Big turbos make more power because they offer less backpressure (which in turn increases the flow at a given boost pressure) and have more efficient compressors (more oxygen molecules for a given volume).

That's it in a nut shell what's so fucking hard to understand. Bolt one on your car and you won't be so damn confused.

A t78 will make more power at 5 psi than the twins at 10 which is why lag with this turbo is a joke as well. Stock wheels will start spinning in second gear at 3.5 rpms lets see you do that with twins.

btw good thread. it's been a while since something interesting has come along. :)

Thanks for the responses even though I feel like I am listening to a broken recording of my own voice.
:djsmile:lol let it suffice to say that I anderstand full and well how reduced backpressure, increseased efficiency (resulting in lower intake temps) - result in greater power at a given PSI. I think we are beyond that now.:bigthumb:

The point I was trying to hammer out is whether compressor flow, namely CFM, has a discernable effect on volumetric efficiency.

WARNING - The following is tedious. Don't read if you are going to whine about how long or complicated this is. :cylonA:

I just had a phone discussion about this exact subject, just called a buddy of mine to catch up but I figured I'd see what his thoughts were since he happens to be a physicist. Granted it's not his field but it was good to beat some ideas around. He didn't really help much besides bringing up the ideal gas law, but he got me questioning how CFM was really measured....

I was getting a bit tripped up with the whole visualization of CFM. I incorrectly visualized 1 CFM as a cubic foot of air (irreverent of density) moving 1ft in 1 minute. As it turns out I was wrong (mostly). I did not realize that CFM measurements are density/temp dependent! IOW, 1 cubic foot of air per minute at 60 degrees may equal 1 inch of air per minute at zero degrees. Temp is never listed alongside turbo flow ratings so how is anyone to know.... ofcourse its seems rediculously obvious now..... lol

To get this clear in your mind, realize that CFM ratings do not account for the differences in density of the the various turbo setups, because it doesn't account for innefficient charge temps. Therefore it is not a realistic indicator of what is going on in your intake!

Apparently the CFM ratings of turbos are standardly listed at a baseline temp of zero degrees (or some standardized temp) via some conversion factor applied after testing.

Now that I have cleared this up the Ideal gas law perfectly describes why more CFM does not necessarily translate directly into superior intake infiltration. Simply put, the volume of air expelled from a large turbo is not any different than that of a small turbo at the same PSI, the only variables are air density, and turbo RPM. If the air is colder the turbo has to spin faster to make pressure, simple as that. Not rocket science by any means.
Now to defeat my earlier theory...:dark: Not very hard with this new info. lol

Regardless of CFM, if the turbos are making the same PSI that means that they are providing the same volume of air. no exceptions. IOW, one will infiltrate the engine just as quickly as the other (ignoring factors like kenetic energy etc). It is tempting to think that the larger turbo with the larger CFM rating is pumping out more oxygen due to its big CFM rating but that is not the case unfortunately.

A good way to visualize the flow from a small vs large turbo is to picture a small beam of lossely packed air being hurtled out from a small turbo at high velocity while a wide trunk of neatly packed air slowly moves out of larger turbo. Again, density and velocity are the real variables here, not volume. How can it be when at the same PSI? It can't!

For me, big CFM number always made me want to think that larger turbos move a larger volume of air at a given PSI. Now I realize why that is impossible.

conundrum solved.

Rob, I think you're missing my point, but then again, I also think you have MUCH more time on your hands than I do (apparently Oxford is kinda lax schedule-wise? LOL). So I'm gonna hafta lay this thread to rest :)

~Ramy

hahaha, on my last days of break my friend. I've written 60 papers since last August. If you don't believe me I can send them to you ;o)

Your points were expertly delivered and pin-point precise. Why add any more when you've already achieved perfection?

I fear it may be a horrible mistake to point this out :), but more air will flow through the engine system as a result of installing a larger turbo, given a fixed boost pressure.

But it isn't BECAUSE the turbo compressor is capable of flowing more. I think it is the causality that was getting confused before.

The bigger turbo has less backpressure, so it leaves a mouse rather than an elephant in the combustion chamber. That leaves "more room" for fresh air to enter the combustion chamber.

And because the larger compressor is generally more efficient at the power peak, it also means the intake temp will be lower. So given a specific amount of space available for fresh air in the combustion chamber, the lower temp will allow more oxygen molecules to fit in that space.

Ultimately, all these things are related in the "engine ecosystem". But that isolates the two big causal factors for getting a power increase after installing a larger turbo while still running at the same boost level.

-Max

whats up guys

I was going to pull some of my quotes, but decided that that would be asinine.

soo

What Max said :icon_tup:

That's a good round-off of the discussion. I think the lack of definition of "Flow" is what has been killing everyone all along. People jump on and make pithy comments to sound profound (not talking about you Max) but they explain squat.

Sometimes I start these threads to share, sometimes its to learn. This time I decided to learn as I shared. lol

I've enjoyed this discussion.

I feel like a should do a wrap-up but I think Ramy's post about covered it.

ltr,
Rob

yea, i even posted it twice.

its probably the shortest response out of all 8 pages of this thread, lol.

haha, nice.

BTW, I only see 3 pages right now. Check my How to: stickie to max out your threads per pg.

Rob

DISCLAIMER: RANT

The problem with discussions about this topic (btw they happen on every forum) is that people post their opinions about this and that, but so much of it is untrue (this thread unexcluded) that it ends up confusing people more than it helps.

I am still unconvinced that most people know what's truly going when they believe they have the right idea because of all of the false information and poor analogies being made in this thread. Seeing all of these equations from physics or thermodynamics 101 aren't needed in seeing something this intuitive.

I mean, how does the calculation of compressor efficiency come into play with the original topic? It is not needed. Can I calculate it? I can derive it, but it doesn't make me feel special. I could come here and try to impress you with the calculation for this and that, but it sure as hell wouldn't be helpful to me if I was trying to understand the difference between the a large turbo and a small turbo at the same boost level.

You CANNOT just add on any sized compressor to a turbine, even though in theory it'd be a great turbo, it's just not the case. Turbine performance/efficiency is greatly affected by the compressor stage, and it needs to be matched correctly. If you just threw any random compressor on a turbo, chances are the turbine efficiencies would be very low.

Also, volumetric efficiency is not THE main driving factor behind why large turbos make more power than small turbos at the same boost pressure. The equation for mass flow rate of the engine is really not that important either. Nor is the ideal gas law or calculation for compressor efficiency needed to understand the original question.

In addition, although an adiabatic process is without heat, in calculating compressor efficiencies, you compare it to an isentropic process, not adiabatic. We can never assume that NO energy is used to heat the air, even in the perfect world.

I stated it in the first post of the original thread I linked. It's that the large turbo supplies more mass flow of air (kg/s, lb/min, etc) than a smaller turbo. (There should be no confusion to what mass flow rate is, mass flow rate is mass flow rate) That really should be the end of story.

Sorry for the rant, it's late.
Kevin

MY Rant Back:

The whole point of these threads like is to share thoughts on the stated issue and arrive at a mutual and well-rounded understanding. The best way for people to actually understand these concepts is for those who are knowledgeable about the subject to share their knowledge (certainly not most of the posters here, including me). I have Corky Bell's book at home, but let it suffice to say that it does not cover this theory adequately nor does the Garrett website or any of the short posts which you have placed previously. Are they good generalizations? Yes, but as you've seen people are left to pick and choose formulas to derive incorrect understandings of concepts which are so much deeper and more fundamental, yet unaddressed. It would be considered a gift to the forum, as well as the Google search function, if some of those people who are fortunate enough to work in the industry and obtain an intimate knowledge of the subject, such as yourself, would explain matters like this in painstaking detail so that there can be no further confusion.

Instead, those with deep knowledge of subjects like this would prefer to post one-liners and "profound" pithy statements which circumvent the explanation of more fundamental processes and relationships which are necessary to arrive at such eloquent conclusions.

Explain it once in great detail and from then on people can use the search function is what I say :icon_tup:. I will personally ensure that the incorrect informationin various threads gets deleted once this happens.:)

In my defense i developed the "franken-turbos" as aids in explanation.

Second, Kento has mentioned twice or three times turbo upgrades that make more power on stock manifolds. Are they changing the turbine wheel as well? clipping?

Mine does. Sortof. Defi gauges are very sensitive, the boost gauge comes with a damper. Without it fitted my boost gauge bounces constantly by about 1psi at max boost, up and down. With the damper its constant.

That's a good observation. I don't see how it could be possible for the manifold pressure not to fluctuate because no turbo is going to be able to instantaneously repressurize the intake once the intake port opens and allows the air to spread over a larger volume.

Wanklin,
I tried to explain it in detail in my first post in the previous thread. I really did. I also don't want people to think that just because of where I work that I know everything about turbos because I don't. It doesn't add to my credibility in any way, that's why I don't state my occupation when I post, but rest assured that when I do post, I don't just assume and make things up which cause confusion.

Kevin

Fair enough Kevin. I can respect that. Please just keep in mind that the first sentence of this thread stated that nothing that I say should be take as gospel. There is a difference between writing a book about a subject and starting an inquisitive internet discussion. Surely we can both appreciate this.

With that being said....

If someone takes the time to read this thread beginning to end he/she will see that the issue is being debated and slowly shaped into a consensus.

I beg to differ. Physics and thermodynimics allow one to understand how machinery operates at a more fundamental level. Otherwise you are just comparing symptoms and treating assemblies as black boxes, you're not actually analyzing problem. It has nothing to do with impressing anyone, though I don't know if I'd say the same about the condescending pithy statements (not directed at you specifically).

Actually the flow calculation was plenty useful as it shows the correlation between RPMs, air density and flow. It also shows how decreased back pressure (increased VE) affects the actual flow of the engine.

That's why we plainly stated that too many mechanical variables were being ignored to make a realistic assumption.

All your doing is telling me what isn't involved, you are saying nothing about what you think is involved.

The ideal gas law does apply to this situation as you are steeling dealing with air molecules that are bound by a law stating that volume, pressure and air density are all interelated and dependene on one another. The simple fact that you are slapping on a turbo does not change the physical properties of 02 molecules, sorry.

You simply cannot push more air into an intake of fixed volume at the same temp without creating greater pressure. Certainly, the ideal gas law does not explain how the compressor wheel velocity plays into overall flow, and flow itself does not directly dictate performance, but that is not the point. The point is that these are all important fundamental concepts to understand if you want to be able to actually conceptualize what is going on. To say that the ideal gas law does not apply at all is misleading.

Right.... It really isn't the end of story. The mass flow rate is dependent on the air density. A turbo rotating at a fixed RPM will move more molecules of cold air in 1 second than it will molecules of hot air. This flow is flow crap is what is confusing people.

I think perhaps you took my statements too literally. When I mentioned that even installing slightly larger compressors on the stock twin assembly resulted in power gains, I didn't mean to imply that just throwing any ol' larger compressor will result in increased power in relation to boost (in fact, in this post, I mentioned using the BNR/M2/Australian SP twins as an example didn't mean that you could just slap on the biggest compressor you could find). I was only pointing out that some people were concentrating on reduced backpressure as the sole reason for a larger turbo's ability to generally produce more power relative to boost pressure.

what??? I disagree completely. Equations are not just math, they are a detailed description of complex events that are not intuitive unless represented by numbers.

In fact you would do EVERYONE a great favor if you would actually sit down and prove how in fact large turbos differ from small ones and what that in means in terms of HP. :)

Rob wins the pissing contest hands down. Stop filling your head with turbo knowledge and go write a paper on why it should be illegal to put a v8 in an FD :balls:

Formulas are often the simplest way of explaining complex relationships:

http://img464.imageshack.us/img464/7...ormulason0.jpg

Damn you just gave me a whopper of a headache :D

I'm gonna go fire one up and wax one of my cars.

hahaha

lol. Sounds good Ftritzorama. I'm of to the union society to hear a debate about why the United States should have never been founded by England.... (As if they founded the US??) Anyway, I might stand up and challenge them a bit, or I may just laugh my ass off. We'll see how it goes. lol Have a great evening gents.

-Rob

if those are true equations: case closed IMO.