I just wrote this in a different thread, so I will copy and paste here, too.
In real life, I work on cars. The way a turbo works is that it forces air through the intake manifold into the piston. A non turbo'd engine isn't very capable of increasing air volume through the pistons because atmospheric pressure won't allow it. The only air it can pull in on it's own is during the downward stroke of the piston (in case you don't know, the combination of fuel, air, and spark is what creates hp, and it's a very careful balance of the three that create a maximum amount of power).
With the turbo literally forcing air into the intake manifold and into each cylinder, it can overcome that atmospheric barrier of air pressure by a few psi. The turbo on my last engine I built created 18psi at full boost, around 5800rpms.
Here is the next dilemma, the bigger a turbo is and the more psi it can generate inside of the intake manifold, the more time it takes to spool up and create that psi. A turbo designed to create 4-8 psi can spool relatively quickly, thus creating power more efficiently at lower rpms (stage 1 turbo). A little bigger, more psi...better midrange power, usually around 10-18 psi. Really big turbos, 20+psi, need more time, thus are more efficient at higher rpms.
Sorry for the rant, I hope someone learned something.
