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i've been looking into tuning recently and here is what i think i know:
dme looks at (a) maf signal (amount of air going into the engine) and (b) rpm, to determine how much fuel to deliver via the injectors. this is done using a 3D, 3-axis map (x=rpm, y=quantity of air, z=amount of fuel required). these are the maps that get modified by tuners to increase performance.
dme then looks at exhaust o2 sensors to see how things went during combustion. depending on the amount of oxygen in the exhaust, the dme either adds or removed fuel by lengthening or shortening the injector pulse.
this information is put into a short term fuel trim map (stft) as a % modifier of the primary map.
if the stft stays stable (ie, over about 50 kms of driving) the data is transferred to the long term fuel trim map (ltft). this is your dme 'learning'.
if you find that your ltft is staying at a constant value (ie, +5% due to intake modifications) tuners can go to the maf calibration map and calibrate it so that your ltft zeros out. you may wish to do this to make room for other modifications, as max ltft is 25%.
so, if you put a 3.5" diameter maf housing on a car that needs a 3" housing, you are looking at a maf that is reading 36% out (pie are square, right?). dme can't adapt to such a significant change. worse, your engine will be getting more air than it thinks it is and run lean - hard on engines (no cooling). you can do it, but a dme remap is required.
however, it also shows that those who state their intake products require an expensive remap (which they also sell) to realise full benefit are wrong, unless airflow is increased by more than 25% (not likely).
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