Michael Johnson wrote:

> 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?

None of those sites listed states that the effect of a dirty air filter
on gas mileage applies to FI engines with MAF sensors. It seems rather likely
to me that those sites are merely repeating what was true with carburated
engines as still being true, without retesting or even remodeling.

Do you have any URLs for tests done with modern engines?
--
Cheers, Bob