What intake should I buy for my Subaru?
There is no simple answer to this. Here's why...
Subaru utilizes MAF-based fueling scheme
MAF stands for Mass Air Flow.
So, what does MAF-based fueling have to do with me?
Basically, there is a wire that is in the intake stream that measures airflow. As air flows across the MAF, it requires a voltage to keep it a certain temperature; more airflow requires more voltage to maintain that temperature.
What is MAF-based fueling?
In the ECU, there is a scaling for the MAF sensor, based on the characteristics of the OE airbox. By altering airflow into the MAF sensor, you need to adjust the scaling. Bends in the intake piping, intake piping size, etc. all will affect the MAF scaling.
I bought a stock-sized intake, so I shouldn't have an issue, right?
Just because the intake is the same size, the way air flows over the MAF sensor will affect the scaling. See the following chart of the Cobb scaling of different intakes on the '08+ ECU:
(Photo courtesy of Phatron @ Phatbotti TKR)
As you can see, the MAF needs to be scaled 25+% at certain MAFv. Both intakes in this example are stock-sized.
What does this all mean, and how does it affect me?
The ECU has some built-in parameters to help keep airflow on target, but they are somewhat limited. This includes both short-term (STFT) and long-term (LTFT) fueling corrections; there is also closed-loop (CL) fueling and open-loop (OL) fueling.
STFTs are an immediate correction, while LTFTs are a mean/average correction (based on STFT/O2 data) for a certain load range (there are typically four ranges).
During closed-loop fueling, the ECU is getting feedback from the front O2 sensor (located in the passenger side of the exhaust manifold), in which target AFR is compared to actual AFR. CL fueling is only applicable during idle, cruise, and light acceleration. Upon heavy acceleration, the car switches to open-loop fueling, in which the car no longer relies on feedback from the front O2 sensor, and solely relies on target/compensation tables in the ECU (and whatever LTFT are applied).
As the ECU learns the characteristics of fueling during closed-loop fueling operation (based on STFT), it will designate a LTFT value to preemptively add/subtract fuel, as necessary, in order to keep target/actual AFR on course. There is a limit in which the ECU can apply for LTFTs (often 15-25%).
Where this becomes a bigger issue is when the ECU switches to open-loop fueling, and no longer uses the front O2 sensor for AFR feedback. There is an area of LTFTs that are only applicable near the transition from CL and OL fueling. What happens is that LTFT is applied to the fueling compensations, without any kind of feedback (like with CL fueling). What happens is that you have a blanket LTFT being applied across the board, without any checks & balances (the benefit of CL fueling). If the LTFT cell only says you need 10% fuel added to it, while you really need 25+ (as per the image above), the car is going to run lean without proper MAF scaling.
Now that I understand the basics, what can I do about it?
There are a few threads which explain what you need to do:
RomRaider • View topic - How To: Closed Loop MAF Scaling
HOWTO: One Man's Way of Scaling a MAF - NASIOC
RomRaider • View topic - How To: Open Loop MAF Scaling