The Zetec is tuned from the factory for 87 octane. The rather average compression # of 9.something is also well suited to run at 87 octane.
So running anything higher will net nothing. Simple physics of engines. No exceptions!
There is no HP that will be gained. There is no throttle response that will be gained.
If you want to run a higher ocatne 89 is fine, but anything more and you will no longer be burning all of the fuel in the combustion chamber (therefore losing power and gas mileage).
Of course if you have engine knock because soemthing is wrong with your engine, then a higher octane gas will help to prevent the knock. But that's like putting a band-aid on a cut that needs stitches.
Aside from that, any improvement you feel from a higher octane gas is like "I washed my car & now it feels faster".
