What sort of compression ratio would be good to aim for?

Edit:
Ive just found this information over on fiestaturbo.com

With forged pistons there are a number of options for the Zetec. The cheapest is to use those originally designated for another type of turbocharged car. In this case the most common source for 2.0 pistons is the Vauxhall C20LET from the Cavalier and Calibra Turbo. Fitted to a Zetec these will give a CR of 7.4:1

The C20LET bore is 86mm, compared to the 2.0 Zetec’s 84.8mm, so a slight overbore of the block is required. This presents no problem as the block is likely to have some wear on it and there is enough material to maintain the wall thickness. This will take the original 1998cc to 2045cc. Also required is modification to the rods to accept the 21mm wrist pin.




The metal Series 3 (Focus) Zetec head gasket is fine for turbocharged use and has been used without problems a up to 2 bar boost. As far as I have been able to determine. The turbocharged Focus RS uses the same gasket as the NA engines.