Can anyone tell me the what size head gasket or decompression plate would be needed to lower the cr on a focus st170 to create a st170 turbo. There's a guy on ebay selling a st170 turbo with stock pistons and he says he used a 3layer head gasket to lower cr and its running 14 psi. But I thought the st170 head gasket was already 4 layers thick and the stock zetec was 3 layers? Also the Americans seem to be safely turbo charging stock st170s ( svt turbo in America) but I can find how they are doing it Wether it's a cometic head gasket or decompression plate? Any help or advice much appreciated cheers