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Originally Posted by Drifter
After posting the above I was able to find a study on clean vs dirty air filters from the Oak Ridge National Laboratory ( https://www.fueleconomy.gov/feg/pdfs...02_26_2009.pdf ) They concluded:
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Results show that clogging the air filter has no significant effect on the fuel economy of the newer vehicles (all fuel injected with closed-loop control and one equipped with MDS). The engine control systems were able to maintain the desired AFR regardless of intake restrictions, and therefore fuel consumption was not increased. The carbureted engine did show a decrease in fuel economy with increasing restriction. However, the level of restriction required to cause a substantial (10–15%) decrease in fuel economy (such as that cited in the literature3,4) was so severe that the vehicle was almost undrivable. Acceleration performance on all vehicles was improved with a clean air filter.
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That is what I expected and have seen.
A carbureted car? Sure, it might get a little richer if you drive around with a clogged air filter.
But reducing intake restriction for for fuel efficiency on an EFI car? Pointless. Any increase or decrease in fuel mileage is because of something else that was changed or because the measurements weren't precise.
I'm not saying that changes to the intake won't make changes in fuel mileage. But those changes affect things like the AFR or fuel atomization (or de-atomization) and such.
If anyone wants to remove restriction from the intake, remove the throttle plate or drive around with it wide open. It's like trying to get a highflow water pump to work with your thermostat stuck shut. First remove the valve that's constantly restricting the flow before trying to increase the flow elsewhere.