Why 10psi from a small turbo DOES NOT equal 10psi from a large turbo

We see the question, "How much boost can I run on this or that motor?" all the time.

BeyondloadedSE even recently added a section to the FAQ sticky regarding that 10psi from say a t-28 is drastically different from 10psi out of a gt-47.

This obviously comes down to the efficiency of the turbo. But what does that mean to someone new to the turbo world.

What makes some more efficient at different boost levels? I hope this discussion goes past referancing compressor maps and gets into a little physics.

What would cause IAT to be higher at a lower boost level? Or how is IAT constant at increased boost levels (up to a certain point)?

My increased curiosity was spawned when Nautilus' non-intercooled turbo had lower or equal IAT at 5psi than at 3-4psi.

I hope this thread can gather some quality information and if it does I would pear it down to include the most pertenant and have it made into another sticky or added to the FAQ sticky. I have some knowledge of the hows and whys posed above but want to try to gather more and better knowledge than I have in one place for the benefit of all.

Just to clarify one thing about the Nautilus test, I thought the IAT comparison was done between an intercooled and non-intercooled system at the same boost with the intake temp increasing by about 50 degrees for the non-intercooled.
Does this change your question at all or did I misunderstand what you are asking?

The test was done with no intercooler at low boost and no intercooler at slightly higher boost.

That test just showed that there is a certain amount of minimum heat generated just by the turbo itself as well as the movement of the compressor wheel. Then raising the boost would normally increase the heat due to additional compression of the air, however the turbine is now more efficient and there is less friction in the air. I imagine part of it is that the airspeed increases making it spend less time in the compressor as well.

Remember that it will have to fall back on a compressor map to help explain. First off you should understand that a compressor map is measured in efficiency vs. pressure ratio vs. mass airflow vs. compressor RPM
That is 4 variables measured in a 2-D chart! The efficiency is determined by the temperature rise in the air at the outlet versus at the inlet. Lower outlet temps with the same pressure ratio, inlet temp, and mass of air would mean the turbo was more efficient. In this case more efficient with everything else 'fixed' means cooler outlet temps. Cooler outlet temps mean more dense air as well so generally speaking Temp and air mass go hand in hand.
The efficiency islands on the compressor map list approximate efficiency.
If everything else was equal then you would go with the turbo that has efficiency islands that are higher, say in the 70+% range in the center than one that is in the 60% range. To truly know the turbo is right you have to map it out and see where the majority of the airflow values fall on the compressor map at the boost pressures you expect to run.

As for 10psi not being the same from one turbo to another? Well that's kinda misleading though true. Given the same engine and same conditions then 10psi yields the same amount of air no matter what turbo is pushing it. The original statement is just an attempt to teach someone that some turbos may not be able to achieve that same pressure at the same conditions. Something would have to change such as rpm, pressure, etc.

Doesnt really change what I am asking. The way I read it they did 3.5 and 5 both intercooled and non-intercooled. The non-intercooled had same IATs at both both boost levels.

Anyway, it doesnt change what information from our most experience members I hope to get from this discussion.

warmonger said:

The test was done with no intercooler at low boost and no intercooler at slightly higher boost.

That test just showed that there is a certain amount of minimum heat generated just by the turbo itself as well as the movement of the compressor wheel. Then raising the boost would normally increase the heat due to additional compression of the air, however the turbine is now more efficient and there is less friction in the air. I imagine part of it is that the airspeed increases making it spend less time in the compressor as well.

Remember that it will have to fall back on a compressor map to help explain. First off you should understand that a compressor map is measured in efficiency vs. pressure ratio vs. mass airflow vs. compressor RPM
That is 4 variables measured in a 2-D chart! The efficiency is determined by the temperature rise in the air at the outlet versus at the inlet. Lower outlet temps with the same pressure ratio, inlet temp, and mass of air would mean the turbo was more efficient. In this case more efficient with everything else 'fixed' means cooler outlet temps. Cooler outlet temps mean more dense air as well so generally speaking Temp and air mass go hand in hand.
The efficiency islands on the compressor map list approximate efficiency.
If everything else was equal then you would go with the turbo that has efficiency islands that are higher, say in the 70+% range in the center than one that is in the 60% range. To truly know the turbo is right you have to map it out and see where the majority of the airflow values fall on the compressor map at the boost pressures you expect to run.

As for 10psi not being the same from one turbo to another? Well that's kinda misleading though true. Given the same engine and same conditions then 10psi yields the same amount of air no matter what turbo is pushing it. The original statement is just an attempt to teach someone that some turbos may not be able to achieve that same pressure at the same conditions. Something would have to change such as rpm, pressure, etc.

Click to expand...

owww great now my brain hurts...Interesting discussion, continue

<<needs to read better. My bad.

I find all this stuff really interesting. I am not sure about dropping the cash right now but it makes for interesting reading.

I believe that Garret states their GT (ball bearing) turbos are more efficient (though have smaller islands) than non-ball bearing turbos. I'm purely regurgitating, so can you please elaborate on that?

One of the major issues here is that you are using just pressure. 10 PSI on the same engine with 2 different turbos and a decent intercooler would probably make the same power unless the one turbo is just way out of its efficient area of operation. This brings into play: density ratio. When people are sizing turbos and figuring out airflow and if a turbo is a good match... You have to go beyond PR (pressure ratio) and work with dr's (density ratio). This is what really determines the mass of air that will be going into the engine.

Now once you've got the compressor side figured out, you still need to delve into the mysteries of the turbine side. This is because the turbine creates a pressure in the exhaust, greater the pressure the harder it will be for the intake pressure to fill tge cylinders. Turbine sides also have efficiency charts but are very hard to find.

OK, thats it for now

Not sure, but it sounds to me like SHOgofast was misunderstanding the difference of 10 psi on two different motors rather than two different turbos, for example 10psi on a 1.8l vs 10psi on a 3.0l. the difference being CFM, and the ability for the 1.8l to acheive 10psi more efficiently than the 3.0l with the same turbo. :shrug:

EDIT: assuming the turbo is not to big for the 1.8l...

I am not confused about 10 psi on different motors. I specifically meant to ask the difference of 10 psi being produced by different turbos.

As warmonger said it is about the turbos efficiency. I was hoping to spur a fairly high level discussion about what contributes to greater efficiecny in different turbos. Why and what makes certain sizes particularly good matches for our platform be it 2.5 or 3.0.

Maybe the function A/R has on what rpm the turbo achieves full boost at.

The stickys presently cover the where to buy, whats been done, things to consider...really well. I felt that the FI forum could benefit from the more technical side of turbocharging.

FYI, its not that I do not know the answer (or at least have a damn good idea) about many of the questions I posed. Its mostly that I definatly do not consider myself competant enough to lay down an explanation on these topics for others to learn from.

Don't forget that when you throw intercoolers into the mix 10 psi on different turbo's becomes a lot more alike. A good intercooler can make up for a lot of inefficiency on a turbo.

Most turbo's really have very small ranges of ideal operation. It's why people are always upgrading to the latest and greatest or going bigger so they can run 1-2psi more.

Really though no technical discussion about turbochargers can be made without referencing a compressor efficiency map at least. Like i stated previously the turbine side of the turbo also has a huge effect on the behavior of the turbocharger. Most of the times to sacrifice to make big power you have to run a larger turbine which allows you to reduce the backpressure in the exhaust manifold but also takes longer to get up to speed... and since turbine RPM is what drives compressor RPM, hence increasing flow and creating pressure inside the manifold... what the turbine does and how it does it is very important.

Now the problem with turbine sizing, like mentioned above, is that turbine charts are very hard to find, and thus most of the sizing is done by trial and error. However you can get a general idea of turbine sizing by looking at the A/R of the turbine and the exducer size.

Turbo characteristics

Turbo characteristics

If you are interested in figuring out why some turbos are better matches than others, then you have to look at the basics.
It all boils down to
available energy
mass
sizing
rpm

Different compressor blades are designed to scoop the incoming air and fling it out into the scroll with centrifugal force. Airflow capacity will be determined by the size of the scroll and the size of the wheel along with the shape of the blades. Small wheels can spool quickly but require higher rpm to push larger amounts of air. Large wheels scoop the air better but become inefficient at higher wheel speeds.
The turbine side provides the energy to turn the compressor. This is a combination of the heat available in the exhaust, volume of exhaust and velocity. A low volume of exhaust such as a small engine produces can be made to have a lot of energy by restricting it a lot at dramatically increasing the velocity. The A/R ratio pretty much tells you how much of the exhaust is directed right against the turbine blades. Low A/R ratio means smaller housing and more exhaust is shooting directly at the blades for best energy. Large housings let a majority of the exhaust flow by unrestricted and use only a portion of the exhaust to turn the wheel.
In the last case you would want that turbine used on an engine that pushes a large velocity of exhaust so it has the energy from sheer volume but with lower velocity and in the end can flow more exhaust with less restriction.
Larger compressor sections must have a lot of turbine energy to spin them or there is significant lag.

No matter what size of engine you pick you can make up for the displacement by increasing the exhaust flow and airflow. For a 2.5L 4 cylinder engine to flow the same air/exhaust as a 5.0L it must do double the rpm range...simple math. So if you have the turbine sized for a 5.0L engine at say 4000rpm optimal efficiency, that same turbine would work well for the 4 cylinder 2.5L at 8000rpm if all else were equal.
So then comes the compromises. You want boost a little faster so you drop the turbine size, but you want the same overall power output so keep the compressor size. Now you can flow the air and spool it up quicker, but you have sacrificed the peak flow numbers it could have made in order to do it.

Therefore sizing the turbine/compressor is a balance of available exhaust energy determined by flow volume (or mass exhaust flow), temperature and velocity.
The intake compressor relies on the exact same basic factors. You can begin to determine what flow's you need by starting with engine displacement and overall power output desired, then start adjusting sizes until you arrive at a combination that works for you.
This is where the compressor map comes in. There is a turbine map to help you with the exhaust flow as well. Remember that no matter what engine it is the same airflow (cfm) will net the same exhaust output. So comparing with another engine of different size but with the same equivalent output you are looking for is as simple as calculating displacement and rpm and comparing compressor maps.