I've read that the bluefin does not suit the Fiesta ST but the dreamscience does.
You won't get much benefit by fitting just a back box, the main downside of the exhaust system is the flexi pipe and the catalyst.
An uprated flexi with a sport cat and a cat back system will give you the best gains, without breaking the bank.
You can de-cat but you're risking prosecution and MOT fail, plus the engine light will be on all the time
Also the spark plugs will be better with denso iridiums as ST has platinums
and the HT leads are a bit cheap, a set of magnecors will improve the spark to the plugs.
After that it's throttle bodies and cams to get any more gains
I can't comment on the suspension, I think it's too hard to begin with.
Don't forget to upgrade the brakes to match the power!!
The sky's the limit really, depends how much money you want to spend
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2011 Focus 3-door Zetec 1.6 TDCi.
Previous <img src="/images/graemlins/stemo.gif" alt="" />s - 2006 Panther Black & 2007 Red. Gone and quite forgotten <img src="/images/graemlins/lol.gif" alt="" />