Is putting a wrx cold air intake onto a 08 Impreza 2.5i with a wrx aftermarket exhaust worth it

Operative language------- Estimated Horsepower Gain 14.28 HP @ 5228 RPM No, I wouldn't do this.

I've pulled dozens of cold air intakes and K&N filters out of 2.5i over the years, as they NEVER are helpful. The air requirements of this torquey, low-revving big 4 NA motor are quite meager. Even a mouse-nest's clogged several year old OE filter doesn't choke these down appreciably. Really, how many times are you ever shifting above 4.5k? (If so...stop it...you're just wasting fuel!.) The header and/or CAT-back you put on probably SOUNDS lovely, but it in no way is improving performance under 4k rpm. GREAT placebo effect, though, and sometimes that's all that counts, eh? As you may learn, any power gains to be had are a function of increasing the mass of air into the combustion chamber...either by vacuum (NA) or pressure (turbo, sc). Reducing airflow back pressure by fancy filters is thus only useful when EXCESS air is being PUSHED into the system. Increasing mass of air is to a smaller extent affected by ambient air temp too; hence some cold intakes are far removed from the heat near the engine...but any bends in the plumbing work opposite to that increase in air delivery too. The best way to impreove your '08 2.5i is with 17" wheels and nice V rubber...or maybe sway bars if you've got $300-$400 to spend lying around. Cheers. Ern

Guru - When I pass in the mountains where I live I regularly run my 2003 Forester up to 6 grand. Would intakes and exhaust upgrades help power at those engine speeds? It runs right up to 6k cleanly with no drop off of power at all now.

Most human perception systems can reliably just recognize +/-5% difference (jnd) in analog continua. (Interesting that audio frequency is an exception, eh?). So will you get +5% more power with a CAI or K&N with a 2.5i. My strong guess is NO.