The Civic SI came with a factory cold air set up.Honda wants a fortune for it,but it would fit and increase performance marginally.I got one from NAPA (long tube)for around $75.00.Mileage went up 1 mpg(documented)and accelerating up an on ramp there is a difference.The down side is the sound these set ups make.They sound like an old four barrel wide open,wooooooooo! I kind of like it myself.Estimated power gain is around 6 hp (knfilters.com)