This is so right, its too bad the only thing 80% of people talk about is peak numbers.

Ive raced so many cars with bigger turbos that could pull me even though they had a peak power higher than mine.

 

First of all, a good post by l2r99gst.

The one thing we seem to be getting muddled up is the "flow at a pressure" thing.

The source for this is that a bigger/more powerful fan (or compressor or whatever) is usually rated at a higher flow at a certain pressure drop. For example a fan will be said to flow 3,000CFM@10psi and another would be, say 5,000CFM@10psi. As a result, one fan moves more air than the other at the 'same pressure'.

While this is true, it's not saying a lot. Each fan or compressor has a set of operating curves (flow vs. pressure drop) and equally important, the system into which it supplies also has a curve (of flow vs. pressure drop) of itsown.

Because of this, while the larger fan will flow more air, the larger flow will also cause a larger pressure drop in the pipe/manifold and therefore if you just simply connect the fans to identicle pipes the flow and the manifold pressure will be different. If you somehow adjust the system such that the pressure near the outlet is the same, the output flow will be the same (providing the smaller fan can provide this flowrate).

I think we've basically concluded here that efficiency is key. More particularly, efficiency at a particular operating point (or range of points). It would be rather silly to have a huge turbo and run the same boost as stock and it would be equally silly to run the stock turbo at crazy psi. It's possible but it won't be as efficient. While you get the same volume of air into the engine, you won't be making as much power. The compressor is driven by the exhaust turbine and to move more air against a pressure takes work. The key is to do as much work from what you lose from the exhaust side and let thermodynamics take care of the rest to give you a net gain.

 

Turbos are all about CFM, the more CFM a turbo can push the more power it will make.

A gt35r will have alot more cfm then a 16g.

Now comparing pressure (psi), the more psi the more heat you are creating.

Now here is where larger turbos differ from smaller turbos.

Smaller turbos will spool up much quicker, and give you a much boarder powerband, but the smaller turbos wil have boost taper, and wont make the numbers like the larger ones will.

The larger turbos move the powerband higher.. You will need to rev higher with a bigger turbo to ulitlize it at its full potential.

 

l2r99gst,


Good post, I just particulary don't concur on the rpms write up or description that you made for the following reason..

The rpms affect and also don't affect the overall flow. I can drive all the way to the red line and never boost a single psi; how?, by pressing the gas pedal to partial throtle and slowly accelerating. While is is not practical to drive that way, it completely challenges any theory about rpm affecting boost.

That proves you right there that the RPM does not neccesarily affect the performance of the turbo. Best way to describe it is that when you are crusing on the highway in 5th gear at 80 mph you are running on pure engine torque but turbo is not even generating boost.

Secondarily, if you were to hit the pedal to the floor, the rpms don't neccesarily mean you will get more boost in direct correlation to the rpms. Each turbo has its own map of efficiency and each engine generates its own efficient torque curve.

Carlos

 

Jordo,

Good info.. the only arguable comment to make is that the spool up is typically directly related to the size but it is not always the case.

We can generally say that a smaller turbo will spool up faster because of the "lesser turbine/compressor wheel inertia" but in reality a bigger turbo can outflow and can spool up faster than a smaller turbo given other factors such:

bigger displacemnet pushing the turbo
better quality bearing (bearing turbo vs non bearing)
turbine/compressor wheels shape
turbine/compressor wheels material
turbine/compressor wheels design (variable pitch/retractable blades)
designs of headers
intake piping


My stock evo 8 turbo dramatically outspools my ex- WRX stock turbo. I can generated 25psi before 3000rpm in the top gears. This are my own observations for many years from having own and driven 13 turbo cars.

my .2c

Carlos

 

I think you need to re-read my post.

I said that RPMs is one of the criteria for airflow through an engine. I never said anything that RPM was related to boost.

Remember this equations from my post?
Aiflow = (Cid*RPM*VE)/(1728*2)



Eric

 

A turbo's CFM rating has nothing to do with how much CFM will flow through your 2 liter engine.

I think you first need to understand airflow through an engine, then you have to understand turbo efficiency. It seems like you don't understand the first part.


Eric

 

Just thought that it's important to point out that cfm is a measure of vol. flow rate.
Technically, what really makes power is the mass flow rate of air that goes in the engine.
Roughly, [boost]x[cfm]/[charge temp] = proportional to [mass flow rate] so if you have greater flow, same boost but higher temp, you're probably not getting a lot more power.

 

man this post got crazy. first of all, no turbo is goin to make 22 psi throughout the entire rev band. its gonna take time (rpms) to spool it. it could be 3500 rpm or 4300 rpm depending on size (several diff measurements). the volume of the air from the bigger turbo is going to be more, requiring more fuel and making more power. the difference will be in PEAK HP. now the power band overall is gonna be way diff. if your car can only handle 22 psi from the big turbo, you're gonna be slower than the smaller turbo'd car in everyway EXCEPT a top end run.

 

No, it will not. The volume of airflow is dictated by your engine displacement, RPM, pressure ratio and volumetric efficiency. The fact that a bigger turbo can flow a higher CFM at a particular boost DOES NOT MATTER.

As mentioned in my first post, you may end up with a higher mass air flow because of VE and temperature differences, but that's it.

I give up. No one seems to be reading.


Eric

 

I2r99gst,

I think people don't understand VE.

Could it be said, since the psi is the same, & the displacement is the same (same motor), the only available mechanical advantage of a bigger turbo (lets forget about the placebo effect) is a more efficient exhaust through the turbo and a cooler intake temp (which in effect means more cylinder pressure).

 

Take a look at post number 15 in this thread:

http://www.dsmtuners.com/forums/show...b+dyno+numbers


It brings together many of the points here and dispels a few of the them. Be sure to consider a turbo as part of a complete system and not an independent pump. Also, there are several turbo size calculators on the web; if you tinker with the variables, it helps to understand the process.

 

l2rsgst99,(Eric) You wrote on post #55:

"No, it will not. The volume of airflow is dictated by your engine displacement"
--------------------------------------------------------------------------------------------------------


^^^^No, displacement not always mandate the flow of the turbo, this is a good example:


A Evo 9 4G63 outflows by design and mechanically internals a 2.0 liter saab 900 turbo engine 8 valves from 1986. Both cars are 2.0 liters right? both cars have the same displacement right?

At the end the CFM output is directly mandated by a trillion aspects from the turbo and also by the carasteristics of the engine pushing the air.

Again, displacement is just a meassurent of bore, stroke and combustion chamber, that is it!! The flow is mandated by many other aspects like :


how fast the engine will rev
size of valves
shape of cylinder heads
etc, etc and off course the trillion other aspects of the turbo I explained in previous pages.

Eric, don't give up.your opinion is highly appreciate and valued, you just will find people that will challenge your theory but that is all. I respect your opinion and I will respectfully listen to your answers.





Carlos

 

Carlos,

If you are going to quote me, use the whole quote, please.

If you go back to the original post, we are talking about the difference between a smaller turbo and a bigger turbo on the SAME ENGINE.

Once again, though, for any engine, volume airflow through that engine is determined by:

displacement, RPM, VE, and pressure ratio

For the mass airflow, now temperature also comes into play. That's when you need to know the efficiency of the compressor, intercooler, etc.

I think you need to go back to the original question that posted in post #1. All of my comments are answering that question. We are talking about different turbos on our engine with no other modifications. We are not talking about different engines, we are not talking about howo much a turbo can flow by itself as a separate entity. We are talking about what dictates flow through the same exact engine and whether or not changing to a different turbo will change that, keeping the same RPM, same boost, same displacement.

As I said before, in that situation, there are only two factors involved:

1. VE
2. temperature, due to different compressor efficiencies

If the bigger turbo is not more efficient than the smaller turbo at the test psi that we are talking about, it will not flow more air or produce more power. The only contributor to more power then will be the slightly increased VE.

If the bigger turbo has a mich higher efficiency at the test psi, then it will produce more power.

The point is, that on our cars, going to a bigger turbo will only give you more power if you are running a psi that is inefficient for the smaller turbo. Of course you can run much higher psi and make much more power on the bigger turbo, but that's not the question posed in this thread. This thread is about the same psi on two different turbos in the same 2.0L engine. In that case, you need to look at the compressor maps to see which will make more power for our engine.

For example, you can make more power with a smaller turbo, like a 50 trim at 15-20 psi, then you could with a huge turbo like a GT42r (or larger) at the same psi and RPM, depending on the efficienies of the turbos at that psi and given RPM. When you understand airflow through an engine and compressor efficiencies, you will understand why. I tried to explain why, but no one wants to read. I even gave compressor calculator links, but no one wants to use it. Speedlimit even linked to technical articles, but no one cares.

How many times can sometime try to teach people?


Eric

 

Yes, exactly. But, the temperature will only be lower if the efficiency at the RPM and boost level that we are testing is better for the bigger turbo. It's definitely possible that the bigger turbo will even produce less power. It all depends on the variables that we are testing and the compressor efficiencies.