here's a really great explanation on reading a compressor map. I'ts a little complicated, but the dummy version is:
Pressure ratio equals boost + 14.7 and then divided by 14.7 at sea level.
The example they use is: your boost gauge reads 12 psi.
So 12 + 14.7 (atmospheric )= 26.7. This is your absolute pressure.
Now divide 26.7 by 14.7 = 1.82.
This is your pressure ratio along the left side of the compressor map.


Make sense?

http://www.turbobygarrett.com/turbob...o_tech103.html