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Originally Posted by BAN SUVS
CAIs basically cause the MAF not to correctly read the amount of air flowing into the engine. most of them will flow more than what the MAF sees, so the car gets leaned out. The WRX will add fuel long term, but when you first get lean, it causes minor detonation which causes the ecu to learn to pull timing, and MBT is more important to making power in an EJ20 than the exact A/F ratio. In closed loop fueling, using the O2 sensor feedback, the ECU does adjust fuel and nothing bad happens. but under WOT/high RPMs when it switched to open loop and goes by the maps in the memory, it's suddenly lean and starts knocking. In open loop, there's not much authority over long term fuel trim, and pulling timing is the first defense against detonation.
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So, two questions:
1) How can the MAF not correctly read the amount of air flowing through the intake tubing? It's a closed system, so any "extra" air that the engine sucks in as a result of less inlet restriction has to pass through the meter. Is it an issue with the physical meter placement, right after a tubing bend in the CAI or something? I could understand that, it's sometimes an issue on mustangs since the meter is placed right next to the air filter, and consequently any aftermarket CAI piping bend that comes in from under the fender... if the meter is clocked so that the wire elements inside are on the inside of the bend, they have trouble reading accurately.
2) You touched on this, but it seems wierd to me; do your cars not apply open loop/long term fuel trim corrections based on what it learns during closed loop? That's what the strategy I'm used to working with does...