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I did not find a lot of information about this subject, but the Load Compensation to Manifold Pressure had a huge impact on my car.
I'm in the process of stabilizing AFR corrections (+ 25% at certain RPM range) issues on a stock map after I switched to an IAG short deck block (the car has no other modifications at this stage).
It took me some time and the replacement of a MAF sensor, fuel regulator, smoke and vacuum tests, until I discovered that the issue should be fixed with the Load Compensation table.
I was aware of the 2600-2800 stumble issue on Subaru 08+ and Cobb has a great write-up about this: https://www.cobbtuning.com/2008-sti-fuel-pressure-regulator-kit-update-technical-overview/
But what I discovered is that the problematic RPM range that requires the additional compensation has moved to the ~3200 range on my car.
Stock map and modified maps are attached for reference.
I'm trying to understand the mechanical reason to why a different engine block can cause such huge change. I can guess that this might occur due to slight change in the compression ratio?
A side note - I tried attaching the fuel regulator to a more stable vacuum line (intake manifold), and I'm still running the car like that, but it doesn't seem to make any difference.