| I think we would all agree that total HP is reduced by a dirty a dirty air filter because it reduces the total amount of airflow available at the MAF under WOT. However, discounting a WOT situation, and not adding boost, how does a dirty air filter affect actually mileage? I say not very much. Because as long as there is enough air available at the MAF to run the car normally, the MAF will provide the proper amount of fuel; the mix of air and fuel will be proper, and the engine will run as normal. If the filter does restrict some air, all that is needed to add more air is to increase the throttle body plates openings. In effect, at less than WOT, the dirty [not clogged] air filter simply acts to reduce the amount of air, which is easily compensated by the throttle. The engine does not struggle to suck in more air as a result of a dirty filter. This concept runs against my intuition, but I had this discussion today, and I had to change my thinking. gordo
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