C. E. White wrote:
> "Michael Johnson" > wrote in message
> news >
>>> Exactly, I don't believe an air filter will change mileage. The auto
>>> makers would be on it and so would the rest of this small world.
>> Ever notice that an engine running with a dirty filter sees a drop in gas
>> mileage? The same principle applies to an OEM filter verses a filter that
>> flows better (i.e. a K&N etc.). There are two things that can happen when
>> air flow through an engine is made more efficient. One is an increase in
>> horsepower and the second is an increase in mileage. Sometimes both can
>> happen simultaneously. If you don't believe me then take your air filter
>> and clog it up and run your car for a tank of gas then put in a new one
>> and see whats happens to your gas mileage and power output. I'll bet the
>> farm that they both will see a substantial increase when a clean filter is
>> installed.
>
> While this was certainly true with carbureted engines, there is no reason to
> think this is the case for modern fuel injected engines. For carbureted
> engines, a clogged air filter acts like a choke and enriches the mixture
> because of the effect on air pressure in front of the throttle plates. This
> reduces the fuel economy. In a modern fuel injected engine, the mixture is
> not influenced in this way. The amount of fuel injected is determined based
> on the MAF sensor and other sensors. These sensors can't tell the difference
> between a restriction to the flow related to the air filter and a
> restriction to the flow related to the throttle plates. There is no
> difference as far as the computer is concerned between the restriction of
> the air filter and the restriction of the throttle plates. The engine speed
> / power output is determined by the total intake restriction (intake tract
> plus throttle opening). The only thing a slightly restricted air filter does
> on a modern engine is require you to open the throttle a slight amount more
> and reduce the maximum power output. The effect on fuel economy for a modern
> engine is minimal. I won't claim it is zero, but I doubt you would be able
> to tell the difference unless the filter was absurdly restrictive.

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

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.

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?