Quote:
Originally Posted by tasdrouille
As air filters accumulate dirt, they also filter better. So do yourself a favor, get an air filter service indicator and only change your filter when you need to.
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Tas is correct.
This phenomena is called initial blind-off. It happens when the largest pores in a filter fill up, or blind-off, leaving the smaller holes to filter the incoming air (or oil). This increases efficiency as the particulates accumulate on/in the filter media. So having a dirty (not ridiculously dirty, we all know what a reasonable about of filth is
) filter is actually more efficient than a brand new filter.
The only thing that changes with the filter is the differential pressure. As holes in the filter blind-off, it becomes harder to get the air (or oil) through the filter, thus pressure goes up. This is compensated for in newer cars with ECUs by using the Lambda (O2) sensor behind the catalytic converter to adjust the stoichiometric mix. It is a big problem with carburetted vehicles because as differential pressure goes up, more gas is added to the mix and stoichiometry is not maintained. That is why you still hear people saying that a dirty air filter is bad for gas mileage, because it
used to apply to cars. It no longer has much of an effect on newer vehicles
, though it still applies to older ones.
So change your air filter when it is damaged or too dirty to see sunlight through. Otherwise it is a waste of a perfectly good filter. And everyone knows how ecomodders feel about waste
.