From some recent posts, it seems folks on this forum have finally figured out that installing a high efficiency air filter, like the K&N, doesn't, and shouldn't improve fuel economy. Older posts on this same forum are filled with folks swearing they should improve mileage, and that they actually do. So, if you're trying to improve your mpg, save your money.

Unless your filter was REALLY dirty, such that you found yourself at, or near full throttle most of the time, these filters should actually make your mileage a little worse. For those of you who think you saw better mpgs, it might be interesting to discuss why you think so. Maybe there was something else you did at the same time.

From some recent posts, it seems folks on this forum have finally figured out that installing a high efficiency air filter, like the K&N, doesn't, and shouldn't improve fuel economy. Older posts on this same forum are filled with folks swearing they should improve mileage, and that they actually do. So, if you're trying to improve your mpg, save your money.

Unless your filter was REALLY dirty, such that you found yourself at, or near full throttle most of the time, these filters should actually make your mileage a little worse. For those of you who think you saw better mpgs, it might be interesting to discuss why you think so. Maybe there was something else you did at the same time.

If you want to save money K&N filters and any air filters that are cleanable are ways to improve performance but not mileage.

In 1996 I purchased a K&N filter ,,still in use today ,,this has saved me from purchasing a new filter 12 times now...I feel I got my use out of it..and saved money ....paper filters plug up too easy...

well, because i am very observant and check my mileage routinely. so, on brand new 2002 honda cr-v, k&n air filter improved mpg by 1 mpg. on the same vehicle, k&n typhoon air intake dropped it down exactly 2.5 mpg, which is 10%. then i restored it back and even improved some, with 2oz of acetone per 10gal of gas.
i have 1.5 year long mileage log and mpg was checked with digital obd2 scanner.

Hi ukrkoz!

That filter really should have made your mileage worse. Have you ever calculated the standard deviation of your fuel mileage numbers? How many tanks of gas did you run with the k&n filter to conclude that your mileage was 1 mpg better?

To kinda figure that out, post your mpgs on your last 10 to 15 tanks. Then, how many tanks did you go through after putting the k&n filter in? What was the mpg for those tanks?

i drive a 99 silverado.... i drove it for about 6 months before i put a k and n cold air intake on it.... after i added that intake i got 2.5 mpg better. so you cant tell me it doesnt help.... you dont know what your talking about... all my friends are into cars and trucks.... we all run k and n intakes on our trucks and cars. and we all get about 2 mpg better... we have 2 mustang gt's with them. a ford power stroke. duramax diesel and my silverado. all have got better mpg..

i drive a 99 silverado.... i drove it for about 6 months before i put a k and n cold air intake on it.... after i added that intake i got 2.5 mpg better. so you cant tell me it doesnt help.... you dont know what your talking about... all my friends are into cars and trucks.... we all run k and n intakes on our trucks and cars. and we all get about 2 mpg better... we have 2 mustang gt's with them. a ford power stroke. duramax diesel and my silverado. all have got better mpg..

If you replace a paper air filter with a new K&N air filter, and get a large increase in MPG you are not doing you regular maintenance...

even K&N says that its the length of use that makes the K&N a better performer..over the paper ...if the paper filter is clean their is no difference in anything...

i drive a 99 silverado.... i drove it for about 6 months before i put a k and n cold air intake on it.... after i added that intake i got 2.5 mpg better. so you cant tell me it doesnt help.... you dont know what your talking about... all my friends are into cars and trucks.... we all run k and n intakes on our trucks and cars. and we all get about 2 mpg better... we have 2 mustang gt's with them. a ford power stroke. duramax diesel and my silverado. all have got better mpg..

If you replace a paper air filter with a new K&N air filter, and get a large increase in MPG you are not doing you regular maintenance...

even K&N says that its the length of use that makes the K&N a better performer..over the paper ...if the paper filter is clean their is no difference in anything...

Hey 99silveradoz71 !

I sure do know what I'm talking about. Ask your buddies WHY the K&N filter should impove mileage. Modern engines should control the air/fuel ratio at a constant, and mileage should be the same.

Older engines without engine management systems are different. Installing a higher flow filter could make the engine run lean and improve fuel economy. So, if you and your buds learned about cars with these older engines, or if y'all were taught by someone who used to work on older cars, they may not have figured out the difference.

Now, this doesn't absolutely mean your mileage didn't get better. And this is what I really want to find out. Your mileage shouldn't have gotten better. So why did it? Is it because you made a math error? Did you remember your mileage incorrectly? Did you make other changes to your truck at the same time? Maybe the engine management system doesn't do a good job of controlling air/fuel and allows the engine to run lean. Maybe it's just the power of suggestion. I don't know.

Have fun proving your friends wrong. Bet money or a case of beer, or something

i check my mpg everytime i fill my truck.... when i put that cold air intake on i didnt do anything else to my truck... before i got that cold air intake i also got a steady mpg every tank fill up... and ever since the day i put my colr air intake on i still to this day get about 2.5 mpg better.... i could notice the diff as soon as i took it for a drive after i put it on. it had a little bit more pep to it. and no i didnt do a math error...i check my mpg everytime i fill up i do it before i even leave the gas station.
i know that a higher flow filter can make you run more lean that is why you get the better mpg.

Now we might be getting somewhere. My understanding is that our engine management systems are what are generally known as 'closed loop feedback control systems'. The O2 sensors provide the feedback, and adjust the fuel to keep the air/fuel ratio constant.

Are you suggesting that the EMS doesn't keep the air/fuel constant, but instead, allows the richness/leanness to move around within a range?

Here's a little test of your car knowledge. When you step on the gas peddle, are you setting the gas flow or the air flow?

Here's a little test of your car knowledge. When you step on the gas peddle, are you setting the gas flow or the air flow?

That kind of depends on what you mean by stepping on the gas pedal. In most cases you are setting the air flow. The PCM/VCM (not EMC) then adjusts the fuel flow as needed and determined by the TPS, MAF, etc.

There was a thread about 3 years ago that had a link to a test run by some well know names (Icouldn't find it to post here) comparing K&N, etc to stock AC-Delco, Fram, Wix, etc. K&N lost big time. But then again, who do you really believe in these test results.

I ran K&N on all my dirt and street bikes and loved it. I've got trophies that will say the K&N's made it happen rather that my skill.

I really ? the K&N's on todays cars. I ran a stock replacement and a FIPK system on my '98 K2500 7.4 Sub. Bottom line was maybe more HP with the FIPK at WOT but none of the two produced more MPG and the FIPK was way too loud. I've been back to stock configuration for 3 years now and do not regret it in the least.

I've seen articles comparing the K&N to other filters, but it was comparing how good of a job the filters did at filtering. Pretty predictable results. In general, the more you filter, the higher the pressure loss, and the lower the flow. Same as filters for home A/C systems. The K&N's let alot of crap through. The K&N's got the same (or a little less) mileage and let more dirt through, but had more horsepower at WOT.

I still don't believe that the electronic controls on modern engines will let them run lean, if they're working right.

[quote=worstmechanic] In general, the more you filter, the higher the pressure loss, and the lower the flow. Same as filters for home A/C systems.

The K&N's let alot of crap through. The K&N's got the same (or a little less) mileage and let more dirt through, but had more horsepower at WOT.

I disagree with K&N filters letting debris thru...I keep my vehicles a very long time ..the filter does a great job of keeping the intake passageways from dirt..I have used a white cloth and found the interior air ducting clean...paper also works the same...

problem is with vehicle owners not following K&N directions on cleaning and oiling...this is where your MAF will get contaminated...then you will find problems with MPG and performance....cleaning the MAF every 20-30,ooomi is necessary to keep the computer working correctly to adjust for maximum effeciency..

If the air filter was very restricted you would not have any power,as the computer would reduce the fuel flow on this type of fuel management to a very small amout so as to keep the mixture ratio correct..

I'll put my nod to K&N as well (note my current stable below). I keep mileage records and when I added a K&N filter to the wife's Envoy, there was a marked increase in mpgs...not an incredible amount, but definitely noticeable.

But at the very least as mentioned before, the savings from not buying paper filters (not to mention the environmental benefits) eventually wins out for those of us who hold onto cars till they die.

I haven't the time to ponder all the ramifications of it, but to me: more air is good. How the ECM determines how it is to be exploited depends on the needs of the driver's foot and what the truck can deliver. Better performance would be more obvious at WOT than slower speeds since more air can be sucked in with less restriction (try breathing hard through a straw versus a snorkel). So most of the time, you won't see big gains in hp. But every little bit helps when you need to git up & go.

I haven't the time to ponder all the ramifications of it, but to me: more air is good. How the ECM determines how it is to be exploited depends on the needs of the driver's foot and what the truck can deliver. Better performance would be more obvious at WOT than slower speeds since more air can be sucked in with less restriction (try breathing hard through a straw versus a snorkel). So most of the time, you won't see big gains in hp. But every little bit helps when you need to git up & go.

I love it! I think the reason so many folks bought K&N filters WAS because folks didn't take the time to ponder whether their claims for improved gas mileage could be possible.

At 60 mph, your engine should use the same amount of fuel and same amount of air whether it has a stock filter, K&N, or no filter. You'll have the same pressure drop through the air intake system in all cases. If you have no filter, you will use less throttle. You'll have less pressure drop through the air filter box, but more through the throttle plate, since it is not open as much.

I'm still really curious about the claims of better fuel economy. What could cause this? Is it all just psychological? Is the richness/leanness controlled at an exact number, or is it just controlled within a range?

The dirty MAF affecting mileage seems a little strange, too. If the MAF reads wrong, it should be picked up by the O2 sensors downstream. Then, a fudge factor should be applied to the MAF reading so that the engine continues to run at the same air/fuel mixture. When you clean the MAF, the engine will temporarily run rich or lean, but the feedback from the O2 sensors should correct it pretty quickly. Folks shouldn't see an mpg improvement for MAF cleaning, either.

Okay here it goes...

The engine makes power by burning fuel. The more fuel it can burn the more power it makes. But it needs air to do that. The air comes from the atmosphere at a certain density through the air intake, which creates a vacuum in the intake system since only so much air can flow through it. The intake vacuum is an air charge lower in density than atmospheric.

When you put say a paper filter inside that intake, that reduces the effective cross-section of the intake, restricting flow, creating a higher vacuum thus reducing the density of the air past that filter. Lower density of air means less mass per unit volume going into the cylinders with each stroke, meaning that less fuel can be burned, meaning less power.

By switching to a filter that provides a less restrictive medium, more air can pass through, reducing the pressure differential created and provides a denser air charge for the engine to burn. Ideally, sucking air through a straight, wide, smooth, no-restriction intake will provide the highest density of air for burning...smooth, non-turbulent flow for even air charging.

Now I know what you're thinking...great we're making more power by burning more fuel...but isn't that BAD for fuel economy? The answer is not necessarily. Remember that the engine makes power in cycles...only a certain amount of torque can be generated with each rotation of the crankshaft. The more power you can get out of each rotation, the fewer rotations needed (i.e. lower rpms) to do the same work. If the engine can do the same amount of work but at a lower rpm, then the engine is more efficient and is using less gas over time.

This basically translates to a little bit less getting on the gas when you are moving...maybe not perceptible...because the car can do the work of moving itself through the air using a lower rpm and less throttle. Of course, alot of people will install a FIPK or fancy filter and then test it out to see if it makes more perceptible power. They stand on it and race it around and then wonder why they got crappy mileage. Why? Because they were looking for power they could feel. Only nitrous will do that, air filters don't. They're a quiet helper, and usually do the most good powerwise when the engine is revving higher and needing alot more air FAST.

If you want to test a filter for mileage improvement, you have to maintain your driving style before and after install for an accurate result...not to mention your vehicle too. The impact it makes is subtle. So, changes and tuneups can affect results, even a pothole can throw alignment out of whack and change the way the car drives. So can colder or warmer air...so can tire pressure...so can many things. What I'm getting at is you either try to control everything or you have to accept that there are periods where it will run better or worse.

Where some filter installs fail to improve the top end is when they are installed into the stock intake. You'll get a modest improvement in the mid-range but then will experience the straw-sucking limitation of the OEM intake as revs go higher.

Well, now that I've rambled on, I hope made it a bit clearer. I'm sure I'll edit this later, but hope this helps!

The dirty MAF affecting mileage seems a little strange, too. If the MAF reads wrong, it should be picked up by the O2 sensors downstream. Then, a fudge factor should be applied to the MAF reading so that the engine continues to run at the same air/fuel mixture. When you clean the MAF, the engine will temporarily run rich or lean, but the feedback from the O2 sensors should correct it pretty quickly. Folks shouldn't see an mpg improvement for MAF cleaning, either.[/quote]

your comments I agree with,,, except the MAF ....If the MAF is dirty it will cause a loss of performance....the MAF response is much faster and also before the combustion chamber ...to use only the O2 which has a delayed control on the fuel/air ratio would not be maximum in effeciency...

I studied this MAF in the service manual , and internet.....I followed the instructions on cleaning...not that I had any problems...I inspected the MAF with a magnifing glass....It looked perfect..I removed cleaned the small sense wires and tested ...I was so surprised at the response of the throttle,very fast response...

later a few days of driving this vehicle I then did this cleaning to my other vehicle and this vehicle reponded the same way....

My MPG I measure very often....what I noticed was a slight 1 mpg improvement as I never got 20mpg with my silverado...never...

I was getting 18-19.5mpg...also I did not change any parts or clean anything...when this was done..

the interval of cleaning the Maf is done now whenever the air filter is serviced...and the product I use Is CRC MAF cleaner...

The engine makes power by burning fuel. The more fuel it can burn the more power it makes. But it needs air to do that. The air comes from the atmosphere at a certain density through the air intake, which creates a vacuum in the intake system since only so much air can flow through it. The intake vacuum is an air charge lower in density than atmospheric.

When you put say a paper filter inside that intake, that reduces the effective cross-section of the intake, restricting flow, creating a higher vacuum thus reducing the density of the air past that filter. Lower density of air means less mass per unit volume going into the cylinders with each stroke, meaning that less fuel can be burned, meaning less power.

By switching to a filter that provides a less restrictive medium, more air can pass through, reducing the pressure differential created and provides a denser air charge for the engine to burn. Ideally, sucking air through a straight, wide, smooth, no-restriction intake will provide the highest density of air for burning...smooth, non-turbulent flow for even air charging.



Probably only true at full throttle. Remember that there is feedback from the O2 sensors, and also, don't forget about the restriction caused by the position of the throttle plate.

Let's say you're cruising down the road at 60 mph using 50% of the throttle (gas pedal 50% down and throttle butterfly valve half way open). Now let's say, that by magic, in an instant, while driving down the road, the filter became less restrictive. The mass air flow sensor (MAFS) would see an increase in air flow. Since the engine electronics are trying to control the air/fuel ratio, it would increase the fuel flow. More fuel and more air causes your speed to increase, so you lift your foot off the gas at little. Lifting your foot off the gas is really just closing off the throttle butterfly valve a little. The MAFS sees a lower air flow, and the engine control keeps the air/fuel ratio constant by reducing fuel flow. Now, the engine is using the same amount of fuel and the same amount of air to go 60 mph as it was before. The only difference is that the throttle may only be open 45%. All you've done is moved some of the pressure drop from the air filter to the throttle plate.

And, do I understand you right? Are you saying that you think you can change how many rpms it take to go 60 mph just by changing the air filter?

....If the MAF is dirty it will cause a loss of performance....the MAF response is much faster and also before the combustion chamber ...to use only the O2 which has a delayed control on the fuel/air ratio would not be maximum in effeciency...


Hey j cAT!

I think you're really on to something. You're right about the response time. At something called 'steady state', which means that everything is held absolutely constant for a long time, a dirty MAFS shouldn't affect fuel mileage. The O2 sensors, which are the feedback for the engine control would eventually get everything to the right air/fuel ratio. But, in reality, things are constantly changing. The MAFS is part of what is called the feed forward, and like you said, is very fast compared to the O2 sensors, which are part of the feed back.

I wish I could see the control program. (I used to write them for other applications.) It probably has a target O2 reading. The engine control program looks like it does a much better job of maintaining the O2 sensor reading when the MAFS is clean. On average, the engine must run rich when the MAFS starts getting dirty, even though it's still trying to control the O2 level.

So, can the opposite be true with the K&N filter? At steady state, it shouldn't have any effect. But dynamically, it may. Maybe the filter affects the accuracy of the MAFS reading in a way that makes the engine run lean.

Are folks cleaning the MAFS at the same time they're replacing the K&N filter? Maybe they're giving the credit for the extra mpgs to the wrong thing.

I saw a website called 'ecomodders' that has alot of folks talking about modifications to improve the fuel economy of their cars. Maybe I'll through these ideas their way to see what they say.

The MAF is very critical in the air/fuel ratio...unfortunately is will throw a dtc only when it is really messed up....

GM and other manufacturers don't mention anything about servicing this sensor...So the internet once again does the job of providing information to get the most out of our vehicles...

I still can't believe how that cleaning improved the engine/throttle response.....

these high air flow filters when used with vehicles hauling/towing heavy loads will probably see an increase in MPG...as the engine is working to produce the max HP....with most owners this would never be an operating environment that they would be seeing any difference...

the reason I got the K&N was because it lasts the life of the vehicle...so you save from buying the paper every 10-12,ooomi..

on the reason why some may see a difference between paper and k&n ....the paper was dirty...do you think they would replace a new clean paper filter with another filter..I don't..then they see a difference..

over time the MAF gets dirty and if the vehicle owner does NOT properly clean and oil the K&N,,,, it is very possible the MAF gets oiled as well and this is not good..

I don't think I did...I said more work could/would be done at a given rpm...i.e. a higher horsepower output, albeit slight. Speed and rpms are more or less governed by the tranny gear ratio and/or the torque converter...don't twist words.

I doubt the average person would think to clean the MAF sensor upon changing out to a K&N filter. But it's not out of the realm of possibility they cleaned it sometime afterwards.

In all my years and following proper cleaning/oiling instructions for my filters, I've never bothered to clean the MAF. Maybe I'm lucky, maybe it didn't matter in my vehicles, or maybe for once I did something right...never seen a drop in performance after a filter service. Just me I guess....

In all my years and following proper cleaning/oiling instructions for my filters, I've never bothered to clean the MAF. Maybe I'm lucky, maybe it didn't matter in my vehicles, or maybe for once I did something right...never seen a drop in performance after a filter service. Just me I guess....

that WAS my exact thinking until last year...Now I'm believer...MAF cleaning is going to be done every air filter service interval...on my vehicles..