Got this from a Ranger forum of all places:

With pics:

http://www.rangerpowersports.com/tech/?p=132#more-132

Text:

One of the most confusing items facing most people getting into the turbo performance game is how to read a compressor map. The average person will look at it, see all the numbers, swirls, that strange “surge limit” thing and respond with, “My head hurts!”.

Well, no more of that for the turbo guys! Being able to read and understand a compressor map is one of the basics that all turbo guys and gals should be able to do. It’s really not too difficult once you get a basic understanding of the equations involved to get there and what is actually going on with all those swirls.

Let’s begin with the numbers on the X and Y axes. On the Y axis you’ll see it labeled “Pressure Ratio”, in effect, that is another way of saying boost. All the pressure ratio is, is boost pressure plus atmospheric pressure, divided by atmospheric pressure, or (boost psi + 14.7)/14.7. If you’ve ever seen a boost gauge measuring “Bar”, that’s what it’s measuring is pressure ratio, or the metric equivalent to pressure. Here’s an example:

Suppose you have a system boosting 18psi and you want to know the pressure ratio (p/r):

(18psi + 14.7psi)/14.7psi = p/r

32.7psi/14.7psi = p/r

2.22 = p/r

There you go, one part of the compressor map mystery figured out. Now on to the X axis, which is airflow in pounds of air per minute. This is where the math gets a little trickier as we need to take into account the displacement, rpm, volumetric efficiency, and boost to figure out the volume of air needed as well as the temperature and density to figure out the weight of that volume of air drawn in per minute. Also, as you can see, with rpm being one of the variables, you’ll need to do this portion multiple times for different engine speeds. Another factor that comes into play is whether or not the system being planned around is equipped with an intercooler what the pressure drop is across it and whether or not the boost is turned up to compensate for that pressure drop. Since it’s becoming more commonplace and standard practice (and just a plain ol’ good idea) we’ll do the math assuming one is being used. If you don’t have or don’t plan on using an intercooler, simply don’t figure that in in the first equation. Okay, let’s begin the example…

The first step is determine the absolute pressure at the compressor outlet (Pco). Using the 18psi example from above and a theoretical intercooler creating a 2psi pressure drop, the equation will look like this:

Pco = 18psi +2psi + 14.7psi (14.7psi is atmospheric pressure)

Pco = 34.7psi

Now the fun begins, our next step is to determine the air density after it has gone through our theoretical intercooler (Di). To do this we need to approximate what a good temp for air exiting the intercooler should be. Let’s shoot for 130°F. Also, we will need to convert to Rankin from Fahrenheit as well as include the R constant from the ideal gas law (PV=nRT) which is 53.3. This is what the equation we need to work with looks like:

Di = (Boost + Atmospheric Pressure) / (R*12*(460 + Post Intercooler Temp))

Now, with the numbers plugged in we get:

Di = (18psi + 14.7psi) / (53.3*12*(460+130°F))

Di = (32.7psi) / (53.3*12*(590R))

Di = (32.7psi) / (377,364)

Di = 8.665 x 10-5 pounds per cubic inch

That wasn’t too bad was it? In the next step we need to decide which rpm we want to use to calculate the Mass Flow rate of the system (Mf). Let’s use 5500rpm which is right near the top of the rev range for most 2.3Ts. We also need to know or estimate the volumetric efficiency of the head, we’re going to estimate it’s a mildly ported head with a VE of about 0.85. Here is the equation we need to use:

Mf = (Di * Displacement in cu. in. * rpm) / (2 * VE)

Mf = (8.665 x 10-5 * 140cu in * 5500rpm) / (2 * 0.85)

Mf = (66.723) / (1.7)

Mf = 39.249 lb/min

Here’s where it gets fun. Compressor flow maps use corrected mass flow not the mass flow we just calculated, so we need to convert. Once again we need to use temperature in Rankin, Garrett uses a compressor inlet temperature of 85°F, which is 545°R, if you find that you have a drastically different temperature than that, this is where to make that correction, considering the heat in the engine compartment and that most people will try to place their filter in the cooler part of the engine compartment, 85°F looks like a pretty good number, so we’ll stick with that. Once again, here’s our equation:

CMf = (Mf * (square root of (compressor inlet temp in R / 545R))) / (atmospheric pressure / compressor inlet pressure)

CMf = (39.249 * (sqrt (545R/545R))) / (14.7psi / 13.95psi (due to pressure drop of air filter))

CMf = (39.249 * (1)) / (1.054)

CMf = 37.24 lb/min

There you have it, now you know how to use the X and Y axes of the compressor map, now all that’s left is how to interpret everything else.

For all the mumbo jumbo plotted on the flow map, let’s start with the surge limit. The surge limit is where the compressor has produced all the airflow it can for a certain pressure ratio and is now pushing against that pressure. Basically, the compressor is being pushed back against and is beginning to stall. I think it’s pretty self explanatory that we want to stay away from the surge limit. Next up, all those swirly lines. Those lines are reference points for the compressor’s efficiency. Compressor efficiency is a measure of how well a compressor can pump air without heating it more than the ideal gas law says it should. Obviously, you want as high of an efficiency as possible, the less the compressor heats the air the less likely you are to experience detonation, the air charge will be more dense, and better job your intercooler will be able to do (lower temps in equals lower temps out).

Now, knowing all that, we can go ahead and plot some points on a compressor map. Obviously, you’ll want to have points from across the rev range, let’s choose 2000rpm, 3300rpm, 4500rpm, and 5500rpm with the same 2.22 pressure ratio (18psi). That gives us CMfs of 13.54#/min, 22.35#/min, 30.47#/min, and 37.24#/min, respectively.

Here you can see the compressor map for the Garrett T3 60 trim compressor that came stock on several of the Ford 2.3Ts. With our boost set to 18psi you can see that it will come on early and strong but run out of air very quickly after 4500 rpm. This compressor would be a decent choice for a primarily street driven (stop and go) car at lower, stock boost levels (10-15psi) but as our hunger for horsepower grows it quickly becomes a poor choice.

Here is the compressor map for a T04E 50 trim compressor. As you can see, it won’t build nearly as much boost as quickly early on, but it more than makes up for that past 3000rpm. Once you get into the meat of the power band, around 4500rpm, you are running at nearly peak efficiency for this compressor. Also, once you get past that, into the higher revs, this compressor should be able to supply a good amount of boost all the way up to 6000rpm. This compressor, in a T3/T04E hybrid configuration would make an excellent turbo for a more performance oriented 2.3T.