For instance, here's a GT3071. I've plotted it for a healthy 3.0L that puts out 250HP at the crank at redline with no boost...
The first thing that you'll notice is that not all of the lines are on the "island." That is bad. In this particular case, it means that the turbo is a little too small.
The different colored lines are RPM of the engine. Going from left to right they are 2k, 3k, 4k, 5, 6, 6.5k, and 7k. On the left had side of the graph, is PR or pressure ratio. That is a multiple of the incoming air. If it is a single turbo, it will just be sucking in air through a filter at atmospheric pressure or about 14.7psi. On a vacuum or boost gauge, this is 0psi or no boost. Now if you follow the PR line up to 2.0, that is twice atmospheric pressure. On a gauge, it will be 14.7psi. With a PR of 2.0 your engine will be taking in twice as much air as it would without a turbo. In a perfect world, that also means that your engine would be making double the power.
The plot above therefore shows 500HP or 50lb/min at 7000rpm with a PR of 2.0. If you continued the 7k line down to a PR of 1.0, it would be 25lb/min or 250HP. The other RPM ranges are best guesses made looking at stock engine dyno graphs.
The compressor map looks like an island. The closer that you get to the center blob on the island, the more efficient that the turbo is. In our case it would be a little over 4,000rpm. The will also be where the most torque is produced. As you get towards the outer edge of the compressor map, the turbo is less efficient and just heats the air. That is bad. Hot air equals detonation. That's why this turbo is too small. Over 6500rpm, we have fallen off the map.
I should also mention that the horizontal line on the graph is flow in lb/min. That is pounds of air per minute. A good estimate is that 10lb/min equals 100HP. So 50lb/min equals 500HP. That is the quick and dirty way to see of a compressor is sized properly to your application. If you know that you are shooting for 500HP, and the island ends at 40lb/min, your turbo is too small. Now if you are shooting for 500HP and your island ends at 65lb/min, your turbo is probably too big.
If your turbo is too big, your lines will be plotted to the left of the "island." That is also bad. The left edge of the "island" is called the surge line. This is where the turbo is barely starting to compress and is VERY unstable. The air surges but doesn't come off of the compressor wheel in a fluid motion.
Now here's a turbo that'd be great for our 3.0L...
It would hit full boost between 2 and 3k rpms, the torque peak would be between 6 and 6500 rpms and you've got a little room to grow.
$1250 for dual ball bearings and it's all yours...
http://www.atpturbo.com/mm5/merchant.mvc?S...tegory_Code=GTB
This is a T3/T4 turbo. It has a smaller T3 turbine housing but the bigger T4 compressor. Perfect for a remote mount setup.
You can also see all of your turbine AR ratios in the above link. For a remote mount, a .63AR would be a good start. For more traction in the lower rpms, a 0.83AR would be a better choice because it would build boost slower.
You can also see all of the different turbine housing types. Personally, I'd go with an internal wastegate just to make life easier. For higher performance, an external wastegate is recommended.
I hope that some of that makes sense.
BP