Psi vs Cfm Options

[KOU In3]

Ok, I've always been led to believe that CFM was more important a measure than psi and that the larger turbos flow grossly more air at a given psi than their smaller brethren. I understand the basic part that the hot air produced when the turbo is overworked leads to a higher psi reading from the same quantity of actual air.

Here's where I start to get confused though: if we keep the intake charge's temperature constant through effecient intercooling shouldn't these values be the same? Basicly if you pressurize the manifold to 20psi using 120 degree air from a 14b or the same temp and psi from say a mammoth BR580 shouldn't this be using the same cfm of air?

Right when I think it all makes sense I talk myself back into the classic fallacy of 'dude it's 20 psi!'. Is the Someone straighten me out here please.

[natedogg]

PV=NRT

temperature (T) and pressure (P) are dynamic variables in our system, but volume (V) is a constant unless we change it so a better name for it might be static variable. R is a constant correction factor and N is a variable based on the amount or mass of gas molecules we are moving.

N=(PV)/(RT) I write the equation in terms of N because that is really what we are concerned with...the mass of air. Its obvious by looking at the equation this way that we want high pressure and volume, but low temperature. Pressure is dependent upon temperature, but temperature is not dependent upon pressure. Imagine an air compressor. The air inside an air compressor charged to 90 psi isn't any warmer than the ambient air outside the compressor, but if you start heating the air inside the compressor its pressure will increase. Apply this to a turbo...increasing the pressure is not increasing the temperature of the air. But the higher pressure fighting the compressor blades causes a lot more friction and this is where your heat and inefficiency comes from. Now this increasing temperature is, like Travis said increasing the pressure within the system and the converging feed back of the increasing temps and pressures leads to very hot air. So with pressure and temperature fighting our efficiency we need to find a different way to increase the mass of air going to our engine. Lets increase the volume. This allows more air at the same efficient pressure and temp. Of course this is done with larger IC and piping. This explains the ideal gas law in our application.

Now where does the turbo come in at? Well, the best way I can think of to explain this is with two fans. A big one and a small one. The small fan has to spin twice as fast to flow as much air as the large one at the same pressure. Ala a small turbo has to spin twice as fast to flow as much air as a larger turbo at the same pressure. Since the small turbo as to spin twice as fast it is heating the air much faster than the larger turbo and therefore becoming inefficient much sooner. The larger turbo is capable of flowing more air at the same pressure because it has larger fins and can just plain move more air with each revolution of the fins.

That's how I understand it anyway. Hope this helps.