Forced induction does nothing to static compression, that is a result of the hardware in the motor. It does however affect critical or total compression.
As a rough guide a 10:1 motor static with 7psi of boost will run at 15:1. This of course does not factor in valve overlap and few other things, but as a rough guide.
static compression is at atmospheric pressure = 14.7psi approx. If you boost by 7psi you operate at plus 50% of atmospheric ie 10:1 plus 5:1 = 15:1
The other thing to consider is that the higher the initial pressure the more "boost" it takes to increase it and eventually you get to a point where you cant efficiently make more power. There is a limit in other words. The higher the start compression the easier it is to make a big dynamic compression, so its better to start with a 10:1 and boost to 15 than to start with 8 and boost to 15. Static compression is also very cheap to make by comparison.
There are more things to consider but this is the simple way of thinking about it. In real life a normally aspirated 10:1 motor runs at around 11.5 to 12.5 anyway as you eliminate the valve overlap and increase the air speeds etc.
T/C and centri S/C dont really make boost at idle (at least not much).
In theory.. a S/C is a fixed multiple of atmosperic pressure as it is crank driven, while a T/C is an exponential booster in that when it add x cfm to the intake there is also x increase in exhaust. This then drives the T/C harder and it makes more pressure resulting in more exhaust etc.
There are lots of other factors to consider and generally speaking I doubt I can be relied on for anything (at least thats what the missus says)
<font size=-1>[ This Message was edited by: tony.k on 1/7/02 10:18am ]</font>
