"Michael Johnson" > wrote in message
...

> A dirty filter will lower gas mileage on EFI engines too. Instead
> of going into a long rebuttal I'll just provide a few links to some
> credible web sites.
>
> http://www.fueleconomy.gov/feg/maintain.shtml
> http://www.edmunds.com/reviews/list/...4/article.html
> http://tinyurl.com/2a9v2
> http://tinyurl.com/2hyeyx
> http://www.mass.gov/Eoca/docs/doer/pub_info/dt.pdf

If by credible, you mean sites that are repeating their granddaddy's
advice, then I guess they qualify. Can you show me one that actual has
any data to support your claim? Or at least an explanation of why a
slightly restrictive air filter might reduce the fuel economy of a
modern fuel injected engine? These sites are just repeating the same
sort of maintenance information that has been out there for 70 years.
Things have changed.

> There are a huge number of sites stating that a dirty filter
> decreases mileage. There is more than just a MAF reading that the
> computer uses to determine the amount of fuel needed. Air density,
> throttle position, air temperature etc. also come into play.
> Excessive opening of the throttle plate on a fuel injected car also
> tells the computer the engine is under a greater load which effects
> how much fuel is delivered to the cylinders.

Sigh, if you are talking about a completely plugged air filter, I
agree that the fuel economy may be affected (heck it might actually
increase since the maximum engine power will be limited). However, for
a modern fuel injected car operated in a normal environment with a
filter changed at reasonable intervals the difference in fuel economy
between a "new" and "used" filter approaches zero. The same is true
for a proper paper filter and a K&N filter. The only sensor in front
of the throttle plate(s) is the MAF. It is called a Mass Air Flow
sensor because it is measuring the mass of air flowing through the
intact tract, not pressure or density, or temperature. The throttle
position sensor is at the throttle and measures it position. All of
the other sensors are behind the throttle plate. So think about what
the various sensors see when the filter is slightly more restrictive.
The MAF is measuring the mass of air flowing through the system. It
doesn't measure pressure, so it won't be affected because the flow is
reduced my a slightly higher restriction at the filter (we are talking
about tenths of a psi difference or less). If the filter is slightly
more restrictive, the throttle may need to be slightly more open to
achieve the same power output. And I mean slightly. Throttle position
sensors are not particularly precise. They are a gross position
indicator. I doubt the difference in the throttle opening related to a
slightly more restrictive filter is significant enough to affect the
engine parameters at all. All the other engine sensors are after the
throttle plate. They will not be affected by the restriction in the
air filter any more than by the much larger restriction of the
throttle plate. AND remember you have O2 sensors that feed data back
to the PCM that is used to correct for variations in the other
sensors. So even if the restriction of the air filter was severe
enough to affect the other sensors, the feedback from the O2 sensors
should allow the PCM to adjust the fuel trim to compensate.

> I'm not saying a K&N filter will give a noticeable improvement in
> gas mileage over an OEM unit but with all things being equal the
> engine with a more efficient filter will perform better. Do you
> think an engine with a dirty air filter would pass an emissions
> test? If so then why not?

As long as the filter is in good shape, it will have no significant
effect on the ability to pass an emissions test. See above for the
reasons. I won't argue that a K&N might provide a slight performance
increase at WOT. It very well might. But for anything but large
throttle openings, the throttle plate is by far the most significant
restriction in the intake tract. The filter is almost not there as far
as air flow is concerned until the throttle is nearly wide open.
Again, I am only talking about modern fuel injected engines. For older
carbureted engines, a restrictive air filter would definitely
significantly reduce fuel economy. And the situation is not clear to
me if you are talking about some of the early speed density type fuel
injection systems (systems without a MAF). For normal sorts of air
filter restriction the PCM of these types of systems would be able to
compensate for a restrictive air filter. However, for a very
restrictive filter, they may not. However, as far as I know, no one
has sold a car with a speed density only system for a decade.

By the way, the DIY Basics sight you referenced is loony
(http://tinyurl.com/2hyeyx). You should follow your vehicle
manufacturers replacement schedule for the air filter. I think that
sight must be run by filter manufacturers.

You should read these sites:

http://www.visteon.com/utils/whitepa...05_01_1139.pdf
http://www.filtercouncil.org/techdata/tsbs/89-3R3.html

Air filters (paper and K&N) are less efficient at removing dirt when
they are new. As the accumulate particles, the filtering efficiency
improves. So changing your filter too frequently (or cleaning your K&N
too often) can actually increase engine wear. You should also consider
that K&N filters loads up with dirt much faster than paper filters
(they have less dirt holding capacity). So while they may enjoy a flow
advantage when new (or when just cleaned), the advantage decreases
rapidly with time.

Ed