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Matthew Wright

Do Air Filters Affect Mileage?

By June 18, 2010

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I was talking with Pat at Tiger Tool recently about whether or not a dirty air filter will actually reduce your fuel mileage. Traditionally, a clogged air filter is thought to rob your engine of not only power but MPGs. But is this theory based on outdated, carburetor technology? Here's how Pat summed it up. It's a little technical for the layman but I'd love to hear what people think:

A MAP based fuel injection system measures manifold pressure to estimate the amount of fuel required by referring to a reference table, doing a few calcs and then adjusting the injector pulse width. The main restriction is the throttle plate downstream of the air filter. Add in the effects of the closed loop system and I can't see why a dirty air filter would reduce fuel economy. I do know that power would be decreased. Also if the program went to open loop or other power mode at reduced vacuum then you would see reduced mileage. I think the whole dirty air filter mileage is a hold over from the carb days where a change in differential pressure across the venture caused a dramatic change in mixture ratio.

Even more so in any system that actually measures air flow through the use of a flap, hot wire or?. The actual air flow is measured and thus upstream changes should have no effect on fuel mileage.
So can we still state blindly that dirty air filters reduce fuel mileage!

Well put, Pat! But I'm sure there is still debate out there. Let's hear it!

Comments
June 19, 2010 at 1:47 pm
(1) Johnny says:

I’m sure lube shops who have a vested interest would disagree. Follow the money.

I do think Pat makes a sound argument.

June 27, 2010 at 6:43 am
(2) mike says:

easy to overlook Pats’ observation of less power, as we are discussing fuel economy. also the computer will read the throt.plate position and use in adjustment. changing the air filter should be determined on a individual basis [if you do it yourself]. if you let someone else do it ‘buyer beware’.
I live in an area with a high pine pollen season so I personally change the filters after the season is over. If you do it yourself it is not so expensive.

June 29, 2010 at 11:58 am
(3) TeeK says:

Oops, looks like the spellchecker strikes again. That should be “Venturi” and not “venture”.

I found Pat’s arguments very persuasive. The computer optimises the engine for a lean burn. Restricting the airflow with a partially-blocked filter will simply cause the computer to reduce the amount of fuel added. Just as Pat said, this will reduce the power available of course.

Then I realised there is an implication in that reasoning. You might be reducing power, but the computer is injecting less fuel into the engine… Might therefore a partially-blocked air filter actually result in LESS fuel consumption through reduced power and so actually INCREASED mileage?

I’m sure it’s not good for the engine, chokeing it like that though, so it is not something that would be good on a regular basis. Does anyone know for certain though if it might actually improve mileage?

June 29, 2010 at 12:20 pm
(4) Wayne says:

Air does not FREE-Flow into an engine. The motor has to provide the power to SUCK the air through the air filter.
More dirt in the filter equals more power from the motor to pull in the outside air.
Throw a blanket over your mouth and try to breath in.
With a dirty air filter, the engine has to work harder just to breath and that equals a loss of horsepower.

June 29, 2010 at 12:34 pm
(5) Bob from Santa Maria says:

The argument of increased fuel mileage with a dirty air filter simply doesn’t make good sense since the constant here is the speed of the vehice and the amount of energy needed to propel and maintain that speed. Bottom line I believe is that to maintain lets say 65 mph, with a dirty air filter more gas pedal pressure would be required to deliver the amount of fuel/air needed to create the same energy that a clean filter and less pedal pressure would provide given that more air is entering the MAP with a clean or K&N type filter. The gas mileage gained I think may be slight if any (1-2 mpg) whereas the power argument is unquestionable to me. I experienced this with my SLK 320 after changing to a cotton gauze filter. I also gained about 1/4 -1/2 inch of pedal while keeping my speed constant.

June 29, 2010 at 12:46 pm
(6) Dale says:

1. If you were to follow the logical conclusion here, assuming power is not an issue, no one ever needs to change his or her air filter.

2. The statement that electonic fuel injection will automatically optimize fuel and air mixture based upon available air flow is a given. However I would say that the intact mileage belief will be limited to what the engine rpm range will be. With a semi-dirty filter the available air is going to be less than with a new one.

3. Regardless of what we all say, for a car still under warranty, I would religiously change the air filter at the manufacturers interval whatever it is.

June 29, 2010 at 12:56 pm
(7) JERRY C. WHITE says:

all ICE engines are sophisticated pumps. Increasing the efficiency of air flow in any manner decreases pumping losses. Don’t forget the value of having clean air going into the cylinders, along with clean oil in the crankcase.

June 29, 2010 at 10:17 pm
(8) gzuckier says:

As pointed out clearly and correctly, the loss of efficiency in the increased restriction of a clogged filter is irrelevant in any situation where the throttle is partially closed, IN ORDER TO DELIBERATELY RESTRICT AIRFLOW to limit engine output and prevent the vehicle from accelerating to its top speed. It might have an effect on those hypermileage vehicles which have no throttle whatsoever and operate by intermittently running a tiny engine wide open to minimize energy lost in airflow, and then coasting. But that doesn’t describe anybody’s street car.

July 6, 2010 at 6:01 pm
(9) John says:

All I can say is that in over 40years of driving, I have never noticed any difference to my mpg when the filter is changed. I have found that climate and using a thin oil have a greater impact on mpg than any other factor.

July 7, 2010 at 12:16 am
(10) adam says:

i noticed even fueleconomy.gov has “debunked” this fuel economy myth.

A clean air filter will not gain you any MPG, _but_ they state it will help your horsepower…..I change mine every 12k….basic paper filter…

July 7, 2010 at 7:39 pm
(11) tom freda says:

put a pillow over your face. Hard to breath correct?
less power.. dirt that passes through the filter gets into the intake then mixed with fuel then into the clean chamber. dirt get wedged between the rings and piston. scoring the walls thus giving you less life out of your engine. less power. correct? let me know if i’m wrong. thanks..

July 8, 2010 at 9:23 am
(12) Jim in MA says:

For a few bucks, you buy the oil & air filter combo at
the A-Zone (whiiirrr-whirrrr) and you feel good about yourself and your steed; which is why electric cars won’t make with my generation, either.
REAL men put fossil fuel in that tank, pal; and no sissie synthetic oil for us!

July 23, 2010 at 1:21 pm
(13) Eric Anderson says:

Sorry to whom ever I offend but electric cars will never replace the gas or diesel vehicle. It should not even get the chance to be called a car. If you can’t hear that sweet rumble an engine makes or the ground vibrate underneath your feet. Then it is no car at all.

January 25, 2011 at 8:10 am
(14) Robin S says:

OK, the system will adjust the fuel flow to compensate for lack of air. This in itself won’t worsen the gas mileage.

But, as it will reduce available engine power, it will mean that more use of the throttle is required for any given speed. This will in turn make the mileage worse if you continue to drive at the speeds you normally drive at.

January 27, 2011 at 10:15 pm
(15) J.P. OWENS says:

A RUNNING ENGINE WITH NO LOAD MAY POSSIBLY HAVE NO MEASURABLE LOSS OF FUEL CONSUMPTION DUE TO A RESTRICTED FILTER.
AN ENGINE PERFORMING WORK WILL DEMAND A CERTAIN AMOUNT OF ENERGY TO PERFORM. A DIRTY FILTER WILL CAUSE THE ENGINE TO WORK A LITTLE HARDER TO MAINTAIN THAT PERFORMANCE USUALLY BY INCREASING ENGINE SPEED. THIS WILL IN TURN COMPROMISE MILEAGE. IF YOU DON’T MIND A REDUCED SPEED, HILL/GRADE CLIMBING AT A SNAILS PACE, OR NO FAST ACCELERATIONS, YOU MAY GET THE SAME MILEAGE, BUT AT THE EXPENSE OF PERFORMANCE.

August 9, 2012 at 10:02 am
(16) Pushkar Chintaluri says:

My car gives 20 mpg. Honda Accord I4 2.3L. That’s ridiculous.

Suspected many things, spark plugs, wheels to start with. But part of the service was replacing the air filter, that got me thinking. Does it increase mileage. Let me state some things here:

1. New air filter – clean air,
2. Computer is working with good data, provides a preferred lean mix of fuel and air.
3. Better combustion, good fuel efficiency.

The corrollary:
1. Cloggy air filter – limited, unclean air
2. Computer has inadequate data, hence mix is not to be trusted
3. The mix can be richer, leaner based on the fuel injectors function.
4. Combustion is not consistent, hence fuel efficiency takes a hit.

My knowledge of cars and ICEs is withering away and I make it sound like the air filter has everything to do with fuel consumption. Hence, I will concede that i will be wrong!

However, here’s my question: Does anybody have any actual data from experiments conducted with dirty vs clean air filters and the effect on gas mileage?

September 9, 2012 at 7:15 am
(17) Marc says:

My theory is that yes MPG doesn’t get increased, but if you as another user put (partially) are trying to accelerate from stop to 45, your throttle position (I have a scan gauge, so I know for a fact) has to stay at a higher position before you taper it down to where you need to maintain that speed. That having been said you’re burning fuel equivalent of going 75MPH for a good minute or two, but only going 0 to 45.

Instead when the hose power is maintained or improved, the “hard acceleration” is shortened to a position that could get you going 60 in lieu, and the duration from 2 minutes to probably a 1 minute.

End in end I’m sure the gain isn’t major, but I’m positive it’s not 0% either.

December 11, 2012 at 4:54 pm
(18) VinZ says:

I did some research. A clogged air filter in a modern car will reduce power and there is a measurable fuel saving. However the fuel saving is not as much as you’d expect, around 1%.
Here is the paper for download:
http://www.fueleconomy.gov/feg/pdfs/air_filter_effects_02_26_2009.pdf

I drive a high performance car and having an extra air filter reduces induction noise a lot. Now in the winter I don’t care about the loss of power – for obvious reason. And it is nice to drive a quieter car.

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