CarbonR1

iTrader: (2)

Equations are derived from intuition,
Intuition is not derived from equations.

If you guys would rather look at equations than think about it logically, then we are very different learners.

I agree that equations can help you learn. Of course they can, but I just don't think they help in this situation and the question at hand.

I don't care enough about trying to HELP fellow FD drivers learn about this question to argue with them about it. It's with good intention that I post, my apologies if I came off as thinking some people didn't know wtf they were talking about. (I did)

Ah well, good times
Kevin

CarbonR1

iTrader: (2)

OK, 1 more shot;

Take a look at these two compressor maps. One is of a GT15, one of a GT40

I marked the difference in flow on the map at the same pressure ratio (15psi boost in this case). This is the reason why they will make more power at the same pressure. The larger turbos supply much more air. In this case the GT 40 supplies more than twice the air mass flow rate than the GT 15 at 15psi.

wanklin

iTrader: (2)

yep, that's because the GT40R is operating in a higher efficiency island = lower intake temps. Both turbos supply the same volume of air, it's just that one intake charge is more dense than the other.

It would take writing a small booklet to describe in depth, all of the relationships that are explained in the list of equations above. If I have time, perhaps I will write it out bit by bit and have the rest of the thread deleted.

Thanks for posting that,

Rob

wanklin

iTrader: (2)

Ohh they're true, got them off of Garrett's website

CarbonR1

iTrader: (2)

Not quite Wanklin, In this case, both turbos are operating near peak efficiency which means they will generate that boost pressure without much difference in charge temperatures. Both turbos do NOT supply the same volume of air. The difference that you aren't quite seeing is that the GT40R is supplying much much more air flow (volumetric flow rate, AND mass flow rate)

maxcooper

That's not a very useful comparison for explaining WHY an engine will make more peak power with a larger turbo, though. If you connect those turbos to an engine, the flow rate for a given PR will be determined by the engine displacement * VE (***). It certainly WON'T be in the center of the highest-efficiency land on BOTH of these charts.

It is a good idea to look at some charts to help understand what happens when you install a bigger turbo. The better VE that results from installing a larger (but still applicable) turbo will move you to the right a bit on the x axis.

-Max

(***) Since the units are lbs/minute, intake temp will show up in the VE value, not the flow.

CarbonR1

iTrader: (2)

While it is obvious that an engine wouldn't be operating at those points I picked out arbitrarily, it is essential to understanding why big turbos make more power at a given boost pressure. Look at the plot for what I pointed it out to be. A large turbo will flow more air. I've been trying to express that since my first post in the last thread.

Intake temperatures are not going to affect volumetric efficiency the way you are describing. sorry, but I disagree. Intake temperatures are absolutely NOT why a large turbo can create more power at a given boost pressure. In fact, large turbos can heat up the intake charge more than a small turbo depending on how your turbo match is.

Gorilla RE

iTrader: (1)

I think a lot of folks on this forum like to hear themselves talk/post. i guess if ya'll want to sound smart to other people on the inter-web than be my guest....but its a lot of blah blah that isn't needed. because it really isn't a hard concept to understand and all these long equations are not needed. but hey who am i right?
-J

maxcooper

CarbonR1, I agree that a large compressor will flow more air on it's own. I even agree that it will flow more air when connected to a given engine (at a given boost level). But without an explanation of why an engine system will flow more air with a bigger turbo installed, the answer is rather incomplete. Just showing the compressor maps is misleading. It gives the mistaken impression that a bigger compressor could somehow magically stuff more air through a fixed orifice (standing in for the engine here) without changing the boost pressure. Of course, the main magic is that the engine system is not like a simple fixed orifice, and that the bigger turbo actually "changes the size of the engine orifice", resulting in more flow through the system.

And since we're generally talking about stock FD turbos versus larger alternatives on cars with other flow mods, I stand by my assertion that intake temps (or more importantly, compressor efficiency) is a significant contributor to increased peak power. (While at the same time conceding that my "roll the intake temps into the VE" comment in my last post was wrong or at least misleading.)

"When you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge of it is of a meager and unsatisfactory kind; it may be the beginning of knowledge, but you have scarcely, in your thoughts, advanced it to the stage of science." -- Lord Kelvin

-Max

GoRacer

iTrader: (2)

All this doesn't matter. The larger turbo will move the power curve upwards, so I consider it meaningless. I tried posting something more realistic in the single turbo section but certainly no feed back like this.

Anyhow a real world comparison would be:
GT35R w/1.06 Turbnie A/R @ 15lbs - VS- GT40R w/.84 Turbine A/R @ 15lbs
Why, you may ask? If I remember correctly both of these combinations will move approximately the same air but will the larger turbo spool earlier, the same or have threshold lag? Now if the larger turbo will produce more hp at the same boost pressure but begin positive boost sooner, then we have something to talk about.

wanklin

iTrader: (2)

Good call I'll accept that, but as Max said, the flow tests only explain what the turbos are capable of, not how they actually interact with an engine.

On an engine: I am just translating what the formulas above are saying.

Charge density is greater for the turbo operating on the higher efficiency island which I think is obvious to everyone. Next reduced backpressure increases the size of the available combustion chamber as well as the volume of the opposing gas (increases volumetric efficiency) which increseases the amount of actual flow. RPMs also play a role as the intake port is available more often (though for less time) and thus results in greater net flow.

I agree that turbo efficiency maps are important, but they are not in themselves intuitive answers to the question: I believe they are useful because there are mechanical properties of a turbo which are too complicated and involved (would literally need to know everything about how the turbo was designed/constructed) that we just rely on the efficiency maps to understand how: the exhaust gas interacts with the turbine, the turbine interacts with the compressor wheel, an dlastly how the compressor interacts with the air. Not that you guys don't already know this....

wanklin

iTrader: (2)

All you pointed out is why turbos make more flow on a test stand....

This is interesting. Would you mind elaborating Kevin?

wanklin

iTrader: (2)

Right.... Thanks for the helpful info.

wanklin

iTrader: (2)

I think Fritz addressed this earlier . Also, doesn't matter for what type of application? Why don't you paste a link....

What ever happened to that electric turbo that Garrett?? was working on?

Gorilla RE

iTrader: (1)

my point is that i understand everyones equations (right or wrong) on the subject but its being overly complicated. and yes to be honest this thread is a good read
with a lot of intelligent posts but if you read this guote VVV it IS the answer and it IS simple to comprehend.

wanklin

iTrader: (2)

OK Monday morning quarterback. It's easy to come in here postfacto with this ignorant rhetoric and claim you know the purpose of life....

Simplistic answers do NOTHING to explain fundamental concepts and relationships which are the basis and the reason for the correctness of such synopses. I still am not convinced that Kevin really understands the difference between an engine and a test stand....

We still love you though

Montego

iTrader: (1)

you call those long equations??? bro that's 10th grade algebra.

fastyoungone

Sticky material? LLLLOOOOOTTTSSS of good info........ GREAT READING GUYS!

wanklin

iTrader: (2)

Not sure if I need to tell you this or not Max since you abandoned this statement later, but intake temp is directly related to flow because itake temps directly correlate to air density, and both of these factors are considered when calculating actual flow (among other things). The 02 constant and intake temp parameter are really a representation of charge density.

This relationship can be seen in the actual flow formula posted earlier.

Rob

Also remember that engine RPM says nothing about the turbo's rotational velocity, which is independently controlled by the wastegate/boost controller.

wanklin

iTrader: (2)

Well, I'm off to the library gents. Have a good evening. Perhaps I'll poke my head in ltr tonight and see how the thread is going. Glad it's wrapping up because my school crunch has just begun. Research paper due Tues and I just found out about it ;o)

take it easy,
Rob

NissanConvert

iTrader: (1)

We haven't explored exhaust pulsing yet!

Gorilla RE

iTrader: (1)

haha yeah youre probably right.......

CarbonR1

iTrader: (2)

Turbine efficiencies are increased due to exhaust pulses ~+2% turbine efficiency from the pulses.

There,
Now which one of you smart asses is going to start arguing with me about this too.

Kevin

wanklin

iTrader: (2)

I should correct myself here. Flow efficiency due to incresed RPMs is subject to the law of diminishing marginal returns as there reaches a point where VE begins to taper off. Basically, the intake charge doesn't move quickly enough to fill the available combustion chamber within the short window of time that the intake port is open.

With that being said a higher PSI charge will travel with greater velocity as it has more force exerted on it, and so it seems that higher boost levels are more effective on the top end.

SPOautos

Lets make it simple, the point of making more power is to bring in as much ambient fresh air into the compressor housing inlet as you can. You have a compressor housing with a inlet, compressor wheel, and outlet. The smaller the wheel the faster it has to turn to pull in a specific volume of air. The larger the wheel the slower it has to turn to bring in the same volume of air. Now when the wheels are close in size you can make up differences with compressor wheel design, fin angle, ect....but lets leave that mess out of it. The wheel takes in the air and forces it against the housing wall to compress it, the faster it turns, and the smaller it is, the faster the velocity, the hotter the air gets. As air gets hotter it tries to expand and makes it harder to compress making it harder for the wheel to intake more fresh air (cfm). The larger turbo can take in the same amount of air into the intake side of the housing, compress it in the housing without generating as much heat, and allowing it to take in more air easier because the air isnt trying to expand as much because its not as hot. Imagine you had a air compressor and you were trying to squeeze fresh unpressurized air into it. The hotter that air is the more area it will take up and the less oxygen you can get into the tank at the same pressure. Likewise, as the air gets compressed in the housing, the hotter it is the harder it is to get more in, the less fresh air the housing can accept.

i guess what im trying to say is the hotter the air gets inside the compressor housing it just makes it harder to accept in more fresh air.

I dont know if that is going to help anyone or not. But thought I'd try to explain some of the scenerio thats going on.

Stephen