Originally Posted by the suicidal eggroll
It doesn't matter if the diameter is the same. The flow pattern is different, which throws off the MAF scaling.
ALL intakes for the 08+ WRX/STi need a tune...ALL of them, there are no exceptions. None of them measure close enough to stock to work with the stock scaling.
I had this exact sediment before I did some heavy reading. Everything I read before indicated this very thing and, if you were to view my posting history, you can see just that. HOWEVER, after reading some very interesting data, I changed my feelings on this. I will try to dig up my sources and post them. I am not one to hand out poor, misinformed, and uneducated information. No matter what, I would never put an intake on and not tune for it. There are companies, however, who have the R&D into their intakes and say they run just fine without a tune. Let me find those articles.