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Old 05-29-2002, 10:49 PM   #1
bingo
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Default What's all this stuff about stock turbo not being efficient at higher PSI!?!?

Thsi may get long, I will try to abbreviate the story. Played with my SPO waste gate actuator and ended up setting it WAY to high. Was off the gauge of my boost gauge (past 20lbs, I would estimate 22 or so). Yes my CEL went on. BUT....TALK ABOUT POWER!! This thing is sick. Now I don't want to hear about turning it down...I know I know, and I will, believe me. I would like to eventually set it at 17 or so (we get great gas here at 94 octane).

My point is tha the turbo has tons of power at higher PSI. Why are people changing turbos to run higher PSI's. Is there something I am missing? The car pulled very strong (spun all four from a partially rolling start). and was insane on the highway.

What am I safe running up to in terms of PSI? If there is no knock and EGT's are constant, other than wear and tear, what are the dangers?
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Old 05-29-2002, 11:00 PM   #2
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Default low effeciency = hot air

You have three issues to deal with.

One at high pressure ratios the effeciency of the stock turbo goes out the window. It will produce high pressures but the output air will be much much hotter than it would be from a properly sized turbo.

Second, the turbo cannot flow enough exhaust gas through its turbine and exhaust back pressure goes up dramatically compared to properly sized turbos. Net effect less effective boost. Hot air at 22 psi in, fighting high exhaust back pressure out will produce less power than a lower cool pressure in fighting less exhaust backpressure.

Third: At high flow rates the compressor side cannot maintain those high pressure ratios like a properly sized turbo can, so your high rpm boost will fall off.

Larry
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Old 05-29-2002, 11:04 PM   #3
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This has kind of been beat to death here (yes, search), but I'm bored and feel like typing.

The consensus amongst both amateur and professional tuners around here is that the stock turbo starts becoming inneficient (read: air coming out is too hot to provide more power than at lower boost levels) somewhere around 16-17psi. Somehow I really doubt you got more power at 20psi. Unless of course you're spiking somewhere around 20 and settling down lower. . . By the way if you're not hitting fuel cut at that high of a boost level, you're lucky. My first guess would be bad gauge. I don't know this for a fact, but I'd imagine your turbo is undergoing A LOT of extra wear and tear at 20psi. Not to mention with the stock ECU, timing will be advanced quite a bit where you don't want it to be. ACK! Your poor car.

As for the reason to change turbos, simple: more air. The higher flow of a bigger turbo will give you more air at the same boost levels as our itty bitty stock turbo.

JD
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Old 05-29-2002, 11:15 PM   #4
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Learn how to read this:

and maybe you'll understand why people are making the points that they do. Dude not to suggest you are an idiot or something, because I'm not, but just because #'s are bigger or your car feels more powerful, does, by no means, not mean that your car is working efficiently... There are more factors than simply, "WOAH THE CAR FEELS INSANE!," to consider. Sorry for coming off soo stern, but I have known a person or two who set their boost levels to 22psi and now are waiting on a new engine, hood, turbo etc etc... b/c a small fire erupted as the backpressure ate the turbo alive... and somehow flames shot thru the intercooler...

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Old 05-30-2002, 12:00 AM   #5
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Well, I guess I am an idiot as the final questions in my point still have not been answered. The only reasonable statement or reasoningfrom you guys is the heat created by the stock turbo. However the heat of the air would be the same regardless of which turbo cretaed it. Heat of the air is created by pressure. You squeeze air to a certain psi and it heats up regardless of how you squeeze it. My point about effeciency is that, in past posts, people have sugegsted the effeiceincy of the stock turbo (or lack there of) does not allow for more power at higher boosts. I am simply stating a fact. My stock turbo IS running well enough at high boost to make tons more power than lower boost. I am not saying it will run as much power at that boost as another size turbo, but it IS NOT running out of steam as has been suggested in other posts on this issue. I did do a search by the way and that comment is getting sooo old. All the old posts suggested it was not worth going to higher than 17 lbs of boost as it would not yield more power. I am saying beyond a doubt it does,

Sherif, your graph looks real pretty, but is incomplete. I guess 4 years of university in Physics and economics wasn't enough. Is the pressure ratio in bar? What are you referencing this against? How about another graph with another turbo to compare to...stock vs aftermarket etc. Alone it means nothing. Perhaps you can enlighten me?

You guys are making the same blanket statements you have read elsewhere without resoning the thought process out.

Fact: Stock turbo set at higher boost does provide some serious power.

Question (again): If it is ok to run a larger turbo at higher boost WHY SPECIFICALLY not the stock turbo as well IF THE EGT ARE GOOD AND THERE IS NO KNOCK?? ARe there any other changes to the motor people are doing with the stock turbo? ECU remap, more fuel, etc.
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Old 05-30-2002, 12:01 AM   #6
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Well, on the stock fuel system, you shouldn't go beyond 16 psi in my opinion, and even 16 psi can be dangerous.

Reason you want to get a bigger turbo is that the bigger turbo can push WAY more air into the motor at the SAME psi as the stock turbo. Higher CFM per PSI over the stock turbo, that's what you aim for in a turbo. There's no need to run high PSI if you have a big turbo. Running high PSI causes temps to raise, detonation, fuel problems, etc. etc.

Jay
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Old 05-30-2002, 12:09 AM   #7
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Diversion. Now we are getting someehre, thank you. Question. When people are changin to a bigger turbo, is the inlet track not the same. How are they running more CFM with the same inlet track at a lower psi? I think I am slowly getting this thought through.
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Old 05-30-2002, 12:12 AM   #8
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Default Re: What's all this stuff about stock turbo not being efficient at higher PSI!?!?

Quote:
Originally posted by bingo
Thsi may get long, I will try to abbreviate the story. Played with my SPO waste gate actuator and ended up setting it WAY to high. Was off the gauge of my boost gauge (past 20lbs, I would estimate 22 or so). Yes my CEL went on. BUT....TALK ABOUT POWER!! This thing is sick. Now I don't want to hear about turning it down...I know I know, and I will, believe me. I would like to eventually set it at 17 or so (we get great gas here at 94 octane).

My point is tha the turbo has tons of power at higher PSI. Why are people changing turbos to run higher PSI's. Is there something I am missing? The car pulled very strong (spun all four from a partially rolling start). and was insane on the highway.

What am I safe running up to in terms of PSI? If there is no knock and EGT's are constant, other than wear and tear, what are the dangers?
I think your last question was answered quite well.. turbo back pressure will create a wear and tear that will not just affect your turbo... try blowing out your manifold gaskets, just to name a few things...

Also... trust me.... after about a day of two of bling.. the car will reel you in... try it if you don't believe me

Jorge (RiftsWRX)
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Old 05-30-2002, 12:17 AM   #9
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Jorge,

I know what you went through. Believe me I have no intention of leaving it the way it is. My question is not if it is bad, or if there will be backpressure etc. I want to know why there would be more back pressure with the stock turbo that would create these problems than with a larger turbo. I guess I am an analytical type of gut. Want to know the science behind it.
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Old 05-30-2002, 12:25 AM   #10
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Quote:
Originally posted by bingo
Sherif, your graph looks real pretty, but is incomplete. I guess 4 years of university in Physics and economics wasn't enough. Is the pressure ratio in bar? What are you referencing this against? How about another graph with another turbo to compare to...stock vs aftermarket etc. Alone it means nothing. Perhaps you can enlighten me?
Maybe I should retract my statement from before. You obviously (to me) haven't done research on turbocharging specifically. Let me start by clarifying that I am not the all knowing god of turbos but I do know a bit. I know enough to know that I don't have to sling my credentials around to make people think I know something. I know that the graph in actuality is quite complete. If you knew how to read the things, you would see that the pressure ratio is a measurement of: (14.7 + BOOST)/14.7 = X. It is not a boost level measurement but a ratio measurement. Secondly the graph at the bottom is quite simply what the cfm the turbo is capable of flowing. Now within the actual data plotted you will see the #'s: 60,65, 68,70,72, 75 & 76. These are measurements for efficiency. Now when you figure out what boost you're running, lets say for arguements sake, 16.5psi or 2.12PR... you draw a line across the graph and see where it falls. Now you calculate the cfm flow of the engine and plot it across this line (to my understanding). As long as you stay within 60% efficiency for most of the power band, you should be sitting pretty, power and efficiency wise. If you take the measurement at 2.12PR you would see that yes at 6000rpms you can extract nearly 400cfm, yet its outside the 60% efficiency range, which is the accepted and suggested range for an intercooled turbo engine. Now take what I explained and apply it to 22psi... and its ridiculous. The turbo is operating soo inefficiently for the majority of the rpm band, it ends up creating more heat, which == less dense air, which == less efficient combustion, which == less power... Take some time, pick up a book like Maximum Boost... like I have

sherif

Quote:
I want to know why there would be more back pressure with the stock turbo that would create these problems than with a larger turbo. I guess I am an analytical type of gut. Want to know the science behind it.
The answer for this can be derived from the info above. The turbo is essentially being "asked" to flow soo much air it becomes inefficient and can't spin as quickly as is necessary to fulfill the "request." Because of this inability to spin as quick as needed the air gets bunched up in the turbine and creates backpressure. A larger turbo does not have this trouble due to design reasons which I'm not really qualified to answer *shrug*
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Old 05-30-2002, 12:27 AM   #11
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a good analogy:

think about a water hose at 15 psi
and then think about a fire hose at 15 psi - which one is getting more water to the garden?

hose=turbo
water=air
garden=engine

pressure isn't everything - volume counts too...
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Old 05-30-2002, 12:32 AM   #12
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oh yeah = turbos also have a manufactureres rating in terms of what they can safely (think longevity here) handle in terms of boost - so you may exceed that for a time - but eventually you are beyond the engineered limits of a piece of mechanical hardware...
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Old 05-30-2002, 01:01 AM   #13
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......gets out a knight's helmet...


...so logically....if I connect a water hose to my turbo, I can water my garden while I drive to work....hmmmm.....
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Old 05-30-2002, 02:01 AM   #14
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Quote:
Originally posted by Diversion
Reason you want to get a bigger turbo is that the bigger turbo can push WAY more air into the motor at the SAME psi as the stock turbo.
Just to restate this general point b/c it appears so many people (not necessarily in this thread, but scattered throughout the board in general) do not understand this:
One of the first basic things you should try to make your mind understand is that PSI should NOT be how you compare turbos or address power they produce. Flow and volume, as stated above, is a more appropriate figure to compare, (amoung others).

Bingo, you seem fairly intelligent and it appears you are thirsting for knowledge about turbochargers. I think the first thing you should do before you start touching any part of your turbo is please purchase a nice book on turbo dynamics;
ie.) Maximum Boost by Corky Bell or Turbochargers by Hugh MacInnes

You will benefit from either of these books. Because, (please do not take this to be insulting), you DO need them! The questions you are asking indicate you truely have no real idea of what some are talking about, just for example the compressor map. I am by no means a turbo expert but have read both books, and I can asure you they are very informative. Good luck.

Last edited by PureWRX; 05-30-2002 at 02:07 AM.
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Old 05-30-2002, 06:04 AM   #15
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Quote:
Originally posted by bingo
Diversion. Now we are getting someehre, thank you. Question. When people are changin to a bigger turbo, is the inlet track not the same. How are they running more CFM with the same inlet track at a lower psi? I think I am slowly getting this thought through.
The bigger turbos use a Way bigger turbine, a bigger turbine has huge blades and it runs in a larger port, therefore WAY more CFM is created at a lower PSI, especially when you compare it to the stock turbo.

For example, let's "assume" the stock turbo at 16 psi is pushing 300 cfm (hypothetically, cause I have no idea what the cfm rate is on the stock turbo)

Well, let's assume you installed a huge turbo on your car. Your new turbo at 10 psi is pushing what equaled 16 psi on your stock turbo. That's the way bigger turbos work.. They push more air through the motor without working as hard.. smaller turbos do spool faster, but have to spin faster to equal the CFM of a bigger turbo.

Basically when you go aftermarket turbo you're giving up faster spool up rate... but trust me you won't miss it when the big turbo spools up, it's insano power. =)

Hope that helps. These guys like to speak all technical and stuff, but that's ok, enter me, the English translator.

Jay
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Old 05-30-2002, 08:41 AM   #16
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Whew....my brain is still smoking!!

Thanks to all. I will pick up the book today if I can find it. There are still pieces missing in my mind but perhaps the book will enlighten me. I'll post back when things become clearer to me.
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Old 05-30-2002, 08:59 AM   #17
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Its too early, my brain can't register all those words and complex sentances.

Efficiency - the degree to which additional turbo work produces additional boost, as opposed to additional waste heat. The less efficient the more heat.

The end-result of inefficiency, cherry-red turbo, turbo bearing failure, underhood fire.

Simple enough even for me at 6:30 am.
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Old 05-30-2002, 10:03 AM   #18
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Quote:
Originally posted by ross@downforce.net
a good analogy:

think about a water hose at 15 psi
and then think about a fire hose at 15 psi - which one is getting more water to the garden?

hose=turbo
water=air
garden=engine

pressure isn't everything - volume counts too...
Good analogy. However I have not even considered upgrading my turbo or done any research on the matter so try and not tear me apart. And I have no real numbers but just a thought and I am just trying to learn. With that said what is the maximum cfm the WRX can use? If the stock turbo lets say could flow 200 cfm at 15psi but the engine can only flow 100 cfm at redline then upgrading a turbo would be to simply run higher boost, correct? That is to say the stock turbo would be over revving itself if used to try and push 20 psi at 100 cfm at redline. However the stock turbo might be able to push 20 psi upto say 4000 rpm and taper off after that, correct? Remember I am not claiming that this is what can happen just thinking aloud. Now what I am trying to figure out is when you really need to upgrade a turbo. How does the stock compare to some of the aftermarket turbos? What can the WRX engine actually flow?

Thanks
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Old 05-30-2002, 12:38 PM   #19
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Quote:
Originally posted by Black 2002 WRX


Good analogy. However I have not even considered upgrading my turbo or done any research on the matter so try and not tear me apart. And I have no real numbers but just a thought and I am just trying to learn. With that said what is the maximum cfm the WRX can use? If the stock turbo lets say could flow 200 cfm at 15psi but the engine can only flow 100 cfm at redline then upgrading a turbo would be to simply run higher boost, correct? That is to say the stock turbo would be over revving itself if used to try and push 20 psi at 100 cfm at redline. However the stock turbo might be able to push 20 psi upto say 4000 rpm and taper off after that, correct? Remember I am not claiming that this is what can happen just thinking aloud. Now what I am trying to figure out is when you really need to upgrade a turbo. How does the stock compare to some of the aftermarket turbos? What can the WRX engine actually flow?

Thanks
I think part of your post hit on something that people here on the list leave out in most discussions. I probably only have mediocre understanding of supercharger theory, but maybe someone on this list can help clarify... (And before you say it, I already have another browser window open to Amazon to try and find some books on the subject)

My question is this:

I was always under the impression that the reason you "see" 15psi on the boost gauge is that the turbo is flowing a certain CFM *in excess of* the amount the engine can consume at the current RPM, and so the turbo is packing the air, making it more dense and raising the pressure in the intake tract.

So, for example, if you had an engine with a turbo that was already running at 100% of it's peak flow at 15psi (leave efficiency out of it for a minute to simplify) and then you made the engine more efficient (say a new intake, headers, downpipe, etc) then the engine would start to require more flow than the turbo would be able to provide, and you'd see the 15psi go down to something less. Does this make sense?

So now, bringing efficiency BACK into the equation - turbocharger efficiency effects the temperature of the air it is compressing. And air temperature is well known as a factor in the efficiency of an internal combustion engine. Therefore, the turbocharger that is running inefficiently, outside of it's "sweet spot", or on the edges of the blob in the efficiency chart shown a few posts above, will be generating excess heat, which LOWERS the effective efficiency of the engine, therefore making it actually require LESS air, which RAISES the pressure in the intake tract between the turbo and the throttle body. So the fact that the turbocharger is running inefficiently actually raises the pressure you see at the gauge, even though it's not making any more horsepower. Does that makes sense?

So what I'm basically getting at here, is that a higher pressure reading on your boost gauge is not necessarily a GOOD thing, it just means that there is a volume of air that the engine isn't using. If the unused air charge is nice and cool, and the turbo isn't working hard to provide it (>75% efficiency say) than you actually have too much turbo for your application and you'll probably be happier with a smaller one that spools up faster. Either that or you have to increase the efficiency of the engine to take advantage of the extra air available.

However if you have a higher pressure reading on your boost gauge and it's nasty HOT air, which is reducing the efficiency of the engine itself, then modding the engine to increase the engine's efficiency makes the turbo work even HARDER to maintain the same boost, which means you're getting even HOTTER air to the engine, lowering the efficiency of the engine, and basically taking back some of the efficiency you got by modding the engine in the first place.

I think that's why you see the guys running larger turbos doing so well in terms of quarter mile times. Their net efficiency (turbo efficiency + engine efficiency) is much higher than someone who is maybe running a highly modded engine but stock turbo (low turbo efficiency + high engine efficiency) or someone running a stock engine with a too-big turbo (higher turbo efficiency + lower engine efficiency)...

Which of course is what any of the good tuners will tell you - the car is a complex system and everything has trade-offs.

Anyway, sorry for the rambling, but I wanted to think through the whole "more pressure = more horsepower" fallacy out loud, so people have a chance to refute it or add more detailed explanation!

-B
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Old 05-30-2002, 12:42 PM   #20
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Default Turbo basics

Quote:
Originally posted by Black 2002 WRX


Good analogy. However I have not even considered upgrading my turbo or done any research on the matter so try and not tear me apart. And I have no real numbers but just a thought and I am just trying to learn. With that said what is the maximum cfm the WRX can use? If the stock turbo lets say could flow 200 cfm at 15psi but the engine can only flow 100 cfm at redline then upgrading a turbo would be to simply run higher boost, correct? That is to say the stock turbo would be over revving itself if used to try and push 20 psi at 100 cfm at redline. However the stock turbo might be able to push 20 psi upto say 4000 rpm and taper off after that, correct? Remember I am not claiming that this is what can happen just thinking aloud. Now what I am trying to figure out is when you really need to upgrade a turbo. How does the stock compare to some of the aftermarket turbos? What can the WRX engine actually flow?

Thanks
I think part of your post hit on something that people here on the list leave out in most discussions. I probably only have mediocre understanding of supercharger theory, but maybe someone on this list can help clarify... (And before you say it, I already have another browser window open to Amazon to try and find some books on the subject)

My question is this:

I was always under the impression that the reason you "see" 15psi on the boost gauge is that the turbo is flowing a certain CFM *in excess of* the amount the engine can consume at the current RPM, and so the turbo is packing the air, making it more dense and raising the pressure in the intake tract.

So, for example, if you had an engine with a turbo that was already running at 100% of it's peak flow at 15psi (leave efficiency out of it for a minute to simplify) and then you made the engine more efficient (say a new intake, headers, downpipe, etc) then the engine would start to require more flow than the turbo would be able to provide, and you'd see the 15psi go down to something less. Does this make sense?

So now, bringing efficiency BACK into the equation - turbocharger efficiency effects the temperature of the air it is compressing. And air temperature is well known as a factor in the efficiency of an internal combustion engine. Therefore, the turbocharger that is running inefficiently, outside of it's "sweet spot", or on the edges of the blob in the efficiency chart shown a few posts above, will be generating excess heat, which LOWERS the effective efficiency of the engine, therefore making it actually require LESS air, which RAISES the pressure in the intake tract between the turbo and the throttle body. So the fact that the turbocharger is running inefficiently actually raises the pressure you see at the gauge, even though it's not making any more horsepower. Does that makes sense?

So what I'm basically getting at here, is that a higher pressure reading on your boost gauge is not necessarily a GOOD thing, it just means that there is a volume of air that the engine isn't using. If the unused air charge is nice and cool, and the turbo isn't working hard to provide it (>60% efficiency, to quote what someone said above) than you actually have too much turbo for your application and you'll probably be happier with a smaller one that spools up faster. Either that or you have to increase the efficiency of the engine to take advantage of the extra air available.

However if you have a higher pressure reading on your boost gauge and it's nasty HOT air, which is reducing the efficiency of the engine itself, then modding the engine to increase the engine's efficiency makes the turbo work even HARDER to maintain the same boost, which means you're getting even HOTTER air to the engine, lowering the efficiency of the engine, and basically taking back some of the efficiency you got by modding the engine in the first place.

I think that's why you see the guys running larger turbos doing so well in terms of quarter mile times. Their net efficiency (turbo efficiency + engine efficiency) is much higher than someone who is maybe running a highly modded engine but stock turbo (low turbo efficiency + high engine efficiency) or someone running a stock engine with a too-big turbo (higher turbo efficiency + lower engine efficiency)...

Which of course is what any of the good tuners will tell you - the car is a complex system and everything has trade-offs.

Anyway, sorry for the rambling, but I wanted to think through the whole "more pressure = more horsepower" fallacy out loud, so people have a chance to refute it or add more detailed explanation!

-B
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Old 05-30-2002, 12:56 PM   #21
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Turbos by themselves don't change air flow volume, they increase density, pack more air per unit volume,. so the same volume of air has more O2 to ignite more gasoline to produce more power. Hope it helps.
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Old 05-30-2002, 01:07 PM   #22
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-B,
Great post! Informative with a smidge of humility A rare quality in these parts As far as my own understanding goes, though I'm no engine/turbo guru and still have much to learn, you hit the nail on the head.
-Leland
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Old 05-30-2002, 07:01 PM   #23
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Pardon my ignorance but im still confused.

I too had the experience of running insanely high boost on my car (did so by accident when testing the car after installing an ABC). Apparently it wasnt hooked up right and I was seeing somewhere in the neighbourhood of .14 bar

Thing is, I thought I had just stepped into a formula one racing car when I got onto the gas. The car was ridiculously fast which brings me to my dilemma in understanding the whole inefficiency theory.

BTW I was going from a TXS Stage 1 to the TXS Stage 1 upgrade (no ABC before) so if you are wondering why i didnt hit fuel cut etc, its because i had a unichip

Regardless of my Formula 1 experience I immediately fixed the connection and set it down to 16psi as thats what everyone recommends. And well, I guess I just follow the crowd.
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Old 05-30-2002, 08:34 PM   #24
Weedy
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Sorry although I thought that I would just chime in here. All of this info is very informative so far! This is what the factory forced induction message board should look like! Keep the info flowing! Thank you!
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Old 05-31-2002, 02:34 AM   #25
Sordid Philosopher
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Default Another silly question

I understand efficiency really well. I understand why a bigger turbo can produce more air etc...

What I don't understand is that if the air outlet is say 2" then why two different turbos, say the stock TD04 running 15 lbs of boost is flowing less air....than a VF30 at 15 lbs with the same size 2" outlet?

They both have 15 lbs boost, they both have to push it through the same 2". I read the "firehose" analogy but if the fire hose has a 1/4" outlet and the garden hose has a 1/4"outlet and they both have 15 lbs pushing the water - ack...same pressure, same outlet = same flow.

How do you get more air with the same boost and outlet size?

The reason I believe that the outlet is the restricting part of this equation is because my back pressure increases as I increase boost.
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