"Michael Johnson" > wrote in message
...

> Does an engine under a heavy load run richer than one that is under a
> light load? The ECU has load tables that it uses to help calculate what
> it believes the optimum air fuel ratio should be taking into account the
> conditions it thinks the engine is operating under. What the computer
> thinks is optimum isn't always 14.7:1. The computer expects the
> operator to keep the filters fresh and a dirty air filter makes the ECU
> think the engine is under a heavier load and thus changes the target A/R
> to run richer than normal. This in turn reduces gas mileage.

Some of what you say is true, but your conclusion are wrong. As I said
before, the throttle position sensor is the only sensor that will even show
a minor variation as a result of a change in air filter restriction. And
throttle position sensors are not precise at all. They are gross indicators,
used primarily to communicate rapid changes in the throttle position (i.e.,
mashing down or letting up) so the PCM will be able to temporarily enrich
the mixture (mimicking the accelerator pump of a carbureted engine) or
change the IAC setting to prevent the engine from stalling as the speed
falls back to idle (like a dashpot).

> ECUs
> aren't clairvoyant and can determine the degree to which an air filter
> is dirty. It only takes the input from ALL the sensors and using
> preprogrammed tables makes A/F adjustments (and many other) to the
> conditions it perceives the engine to be operating under. Determining
> engine load is a very important component is what it uses to set the
> target A/F at any given moment. This is why they put throttle position
> sensors on engines nowadays. The new Mustangs are somewhat different in
> that there is no longer a direct wire connection to the throttle plate.
> The computer senses the position of the accelerator pedal and then
> sets the throttle opening accordingly.

Not just Mustangs are fly by wire. But again, you are missing the key truth.
An air filter, even a used one, is a minor restriction comapred to other
elements in the intake system. When crusing at a steady speed (say 60 mph),
the air drop across the engine air filter is going to be less than 0.3 psi.
The pressure drop across the throttle plate will be on the order of 7 psi.
The difference in pressure drop between a clean K&N filter and a reasonably
dirty paper filter is probably less than 0.1 psi. Is it your claim that this
small change is going to upset the PCM so much that it can't maintain the
proper fuel to air ratio? There will be a bigger difference in the pressure
after the air filter if you drive the car from sea level to the top of a
5000 ft mountain that any change in pressure related to normal changes in
the filter restriction. If you truly believe this, why doesn't installing a
K&N upset the PCM parameters?

> I have a TwEECer chip in my '89 LX that lets me program almost all of
> the EEC-IV operating parameters. I know load tables exist and they
> affect A/F as do many other sensor readings. There is a lot more going
> on in the ECU than reading air in and making a simple computation for
> fuel required. Things like engine acceleration rate, load, etc. come
> into play in a big way. When something like a dirty air filter causes
> readings of the throttle position sensor to be out of the range it
> expects for a given driving condition then gas mileage can, and will, be
> affected. The computer doesn't know the air filter is dirty and tries
> to run the engine in a manner that isn't optimum for gas mileage. It
> thinks you are doing something like climbing a hill, or accelerating,
> and delivers fuel accordingly.

Again, the throttle position sensor is just a gross indicator. The change in
the position of the throttle related to normal variations in air filter
restrictions will be trivial at cruise speeds. As the throttle angle changes
from 4 degrees to 90 degrees, the tps ratio of output voltage to input
voltage will go from around 0.2 to 0.98. The accuracy of the output is on
the order of +/- 20%. There is no way a reasonable change in the restriction
of the air filter is going to cause a greater change in the output of the
TPS than normal variations inherent in the design of the tps. Automotive
throttle position sensors are not highly accurate. And the PCM is able to
use the feedback from the O2 sensor to compensate.

> The A/F doesn't remain constant across the entire load range an engine
> can experience. As the load increases the target A/F decreases. This
> is programmed into the load data tables of ECUs. If the A/F didn't
> decrease then cylinder temperatures would get too high and start melting
> things like piston tops.
>
> To further make my point does an engine get better gas mileage going
> downhill or uphill at the same speed? Is that because in one condition
> the engine is under a heavier load than the other? If the rpm rate is
> the same for each condition then why is the mileage different? It is
> because the throttle has to be open more going uphill to get the air
> necessary to make the power needed to overcome the elevation increase.

Ths had nothing to do with wether or not the air filter restriction has any
affect on fuel economy.

> The data tables for engine load the computer uses are very specific to
> the size of the throttle body put on the car. It relies on these tables
> in conjunction with throttle position readings to determine what target
> A/F is used from the load table. It also uses the O2 readings to fine
> tune the A/F but only to meet the target A/F from the load table. A
> dirty filter requires an increase in throttle opening which triggers the
> computer to operate from the higher end of the load tables. Hence the
> engine runs richer and gets lower mileage.

The change in the throttle opening related to normal changes in air filter
restriction is trivial. You are talking about gross changes that are far
from normal.

>>> 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
>
> So if the filter is dirty enough then it will affect whether the
> emissions test is passed? This means the A/F ratio isn't optimum,
> doesn't it? If what you are saying is true then the computer should
> compensate and make the A/F optimum thus allowing it to pass the test.

Exactly. I can certainly imagine cases where a filter that is severely
contaminated could cause a modern fuel injected vehicle to fail an emissions
test, but this would be an exceptional case. For any reasonably well
maintained vehicle, with an air filter changed per the manufacturers
recommendations, you aren't going to fail an emissions test because of the
air filter. If you want to hypothesize a very restrictive filter, all bets
are off.

>> 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.
>
> You keep saying for "normal sorts of air filter restrictions" when a
> restriction is a restriction. A filter just doesn't not affect mileage
> one day and then suddenly becomes dirty enough to affect it the next.
> It is a gradual progression that happens continuously and in most
> circumstances is too slow for the driver to perceive.

By normal, I mean real world situations. Again, if you want to theorize
about some wacky almost plugged filter, then all bets are off. Once again -
unless you are operating near wide open throttle, any restiriction in the
intake related to the air filter is trival compared to the restriction of
the throttle plate.

> Also, the basic operating parameters of an engine doesn't change because
> it is fuel injected and computer controlled. The computer actually
> mimics the old carburetors, governors, points etc. by using sensor
> readings. The ECU controlling today's engines isn't a HAL 9000 that
> thinks like a human. If it is getting garbage input from the sensors
> then it spits garbage out to control the engine. The effect of a dirty
> air filter on a fuel injected engine is the same as one with a
> carburetor. On both engines the dirty filter puts the engine under load
> and it is this that decreases gas mileage in both cases.

THINK!!!!!!!!!!!!!!!!!!!!!! Why does a dirty filter impose any more load on
an engine than a partially closed throttle plate. Do you understand
carburetors? Do you know why they have a choke plate in front of the
metering jets. Do you understand how the choke enriches the mixture. Can you
see why for a carbureted engine a dirty filter might act like a choke and
affect the mixture. Don't you understand that none of this applies to modern
fuel injected engines? Carburetors depend on the Bernoulli principal to
meter fuel. The fuel in the bowl is under atmospheric pressure. The pressure
in the venturies is related to the flow through the venturies. If you place
a restriction in front of the venturies, you will pull an artificially high
vacuum in the venturies (higher than created by the Bernoulli principal),
drawing more fuel into the air stream. Anything (like a choke, or a plugged
air filter) affects the balance between the pressure on the fuel in the fuel
bowl and the pressure in the venturies will affect the fuel to air ratio.
This is why a clogged air filter can greatly affect the fuel economy of a
carbureted engine. A fuel injected engine determines the amount of fuel my
measuring a lot of parameters. None of these parameters is going to be
significantly affected by normal variations in the filter restriction.

>> 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
>
> I searched the PDF for "gas mileage" "mileage" and "mpg" and got no
> hits. It's a little too long for me to read through but it seems to
> address filtering efficiency for removing particulates and not the
> effect of dirty air filters on gas mileage.
>
>> http://www.filtercouncil.org/techdata/tsbs/89-3R3.html
>
> This also seems to be addressing filter efficiency for removing
> particulates and not gas mileage efficiency.

True. I just thought you might find it interesting. They were meant to
refute the idea that it was a good idea to change filters based on the
advise at the DIY Basics site you referenced. I thought it was bad advice.

>> 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.
>
> I have no doubt that a K&N filter lets more particulates by (and
> therefore more air itself) than an OEM filter. Where the debate starts
> is whether is has any appreciable impact on engine longevity for the
> average vehicle. I believe it doesn't based on my own experience. My
> '89 LX had had the same filter installed for over 130k miles and I had
> the heads off at around 150k miles and saw no appreciable cylinder wall
> wear. It also doesn't noticeably use any more oil than when it was new.
>
> The air filter debate is similar to the synthetic verses conventional
> oil debate. IMO, there is no appreciable difference in wear from using
> any oil that is changed every 3,000-4,000 miles. The contaminants never
> get a chance to build up in the oil to cause a problem whether the oil is
> synthetic or conventional.

This time I am not trying to debate how well a K&N filter "filters." I am
only arguing that there is no reason to expect a K&N filter to increase the
fuel economy of a modern fuel injected engine (compared to a paper filter in
reasonable condition).

Ed