Why turbo cars use low compression?

I could never figure this one out, most turbo gasoline cars use lower compression setups, but wouldn't it be just as effective for a engine to run 15psi of boost with 9.5/1 compression compared to 20psi with 8.5/1 compression, If fact wouldn't the higher compression has the advantage, due to the fact of turbo lag and being a lot safer on the turbo. Also is there a mechaincal advatage to having lower compression. I figured if the pressure in side the cylinders at the point of ignition on the example I stated recently are equal to one another then why is lower compression set up more popular?
Why turbo cars use low compression?
i believe it has something to do with detonation or the air/fuel mixture igniting before the spark plug fires. if a an already compressed volume of air is compressed further (in a cylinder chamber), it raises the chances of detonation which can severely damage your engine
Why turbo cars use low compression?
Consider the fact that with a turbo, it is pulling more air and fuel into the cylinder then it would normally have. Basically you are stuffing a cylinder (just as an example) with 500cc's of an air fuel mixture into a space that normally holds 350cc's. Something has got to give to compensate for the extra pressure and stress on the block, usually it is the side of the block. By lowering the compression and raising the boost psi, you make nearly the same amount of power as a high compression rating and less boost. You also lessen the amount of stress you put on the block. Since nobody like to crack their block in half. Also as a side note the timing is retarded to prevent any for of premature detination, not compression.

tanning