Re: Mustang GT and K&N air charger

C. E. White wrote:
"Michael Johnson" <cds@xxxxxxxxx> wrote in message news:XtydnbbcFtUV9xPanZ2dnUVZ_uWlnZ2d@xxxxxxxxxxxxxxx

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).

Does the load tables in the ECU increase A/R as the load increases? Does the throttle position sensor help the ECU determine the engine load? Does a dirty filter require the throttle to be further open to make the same amount of power as it does with a clean filter? Does running the engine at a lower A/F decrease gas mileage?

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?

You are splitting hairs. An air filter can operate within parameters for 15,000+ miles but it is progressively getting more resticted and progressively decreases gas mileage. If you chose to you could drive around without an air filter at all and you'll get better mileage that if you used an air filter. EFI cars have BAP (Barometric Air Pressure) sensors to allow for adjustment of A/F due to air density changes from varying altitudes. This tells the ECU that the engine isn't necessarily under an increased load because driving in high altitude areas requires the throttle to be more open compared to near sea level driving.

The core question is does a dirty air filter decrease gas mileage? It does. How much depends on the amount of dirt in the filter and its restriction to flow. The highest rate of flow for an air filter is when it is new. After that it decreases and therefore mileage decreases. The reason is that as the filter gets dirty it causes the ECU to use ever more richer target A/F ratios based on its hard wired load tables in the program code because the throttle position has to be progressively more open as the filter collects more dirt in order to make the same amount of power.

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 throttle position readings are much finer than you think. The load tables (there are more than one) in the ECU have numerous rows and columns to read from. Going from 0.2 volts to 0.98 volts is is a large range when you are reading to the hundredth of a volt. If the air filter restriction is enough for the ECU to jump just one slot then mileage has been decreased. There are enough "slots" that it can read for the gradual drop in mileage to be imperceptible from one tank of gas to the next. However, if you compare mileage over a greater time interval you can see a difference. Most people use visual inspection to determine when the filter gets changed and not mileage decreases. If it looks dirty then it is probably time to change it before the mileage decrease becomes too high and/or noticed.

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.

Yes it does because a dirty air filter mimics the engine being under a load from the viewpoint of the ECU. That is how the ECU reacts and why gas mileage falls.

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.

If you change the air filter before it gets too dirty then you won't notice the small decrease in gas mileage that occurs over the filter's useful life. What happens to gas mileage when you fail to change the air filter on a timely basis?

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.

So you are agreeing that a dirty filter does decrease gas mileage (or effect emissions) and the degree it is decreased depends on how dirty the filter may be?

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.

As an air filter becomes dirty it gradually decreases mileage. It is a trade-off between acceptable mileage decrease and the economics of changing the filter. In the era of quick lube places checking filters with an oil change, most people never have an air filter get so clogged that it causes a drastically noticeable change in mileage or black smoke coming out of the tail pipe. None the less, gas mileage does decrease over the life of an air filter.

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.

You are giving too much credit to an ECU's ability to "think". It operates from tables and not from computing Bernoulli's Principle thousands of times a second. It reads sensor data, looks up a value(s) from a table and then outputs commands based on those table values. Those tables don't take into account a dirty air filter in any way. If the air filter is dirty enough to cause the throttle to be more open then it can only read this input as the engine being under a load. Then is reads from the load table to run at a lower A/F ratio. This decreases gas mileage. Sometime by a little and sometimes by a lot depending on how dirty the filter may be. The ECU doesn't have "the filter may be dirty" parameter anywhere in its programming.

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/whitepapers/2005_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.

They all want you to replace consumable parts more often then needed. That makes them more money.

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).

It depends on if the K&N flows enough for the ECU to think the engine is under a lighter load than when using an OEM filter. If it does then mileage will increase. I would guess that a K&N would decrease mileage at a slower rate than an OEM filter would because it will flow more air if both filters receive the same amount of dirt. Whether it is noticeable is up for debate.
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Relevant Pages

  • Re: Mustang GT and K&N air charger
    ... it believes the optimum air fuel ratio should be taking into account 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 ... show a minor variation as a result of a change in air filter restriction. ...
    (rec.autos.makers.ford.mustang)
  • Re: Mustang GT and K&N air charger
    ... it believes the optimum air fuel ratio should be taking into account 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 ...
    (rec.autos.makers.ford.mustang)
  • Re: Mustang GT and K&N air charger
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    (rec.autos.makers.ford.mustang)
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