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Here's my take on fuel consumption with a restricted air inlet.
The A/F ratio delivered by an EFI system is maintained at close to stoichiometric (14.7:1) by sensing the air flow signal through the inlet. If the air flow inlet is restricted, for a given throttle opening, the engine delivers less power.
At WOT, EFI systems are usually in an open loop mode. In this mode the ECU only relies on an internal map to determine WOT A/F mixtures and not the O2 sensor nor other system sensors. Thus, the mixture will be richer than designed due to the restricted inlet.
At throttle openings less than wide open, to obtain the same equivalent power with a restricted inlet, the driver can increase the throttle opening to allow more air into the engine. The A/F ratio will remain the same but the total fuel volume increases proportionally with the increased air flow.
Whether a restricted air filter uses more or less fuel depends on how the operator (driver) reacts to the restriction. He may settle for the loss in power and vehicle performance and thus use less fuel or compensate with the throttle to maintain a previous level of performance (vehicle speed, acceleration rate, etc.) and thus consume more fuel.
EFI systems rely on many sensors to determine/maintain a target A/F ratio. The air flow sensor is but only one of many sensors. For example, the ECU may determine that a lower-than normal air flow signal along with a wider-than-normal throttle position sensor opening may be considered an abnormal condition and trigger a diagnostic trouble code (DTC).
I may be incorrect, but read through this a couple of times and see if it makes sense.
Sherwood
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