[Drifter]

Originally Posted by Drifter

I can see where a higher flow air filter would make a difference at higher loads, but at light cruise why would efficiency differ throttling with a filter vs a throttle plate?


Edit: unless maybe the ECU were to interpret the higher throttle position as demanding more fuel enrichment & less ignition timing even though actual power demand was equal?

Originally Posted by Isaac Zachary

I have always been skeptical of low restriction intake filters helping a gasoline engine with anything other than power at WOT. That's because they don't help "the restriction" from normal driving. "The restriction" is the throttle plate itself. Even going from a 6 month past due dirty stock filter to no filter at all won't change the overall intake restriction one bit.

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:

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.

Modern Fuel Injection:



Dodge Charger with multi-displacement system:



Carbureted vehicle:


Modern fuel injected car with a severely clogged filter (almost undriveable - the car was barely able to meet the acceleration requirements of the EPA city mpg test):

In the United States, city fuel economy is measured using the Urban Dynamometer Driving Schedule, also known as the Federal Test Procedure (FTP).

Highway fuel economy is measured using the Highway Fuel Economy Test (HFET).

Another relevant test is the US06, an aggressive (high speed, high load) test used to confirm emissions compliance during aggressive driving