Catalists

[kenmorton]

What sort of power increase would you expect to see on a 2l ford zetec if the cat was replaced with a straight through silencer ?

[alfa dave]

None - the catalyst on a 2l Zetec is not the back pressure limiter. You would need to change the manifold too then you could realise something though without changing the calibration (assuming you're running EFI) the results would be marginal.