The ST170 has the same basic block as a 2.0L focus. The rotating assy is different - rods and pistons are sinter-forged. Compression ratio is 10.2:1 with standard head, whereas a standard 2.0L is 9:1, so a little more turbo friendly. You will get away with 250 on standard comp, just, but the boost required to run any more power just results in horrendous detonation at that comp ratio.

Cheapest future proof way of building a zetec turbo? Buy a 2nd hand 2.0L - @£250; new FRS short-engine from ford - ~£800 trade; EFI inlet; management of some kind; focus rs 'ford motorsport' tubular manifold from ebay; turbo, injector etc of your choice.

Chris