What did you do to your Fiesta ST today ?

[SteveS]

Ever since changing to step colder ngk plugs I've been having misfires under full throttle. I gapped to .026(originally .030) and it's much improved but still misfiring a under wot. I'm in dizzy e30 tune. He suggested I gap to .028 which still wasn't enough. Should I go back to OEM plugs?

If step colder plugs cause misfires, yes, the answer is to go step hotter. The fascination with step colder plugs eludes me. It is not a magic formula for power. If, after your tune, the engine was running fine without preignition, and a plug check did not show signs of plug overheating, then there is absolutely no reason to change to colder plugs because the only expectation would be that you would have misfires, fouling, and eventual problems from unburned fuel.