Turbo Size.. a psi is a psi?

[xioca]

Alright so I got in to a discussion with a guy at school and he's going on about how a psi is a psi no matter what size the turbo was. I've read around and thought it depended on cfm, because 15 psi on a t25 isn't the same as 15 psi on a 16g, right? If so what would be the best way to explain it ?

 

[v2ner]

You are partially correct. 15psi on a T-25 is different than on a 16G. The only difference is that at 15psi, the 16G will put out more cfm than the T-25.

 

[drivemusicnow]

This is a complicated thought process to understand, and I know that I am not very good at explaining it.

Here are the three parameters to keep in mind:

Air density of charge: A larger turbo will flow cooler air at a specific CFM value than a smaller turbo will. For example, 20 psi on a T-25 will blow HOT air, while 20PSI on a 60-1 is going to be average intake temp.

CFM: This is a measure of how much FLOW there is. Pressure (PSI) is technically resistance to flow. If you are building up pressure, it means that the air is NOT able to go into the cylinders.

PSI: Given the same length and volume of intake tract, 30 psi (let's pretend for a moment that the system is static) is the same physical volume of air no matter what turbo you are running, however different temperatures will mean that air has a different DENSITY. The density of the air directly effects the MASS of air (ie not volume) that enters the cylinder.

What this essentially is getting at, is that the important factor in determining which setup will make more power is by determining which flows more MASS of air into the cylinder. Volume, and pressure are not directly related to performance, as they do not take into account the density/temperature of the air.

 

[xioca]

Alright, that makes a little more sense. Thanks for the help in explaining it to me.