Tuners say stock Sti airbox is very good...no need for cai unless you've done front mount and big turbo like looking for big hp big mods. They done lots tests and it only makes like 2-3hp difference on some setups(on some no difference) unless what I said above.

Unless thats just for the 06/07 airbox

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06 STi: 337hp @23psi on E85...
06 STi: 309hp @22psi on 98ron...