Trying to learn how to read compressor maps

[Mitchocalypse]

I'm just trying to learn how to read compressor maps and I sort of get it.. minus one concept. Lets just take a theoretical car, with a goal of making 500rwhp. Now I know that there is a lot of anti H-C people on this forum but we'll just use his assumption (or fact?) that maximum rwhp = airflow (lb/min) * 10 / 1.3. (I think i remember seeing this.) So if we want 500 rwhp, multiply 500 by 1.3, then divide by ten. This gives about 65 lbs/min of air. Now going to the compressor map (pic)



follow the dotted red line up to pressure ratio of just above 2.5 (at atmospheric pressure of 14.7 psi, a 2.5 pressure ratio = 22-ish psi of boost .... as far as I understand). If I'm wrong about this, just correct me and read on. But if thats correct, that means ideally, I would want to be running full boost at right around that point. A good tuner will make it so i stay constant at 22 psi, but my question is.. what determines the amount of airflow at that psi (lbs/min)? I mean.. the map shows it can boost at 22 psi, but flow different amounts of air.

My assumtion would be due to heat changes. im pretty sure the bolder blue lines represent the angular frequency of the impeller so my first quess would be that heat is causing the air to be less dense. It flows the same volume of air but it is hotter and therefore less mass in the same amount of volume. But I guess what i really want to know is, how do get the compressor to operate at the exact point I want it to? Obviously the pressure ratio can be controlled, the angular frequency as well to a certain extent, but how do you control the temperature (if my assumptions are correct) at the outlet of the compressor. What I seem to be reading is a 3 dimensional plot of which only 2 dimensions can be controlled and the third just decides itself but there must be someway to pinpoint where you want to be on the compressor map..

(Sorry if this was unclear, I was just explaining my thought process as I went along)

[Slides]

You "control" the pressure/shaft speed via wastegate control, where you actually end up depends on the total intake restriction presented by your engine at whatever rpm it is at.

The efficiency lines dictate the % of theoretical compression efficiency you achieve, the rest being delivered to the air in heat.

You cannot just "pick" a point to operate at.

[Mitchocalypse]

Originally Posted by Slides

You "control" the pressure/shaft speed via wastegate control, where you actually end up depends on the total intake restriction presented by your engine at whatever rpm it is at.

The efficiency lines dictate the % of theoretical compression efficiency you achieve, the rest being delivered to the air in heat.

You cannot just "pick" a point to operate at.

Ah okay what you said makes perfect sense.

Is there a chance that there is a nice table of volumetric flow rates of a 13b-rew at different rpms on the internet, or should I just try to calculate a table like that myself?

The other thing that is a little iffy is how someone can say 65lbs/min is required to make a certain hp. It depends on temperature as much as mass flow rate. Guess I need to look into it a little more.

Edit.. crap, no it doesn't!! i had my mind set that lbs/min was a volumetric flow rate. My bad. and its only been 2 and a half months since my last fluid dynamics course =P