost. If left, unrestricted, it will flow more air, volume, mass whatever per revolution. PSI is whats built up in the charge piping as this is FORCED induction. If you have a free flowing event, nothing is forced. Both parties are correct to a certain degree. But the efficiency of a 35R at 15psi is much more so then the k03. Its taking less revolutions to do so and consuming less energy...

      All thom and agtronics are saying is that K03S and GT28RS are flowing same amount (VOLUME) of air inside the engine...and I say bigger turbo is flowing more air inside the engine (lets forget what happens inside the engine and stuffing all that air inside the cylinder, no one is arguing this...you can't put 1000000 cf of air inside 1800cc engine. We are talking turbos here.


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      08-27-2009 04:49 PM #33
      Quote, originally posted by AL@PagParts.com »

      You can, its called surge as its pushing past the turbo..

      just talking about general turbo flowing into engine here, yes its possible to cause surge / backflow / compressor reversion but thats not really in scope of what we're talking about here.

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      08-27-2009 04:50 PM #34
      Quote, originally posted by thom337 »

      just talking about general turbo flowing into engine here, yes its possible to cause surge / backflow / compressor reversion but thats not really in scope of what we're talking about here.

      You cant talk about diff turbo sizes and not talk about that on the same engine..

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    35. 08-27-2009 04:50 PM #35
      Quote, originally posted by thom337 »

      How can you fill something more than 100%? A turbo doesn't not keep filling something that is full, it causes a pressure rise which increases the DENSITY of the air in the engine. Turbos are all about CHARGE DENSITY.

      In regard to volumetric efficiency, I have seen some people regard turbos as simply increasing the volumetric efficiency beyond 100%. However, it then becomes somewhat of a mis-nomer as it doesn't make sense to fill something beyond 100%. Its better to describe it as 100% full, but with a higher density. The only time I see +100% VE used is when calculations are being made using atmospheric density in which case this makes sense.

      Volumetric efficiency (VE) is used to describe the amount of fuel/air in the cylinder in relation to regular atmospheric air. If the cylinder is filled with fuel/air at atmospheric pressure, then the engine is said to have 100% volumetric efficiency. On the other hand, super chargers and turbo chargers increase the pressure entering the cylinder, giving the engine a volumetric efficiency greater than 100%.


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