Demon tweeks have got a better website now, much easier to find stuff that you can use for your car instead of finding a nice product and seeing it dosen't fit your car
just thought i'd say, you have to select MK3 fiesta and it tells you whether it will fit the Si or not 
but they've got this on there, now obviously I don't need it, just as a general chatting point.. do these actually make a difference?
http://www.demon-tweeks.co.uk/Performan ... 2979/28507
im guessing it for higher performace engines which need more fuel to match the airflow and as its adjustable it'll allow to increase or decrease airflow and make the fuel to suit...
That or there just a con, probably are for cars with just a K&N or something lol