I felt there needed to be a tech article on this subject to help people understand turbo compressor maps and figure out what type of turbo they will need for there perfomance needs. At first glance a compressor map is quite confusing, but they offer alot of information and hopefully after reading this you should be able to get a little bit better understanding of them.
Here we have map for a T04E-50 garret turbo, this is just for learning sake and not the turbo I am recommending.
The numbers you see on the left, listed vertically from 1.00 to 3.60 is the pressure ratio. Let's say you are boosting at 7psi. This is roughly half of the sea level atmospheric pressure (14.7psi), so your pressure ratio is 1.5. if you're boosting 15psi(rounded up for simplicitys sake), your pressure ratio would be 2. The formula for figuring pressure ratio is: pressure ratio=(14.7+boost pressure)/14.7
The numbers you see on the bottom that start at 0 and end at 50 is the flow rate of the turbo measured in lbs/min(some are in cfm and some are in cms). This is the amount of airflow that you are wanting to push through your turbo.
Now that that is done look at the map again, the next thing to look at are the lines that make it look like a thumb print. Those line are effciency lines. The outer one being 65% effciency, and the inner one being 78% effcient(the effciency % differ from map to map, but should be marked on the lines). The area inside the 78% ring is the most effcient area or "island" of the turbos range. Getting your turbo to spend most of it's time in there will give you the most power at the lowest charge temperture.
Let's say you were boosting at a pressure ratio of 2.20 and were moving 30lbs/min worth of air. Follow those two lines together and you will find yourself almost right in the middle of the most effcient island. This is the ideal area to run your turbo. But let's say you want to boost at a pressure ratio of 2.6 and were moving the same 30 lbs/min worth of air, that would put you right between 78% and 76% effciency which is still good, but once you drop below 70% you might want to reconsider your turbo selection. You set your boost at 2.0 pressure ratio, but want to move 45lbs/min of air, this puts your point outside the turbos effciency range, this means that your turbo is to small, you could boost more to get the effciency up but notice how much more you would have to boost to increase the effciency.
This brings us to another bit of infomation; turbine speed. The horizontal lines that slope down towards the right of the map. That is the turbine speed measured in x1000rpm. Notice how at higher pressure ratio the faster the turbine spins and the lower the effciency and how suddenly the effciency drops compared to lower pressure ratios, going outside the top line on the turbine speed is were you decide to go with a bigger turbo(to much speed and you will just be making the turbo work harder and not as effcient). Now to the far left is the surge line, this is the point were the turbo starts to make boost at a given pressure ratio and turbine speed, you do not want a point on the left side of this line doing so will cause your turbo to surge at WOT, and when I mean surge I mean the turbo starts to spool but doesn't have enough turbine speed to build more boost and the car will start bucking madly and your boost gauge will start bouncing, this is murder on a turbo. It is imperative to size the turbo correctly so that this does not happen. In closing you have to figure out how much power you will be making at what boost levels and plot those points on the chosen compressor map to see where they fall in the effciency range of the turbo selected. I hope this is of some help, PM me with any questions that I haven't answered here or that you might have. Happy boosting.