And I've gone crosseyed.....

 

There is a lot of misconception in this thread, but if you reread the posts by Ludikraut, he has it right.

I only went back one page in this thread, but I would like to clear a few things up and mention something that may make it easier to understand.

First of all, 30 psi is 30 psi, given the same VE and temperatures. Bigger turbos move more mass of air because the VE is larger and the T is lower. It has nothing to do with the CFM capabilities of the turbo. The larger turbo is pushing into the same fixed volume as the smaller turbo (the engine). What a turbo can do has nothing to do with what it will do on a certain sized engine. That's why you have to plot points on the compressor map to find what CFM, or mass airflow you can achieve.

The equation that is missing in this thread is the airflow through an engine:

Airflow (CFM) = PR[RPM*V.E.*Cid/3456]

PR = pressure ratio, which is (boost pressure+14.7)/14.7
Cid = cubic inch displacement, or 122 for our stock 2.0L

Temperature comes into play because that equation is for volumetric airflow, or CFM. To get mass airflow, n, from a volumetric airflow, CFM, that is where PV=nRT comes in.

n (lbs/min) = P (psia) x V (CFM) x 29 / (10.73* T)

So, if you combine those two equations, you get the following, which will be how much mass airflow you can flow through an engine:

n (lbs/min) = P (psia) x [PR(RPM*V.E.*Cid/3456)] x 29 / (10.73* T)


As you can see, when comparing two turbos at the same boost: pressure, RPM, and Cid are the same

There are only two variables: VE and T

Those two variables are the only reason why a bigger turbo will move more mass air than a smaller turbo at the same psi. The VE is higher due to bigger exhaust housings and less restrictive flow and the T is lower because of more efficient compressor wheels, as shown by the compressor maps.

I think a lot of people get confused because they see people with huge turbos at 30 psi flowing a ton of mass air and wonder why their little stock turbo can't do that. Well, for one, a stock turbo can't hold 30 psi to redline. Secondly, even if it could, the temperatures would be much higher. Lastly, the VE would be much lower.

A better test would be to test the stock turbo at 10psi compared to a 35r at 10psi. In that case, efficiency shouldn't be an issue and only VE comes into play. I would be willing to bet the stock turbo would be very close in mass airflow and power at those levels, depending on the VE differences from the bigger turbo.


Eric

 

This is a SBR GT35R @ 11 PSI with BR Cylinder Head and Cams.
This was the first dyno run with this setup full throttle was @ 3350rpm

3500rpm = 4psi
4000rpm = 8psi
4500rpm = 10psi
5000rpm = 10psi
5500rpm = 11psi
6000rpm = 11psi
6500rpm = 11psi
7000rpm = 11psi
7500rpm = 11psi

Boost is controlled via AEM, a ball and spring type boost controller might achieve quicker spool.

I would say that 11 psi on a stock turbo wouldn't be as high, but I could be wrong.

 

Ehhh... Why is this formula here?
This formula can be used to figure out the volume in CFM that goes into the turbo i suppose, yes. But not what you're trying to do... You do realize that the volume (that you are trying to calculate here) will be increased by higher temperature and decreased with higher pressure?

PV=nRT is V=nRT / P ,therefore the higher the pressure - the smaller the volume. The lower the temperature - the smaller the volume. Forget about this gas law.




Difference maker is the air density. Density = Pressure/(Constant x Temperature)
That's what puts "more molecules" in the same, neverchanging engine volume, therefore requiring more CFM of the outside air.

That's where a good intercooler is very important by the way. The density will be lower at the same PR with a bad intercooler, especially if your turbo is farther away from the efficiency range (which means it runs hotter, 50% efficiency means it runs 2 times hotter than it should for example).

Aaaanyway, one would figure out how much CFM can be put through an engine at a certain temperature and rpm, put that number on the compressor map at the PR you want to run and see if a turbo can physically do it or not. And how efficient (read hot) it will make the air, therefore dropping the density of air.

The fact that 35r is rated at 700 and 16g at 500 (or "can put more molecules of air") for example doesn't matter. Your engine can only take what it can take at certain boost (PR) and VE.
So, again, if the pressure and the temperature and the rpm's are the same, any turbo will flow the same amount of air. If it physically can of course (you have it spooled; it's not in the choke area; etc)

Is what i think.

 

Right on Eric, but, for the more intellectually challenged people, lets give a simple example using air compressors...

Pretend the 16G is a 5 gallon air compressor. It will hit 30psi easily.

Pretend the 35R is a 200 gallon compressor. It also will hit 30psi easily.

How are they different?

There is a TON more air available from the 200 gallon compressor.

More air, more flow, lower temperatures, etc...

The 200 gallon air compressor has the potential for much more flow than the 5 gallon.

So, the 35R has more potential for flow than the 16G. BUT, BOTH can hit 30psi easily.

The difference is the "reserve" behind it, hence, the flow. But, 30psi IS STILL 30psi.

The difference is in the flow.

(Disclaimer: This is a nutshell version.)

 

This is like saying putting a bigger fuel pump will give you more power cause "it can flow more" gas...

 

Nice post l2r99gst.. that clarifies my understanding even further so thank you.

Tell me something though... what is it about the smaller turbo that heats the charge air more? Is it simply the fact that the RPM's must be higher to support the same manifold pressure? Or is it that the housing itself is smaller and heat just soaks into the charge air more? Or is it the fact that VE is slightly lower and therefore less mass air flow is occuring and thus that creates the additional heat too becuase the charge air spends more time going through the compressor housing?

I'm guessing it is all of the above but just curious on your thoughts.

In a day or two I'd love to do a thread on compressor maps and get your input.

 

Looking at the compressor maps closer, the stock turbo, even at a low 11psi, would reach approximately the 66% efficiency area. The GT35r would finish in the 79% efficiency area, so yes, I would agree that the stock turbo numbers would be lower. Also, since the VE is much higher with the bigger backside and turbine wheel of the 35r, etc, that would also raise the numbers.

I didn't realize that even at 11 psi our stock turbos reach a relatively inefficient area quickly. My whole point is though that if you are at the same efficiency on the compressor map of a small turbo and a big turbo at a particular RPM and PSI, then your numbers are only different due to VE at that point.

The bigger point is that from a smaller to a bigger turbo, you get more mass air and more horsepower solely because of temperatures (efficiencies) and VE.

Looking at a bunch of compressor maps I have, a t04e 50 trim, for example would still be in about a 75% efficiency island as compared to the 35r at 79%, so it's numbers wouldn't be too far off, unless the hotsides and turbines are dramatically different in size.


Eric

 

On the compressor side of things, the smaller turbo, such as our stock turbo, heats the air up more simply because the compressor wheel isn't operating in a high efficiency area of the compressor map. The smaller wheel must spin much faster to produce the mass airflow and as it approaches the choke limit of the wheel, the efficiency drops very quickly. The lower the efficiency, the more the air is being heated. Look at the speeds of the wheels on the compressor maps of the stock turbo compared to a larger turbo as well as the efficiency islands and you will see what I am referring to.

 

dont overboost a turbo

 

.

This is true.

If we have 100 moles of air exerting same pressure (30psi) as 1000 moles of air in SAME volume (1 cubic inch). The 100 moles of air have alot more thermal energy to bounce off that small space to generate 30psi.

Whereas the 1000 moles of air don't have to work as hard.

The T on the 100 moles of air must be greater than the 1000 moles.

 

Actually it's nothing like saying that. It's only using the example that the 35R can produce the 30psi just as easily as the 16G, but there is a lot more flow potential from the 35R than the 16G. There is nothing wrong with what I said, it is simply a severely boiled down explanation. In essence, it can provide the cylinders with more air, more oxygen content, and thus the ability to make more power. The example of using fuel has nothing to do with it, which is why I didn't use it as the nutshell simplification because adding more fuel will not necessarily add more power. If you add more air to the engine, it will, provided all other necessary things like fuel are present. This is the exact reason why Nitrous is used, simply to add more oxygen content to the cylinder than normally possible. Re-read the line where I said that this explanation was for the more "intellectually-challenged." :-)

 

The problem is the above answer perpetuates or confirms the wrong answer to the question we have been debating. Yes a 35R has more flow potential... but we are not speaking about potential. The question is only for 30 psi versus 30 psi for the two turbos. Now if the stocker and the 35R are both making this boost, the respective flow potential the 35R has beyond this is irrelevant.

The 35R makes more power because of the reduction in exhaust back pressure and also the reduction in charge temperature because of increased compressor efficiency.

 

It doesn't make more power at 30psi because of less backpressure, but yes, because it's more efficient at putting a lot of air at high rpm into our engines. Efficient means one thing and one thing only - it heats air at a lesser degree. Air not as hot-> Higher density->more power at same pressure ratio.

But a small turbo will make same power as 35r at 30psi in its efficiency range (as in with the same outlet temps).

Look at dynographs - which turbo makes more power at 30 psi at 4000 prm on our engines? A green or a 35r? The green does. 35r can't even spool by then.

At 7000rpm 35r does. Because at 30psi you'd be off the green compressor map, it's not possible. Because a compressor rotates so fast it moves the air close to sonic speeds. At this point no airflow increase is possible. Plus heat keeps going up.

 

It's both temperature and VE. Anything that increase VE will produce more mass airflow at the same psi. Things like cams, ported parts, and larger hotsides all increase VE.