Quote:
The simulated clogged filter significantly affected vehicle performance, increasing the time to accelerate from 20 to 80 mph by 0.6 to 1.7 seconds on the three vehicles. The Outlet DP measured for each vehicle was sufficient for setting a common air filter indicator to the “change” or “clogged” position. For each vehicle, the Outlet DP at some point exceeded 5 kPa and showed an increase over the clean filter in excess of 2.5 kPa, a common standard for defining a dirty air filter.5,9–11
Despite the filter restrictions, however, no significant changes in fuel economy were observed. Each vehicle was run through at least three rounds of FTP, HFET, and US06 tests with the new air filter, and the same protocol was repeated with the clogged air filter. The tests were conducted on consecutive days for each vehicle. This format was used to allow for the required soak time to perform a cold FTP each morning. The resulting fuel economy data for the vehicles are shown in Figs. 3.8, 3.9, and 3.10. Range bars in the figures show the minimum and maximum of the tests for each case, while the columns show the average. Test-to-test repeatability is within about 1.5%, and all of the variances between the new and clogged air filter cases are similarly within about 1.7%. The baseline fuel economies for the vehicles were all within 0–6% of unadjusted EPA certification database values (U.S. Environmental Protection Agency) for similar vehicles.
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Summary: In fuel injected cars, a computer monitors the air/fuel ratio and puts in less gas when you get less air, so FE isn't harmed, but performance is.
In carburated cars, a clogged filter alters the air/fuel ratio and degrades both performance and economy.