Std silver-top springs are crap for turbo usage and typical/tired ones will float at around 170-200bhp.. Pipers uprated single spring for the zetec is fine to about 300bhp.. after that you need double.. It appears for the blacktop a company called Ferrea sell a double spring conversion with titanium retainers etc.. about £300 for the set - BUT their website says you need to use their valve guides too.. not sure why.