F18Rep

Thanks guys, man I thought I was gonna have to get out that old physics book there for minute.

(More boom, more zoom)... will have to add that to my basic knowledge, will put it right after ($hit, shinola) ... Bruce

FormulaOne10

Think of the engine cycles when discussing compression ratio. As was said before, boost doesn't affect CR. This is because by the time you are on the compression stroke I would hope that the valves in the engine are closed (no boost in other words). However since you began the cycle with a lot more air in the cylinder, you don't have to compress it as much. Think of what compression does...it increases pressure (which in turn does many other things, temp, volume, molecular collisions etc.) to a point which is ideal for ignition-combustion.

Now here is where you can see why you usually need lower compression in a turbocharged engine. When you force air into the cylinder, you are already performing an initial compression. Then the air is compressed again on the compression stroke. This two level compression makes the air/fuel more prone to autoignition-combustion (PV=nRT). If the compression was too high, you would be combusting the fuel early due to autoignition. This is also why you would use a higher octane gasoline for forced induction.