compression ratio

this may be a question from a stupe

but what is compression ratio : ( I think I have an idea :| )

and how does it effect performance : ( I have no an idea )

It's the (idealised) ratio of chamber pressure when at TDC to atmospheric pressure.
Correct me if I'm wrong, but AFAIK it's worked out by the formula

PV=nRT

or assuming T, n, R are constant, and normal letters are initial states and italics are final states,

PV=PV, P=PV/V
where you know
P (atmospheric), V (chamber volume + displacement for each cyl.), V (chamber volume) and want to find P.

and then CR is
P/P

XT500, you forgot to explain how it effects performance?