c/r
 

what exactly is the compression ratio? can i get some info on that? why is it so very importan?

Compression ratio is the ratio of the total volume of the cylinder and combustion chamber with the piston at BDC (bottom dead center) and TDC (top dead center)...

can you run that by me one more time?

ok when the piston is at the bottom of the stroke(BDC) that is the 1 in the compression ratio ex. 9:(1) and when it reaches the top of the stroke(TDC) that air is compressed 9 times smaller than it original volume hence the 9 in 9:1 . the higher the compression ratio the more power there is to be had but you'll also risk detonation(the air and gas self igniting at a bad time) which will ruin an engine unless you run race gas which has a higher octane(higher octane= more resistance to detonation).
On the street 11:1 is as high as you want to go, but with a turbo you'll want a 8.5:1 or so compression. Hope this helps.

Why do we use lower compression with FI?

a turbo forces more air into the cylinder so in a way it increases compression plus the if your intercooler sucks then the intake charge will be hella hot and again give you another chance for detonation.