I think AEM is the way to go. I wouldn't touch Injen with your stock intake!
I ran an Injen CAI for about a 6 months until the AEM came out, did a comparo....no doubt in my mind AEM is a better designed and built unit.
I haven't seen a K&N other than the molded plastic intakes they make for SUVs. For some reason, I have a feeling their intakes wouldn't be as well designed as AEM. IMO, K&N is just jumping on the import bandwagon to grab a few benjamins.
Actually I wouldn't use a CAI. Remember, on a turbo car you want the shortest path possible. That is more important than having colder air since the air will get heated up and cooled down anyway. Even if you have a 20 degree difference in 8inlet air temp, I doubt it will be more than 1 degree of difference once it gets to the throttle body anyway. Secondly the piping on the cold airs (any of them) will cause some wierd resonances and create a very off AF mixture curve. Unless you plan on dyno tuning it with a UTEC or similar AF device I say stick with stock intake and spend the money on mods that will actually make a difference. Uppipe/downpipe/exhaust will be a much better place to start.
i don't think any intakes are out for the 04 STIs yet, but u can definitely get a custom job in getting a HKS style foam filter in
foam filter works best and cleans itself with a few drops of detergent
Should the intakes from a WRX fit? I didn't think the intake tract of the STi was any different.
Any intake system is going to replace your stock intake box (all that black plastic crap under the hood....and the tons of it you can't see). It will simply be a tube that runs to the MAFS or near it and then the intake is on the other end of it. CAIs and short rams are only different in that a cold air runs the tube all the way outside of the engine bay...so that it draws cooler air in.
AEM is good one. in the fender the best. Frist i removed the junk, just ran the stock filter box and KN panel filter. Now i have the AEM in the fender lots better. Cleans up the engine compartment real nice :lol: