This has been discussed more than once in more than one forum, but it’s a pain to go looking up old threads when the problem comes up in conversation.
What’s the FMIC rich dip?
Many people with turbo Subarus, draw-through (stock-style) MAF sensors, and front-mounted intercoolers have reported a “rich dip” in their AFR that starts right after stomping the throttle, and ends about a second later. A change of about 1.0 AFR is typical. I have personally observed this after setting tip-in to zero, just to verify that tip-in isn’t the problem.
This will happen at any RPM. If you consistently do pulls from 2500 RPM, you can tune around the problem without realizing it. But you’ll find that stomping the throttle at (for example) 4000 RPM will cause a rich dip. This cannot be tuned out via MAF scaling or even via the primary open loop fueling table. If you did tune it out for 4000 RPM throttle stomps - by leaning out the cells where the rich dip happens - you’d find that stomping the throttle at a lower RPM causes the mixture to go lean when you cross into the same cells where you were previously rich.
A picture is worth a thousand words, but I don’t have one handy at the moment.
What causes the FMIC rich dip?
Not MAF scaling - the same MAF voltage will yield different AFRs depending on how many milliseconds have elapsed since you stomped on the throttle.
Not the fuel table - the same RPM/Load cells will yield different AFRs depending on how many milliseconds have elapsed since you stomped on the throttle.
The leading theory is that it’s caused by the buildup of pressure in the charge pipes and intercooler core. While pressure is building, there’s slightly more air entering the pipes and core than there is exiting. The MAF sensor sees the the larger volume entering, and the corresponding amount of fuel is slightly excessive for the slightly-smaller amount of air that actually enters the cylinders. Once the intake tract reaches equilibrium, the air flow measured at the MAF sensor equals the air flow entering the cylinders, and the AFR returns to normal.
How can you fix the FMIC rich dip?
A few people have reported that switching to a blow-through MAF setup (where the MAF sensor sits between the intercooler core and the throttle body) solves the problem. I’ve got a blow-through charge pipe fabricated but I haven’t yet done the wiring to hook up the sensor on the far side of the engine bay. I’ll get this done when snowboarding season is over, if not sooner.