I have actually experienced the opposite with my previous car Liberty GT. I lost a little bit of power when I went from the stock box and aftermarket panel filter to a CAI.

As for JDM cars vs ADM cars I would suggest that variables such as temperatures (much cooler temps in Japan than WA generally speaking), altitude, not mention fuel quality have some bearing on this topic. A short ram in WA is going to suck hot air big time. The companies who make and sell CAI will tell you what they want you to hear in order to sell you there product.

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Funny looking German Subaru