If it affects your airflow, then it affects your AFR (logically speaking)...in which case a tune should happen. In this case, I'd say stick with the OEM filter.
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This is a difficult question to answer and I think a bit of education is required to address the logic of the question.
Firstly, it's not the additional airflow that requires a tune when installing an intake (not usually) -- this is a common misconception. The OEM ECU is generally able to compensate for airflow cells that are far above the stock peak consumption. The biggest reason for ECU recalibration is the shape of the curve itself. In order to achieve the correct Air:Fuel ratios for proper fuel combustion, the ECU must convert a voltage signal from the MAF sensor (typically hot-wire type sensor) into an airflow measurement -- usually g/s. The hot-wire sensor can only measure a single point inside of the intake, so the overall MAF in g/s is correlated to sensor voltage through a calibrated look-up table.
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(2015 STI map)
This correlation is very sensitive to external factors. If the MAF housing diameter is changed, the lookup table will have a scale error proportional to the ratio between the two areas contained by a 2D cross-section of the housing.
Some maths:
A drop-in filter does not change the housing or the MAF position, but it can change the way the air enters the MAF housing, which may perturb the calibration. How much the calibration is perturbed depends on the way the airflow is disrupted and very difficult to characterize.
Some notes for the curious -- This is the way by which companies like ETS claim horsepower gains by intake alone. Most of the gain in power comes from leaning out the MAFv relationship. Consequently this is also the majority of gains that are made by a "Stage 1" ECU calibration. It's a cheap and dirty way to gain power, but at the expense of all other compensation tables that rely on true load calculation. I also mentioned earlier
Firstly, it's not the additional airflow that requires a tune when installing an intake (not usually) -- this is a common misconception. The OEM ECU is generally able to compensate for airflow cells that are far above the stock peak consumption. The biggest reason for ECU recalibration is the shape of the curve itself. In order to achieve the correct Air:Fuel ratios for proper fuel combustion, the ECU must convert a voltage signal from the MAF sensor (typically hot-wire type sensor) into an airflow measurement -- usually g/s. The hot-wire sensor can only measure a single point inside of the intake, so the overall MAF in g/s is correlated to sensor voltage through a calibrated look-up table.
(2015 STI map)
This correlation is very sensitive to external factors. If the MAF housing diameter is changed, the lookup table will have a scale error proportional to the ratio between the two areas contained by a 2D cross-section of the housing.
Some maths:
The calibration is also sensitive to the position of the MAF sensor within the MAF housing. If the position of the sensor changes within the housing, either a scale or nonlinear error factor is applied to the calibration.
A drop-in filter does not change the housing or the MAF position, but it can change the way the air enters the MAF housing, which may perturb the calibration. How much the calibration is perturbed depends on the way the airflow is disrupted and very difficult to characterize.
Some notes for the curious -- This is the way by which companies like ETS claim horsepower gains by intake alone. Most of the gain in power comes from leaning out the MAFv relationship. Consequently this is also the majority of gains that are made by a "Stage 1" ECU calibration. It's a cheap and dirty way to gain power, but at the expense of all other compensation tables that rely on true load calculation. I also mentioned earlier
This is not strictly true, since there are cases where the additional airflow will create load conditions where timing and fueling are not calibrated. In these situations, the ECU will extrapolate the timing and fueling to best map the load condition, but it may not be adequate.
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