Originally Posted by achTTung

Its all about the efficiency island.
You can make more boost even with the smallest of the K03 turbos, but it gets to a point, where you're making so much heat, it offsets the pressure.
Hot air is less dense air, so even though the pressure is higher, there isn't more air there.

Heres a random example of a map, you want to try to run your car so that your as close as you can to staying inside that island in the middle, this is where you're turbo is making the best use of the air flowing thru it.
Make sure you always stay away from that surge line, thats the point where you'll start doing damage to the turbo.
Surge is the situation when the compressor "spits out" more air than the engine can swallow, which causes a backup of air at the intake and it actually creates reverse-flowing pressure waves that can bedamaging to the turbo.
All that air pushing back thru the turbo can be equated to thowing a stick in the spokes of someones bicycle.
It stops/slows the spin very quickly, and thats a lot of stress.



So, with that said, the best way to go about choosing a turbo is with a little research.
What kind of power do you want the car to make, realistically, and to get that, what A/R ratio, and boost pressure you're gonna need to do that, so that you can keep your efficiency up.
Now, if your only choosing between a selection of turbos that were designed for similar 1.8T engines, just about all that work has been done for you,
these are turbos hand picked for the engine, so pretty much most of what i just said can be ignored, but its still good to know what exactly your turbo is doing.