[bdkevoIX]

The debate regarding this topic has come up over and over again in various forums, but I have yet to see a satisfactory explanation other than "big turbo flows more air at given psi". So I am going to attempt to explain this phenomenon while trying to include real world variables such as turbine backpressure, CFM, compressor efficiency, etc.

Now I am not an engineer with a degree in thermodynamics or mechanical theory, so while I believe my rationale is close to the truth, I don't believe it to be the absolute truth. So feel free to correct me or add as you wish.

For simplicity sake, let's say we have an n55 engine, GT35r and n55 stock single turbo in this comparison. As everyone knows, turbo has two main components: compressor and turbine. Let's break it down into components and evaluate it separately.

Compressor:
The GT35r compressor is larger than n55 stock turbo's. This means GT35r's compressor has greater surface area. Let's assume that two compressors are rotating at identical speed, let's say 25,000 rotations per minute. The compressor with larger surface area will obviously move higher volume of air than the smaller compressor. Also, while doing this, the work done by the larger compressor is translated into heat in lesser degree than the smaller compressor, thus giving it higher efficiency.

CFM: (cubic feet per minute)
It's a non-SI unit that measures the volume of gas that passes a given point in one minute. We have established that GT35r's compressor moves higher volume of air. We also know that this air has to pass through the intake manifold that's fixed in volume. Since the higher volume of air has to pass through the fixed space (intake manifold) in one minute, the velocity of the air is higher compared to air generated by the stock turbo which has lower CFM, thus lower velocity.

PSI: (pounds of pressure per square inch)
Pressure is measured at intake manifold. This pressure is generated by air molecules moving rapidly and hitting the inner surface of the intake manifold.
This kinetic energy of the air molecules are proportional to the temperature.
In other words, higher temperature will allow air molecules move at faster speed and create higher pressure.
Real world example of this is a can of butane gas. When you heat this up what happens? The pressure inside the can increases due to higher kinetic energy and the can blows!! But if you were to blow the can just by shoving in more butane at room temperature, you will have to put in ALOT more butane to do that.
We have established that stock turbo compressor blows hotter air compared to GT35r compressor. The hotter air has higher kinetic energy. So it's relatively easier for stock turbo compressor to generate 10psi of pressure because they are moving around so fast bouncing off of the surface of the intake manifold. The GT35r compressor, on the other hand, blows cooler air. So it requires more air to generate the same 10psi of pressure.
Since we know that density = # of moles of substance/volume, we know that the air from the GT35r compressor has higher density.

Turbine back pressure:
The turbine wheel is in the right smack in the middle of the exhaust flow, creating resistance. When the piston engines completes it's combustion cycle, it has to let out exhaust gases. Since the smaller turbine creates more resistance or back pressure, the motor cannot effectively expel all of the exhaust gases it produced. Large turbine wheel of GT35r reduces the resistance (back pressure), enabling more exhaust gases to be expelled at the end of the combustion cycle.

So let's put it all together.
What happens when the intake port opens? :
So let's assume that we have an n55 engine that's operating at 4000rpm.
The intake port of the n55 engine with stock turbo and n55 engine with GT35r are opened for same period of time. Intake manifold pressure for both reads 10psi. Few things happen in the car with a GT35r:
1. The air from GT35r moves at higher velocity into the cylinder due to higher CFM, so more air can go in before the valve closes.
2. Not only more air has entered the cylinder, the air from GT35r is denser because of higher efficiency of the compressor.
3. There is less exhaust gas remaining in the cylinder due to reduced exhaust back pressure. This allows more fresh air to enter the cylinder.

So, during the short period the intake valve is opened, air from the GT35r goes into the cylinder FASTER, and MORE air (oxygen) is available for combustion because the air is denser and there is more room for refresh air.

This is why a larger turbo makes more hp at given psi than a smaller turbo.
Now, go and upgrade..

[GTR-Dad]

I applaud your effort to answer a question that seems to be asked universally in the tuner community.

There are a few holes in the logic (Large and small compressors don't rotate at the same speed. Pressure in the intake manifold is controlled mainly by volumetric flow rate and resistance to flow through the head. Relationship between volumetric flow rate and mass flow rate is strongly affected by temperature.)

The trouble with trying to write a desciption of a relatively complex process is that if it's short enough for the average web user to read, you can't adequately describe the process, and if it's complete - well, no one will read it.

Here's a link to an excellent description turbocharger optimization for a given appilation. http://www.turbobygarrett.com/turbob...imization.html
And surprise, surprise... it's not just to strap a bigger turbo to whatever engine you happen to be running.

Cheers,
Dan

[OpenFlash]

In very basic terms, a "bigger" turbo will move more airflow with less heat generated during the compression stage. This is due to the compressor wheel being sized to move the appropriate amount of air within the "happy" compressor RPM range.

And, more importantly, once this air is combusted and turned into exhaust, a turbo with a "bigger" turbine wheel will spin at an appropriate RPM and will induce less power-robbing exhaust backpressure. The less backpressure on the exhaust side, the more air that can be pumped through the engine at any given boost pressure.

So you add up the effects of a cooler air charge with less back pressure and you can see why different turbos can make different power at different power levels.

It also explains why comparing turbos by comparing only compressor maps is meaningless. The turbine size/configuration is arguably more important.

Notice I put "bigger" in quotes because bigger isn't always better when talking about turbo tech/aero.

Shiv

[fruition3000]

I always wondered if the N55 has the same potential as the N54. I got the N54 partly because I think that, there would be more turbo lag introduced with the N55 once tuned.

[Alexander]

GT35r or bust

I had one on my VQ35DE and damn was it beautiful.

[bdkevoIX]

Originally Posted by GTR-Dad

I applaud your effort to answer a question that seems to be asked universally in the tuner community.

There are a few holes in the logic (Large and small compressors don't rotate at the same speed. Pressure in the intake manifold is controlled mainly by volumetric flow rate and resistance to flow through the head. Relationship between volumetric flow rate and mass flow rate is strongly affected by temperature.)

The trouble with trying to write a desciption of a relatively complex process is that if it's short enough for the average web user to read, you can't adequately describe the process, and if it's complete - well, no one will read it.

Here's a link to an excellent description turbocharger optimization for a given appilation. http://www.turbobygarrett.com/turbob...imization.html
And surprise, surprise... it's not just to strap a bigger turbo to whatever engine you happen to be running.

Cheers,
Dan

Precisely the reason why I said "let's assume" for simplicity sake.

[GTR-Dad]

Originally Posted by bdkevoIX

Precisely the reason why I said "let's assume" for simplicity sake.

Thumbs!

Originally Posted by shiv@vishnu

In very basic terms, a "bigger" turbo will move more airflow with less heat generated during the compression stage. This is due to the compressor wheel being sized to move the appropriate amount of air within the "happy" compressor RPM range.

And, more importantly, once this air is combusted and turned into exhaust, a turbo with a "bigger" turbine wheel will spin at an appropriate RPM and will induce less power-robbing exhaust backpressure. The less backpressure on the exhaust side, the more air that can be pumped through the engine at any given boost pressure.

So you add up the effects of a cooler air charge with less back pressure and you can see why different turbos can make different power at different power levels.

It also explains why comparing turbos by comparing only compressor maps is meaningless. The turbine size/configuration is arguably more important.

Notice I put "bigger" in quotes because bigger isn't always better when talking about turbo tech/aero.

Shiv

Wow. Short, accurate, and understandable!

[Express]

Originally Posted by Aonarch

GT35r or bust

I had one on my VQ35DE and damn was it beautiful.

i come from the boosted G family too!! i had a vortech with a 2.87 and at 11 psi it rocked my socks.