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Old 05-15-2008, 08:36 AM   #1
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K & N Air Filters on Diesel Engines

I recently installed a K & N Air Filter in my 2004 Silverado Duramax. On my first trip (not towing) my truck rerturned 16.8 mpg. On my second trip it returned 18.0 mpg. In both cases it was just a short 60 miles Interstate trip. Prior to installing the K & N my mpg was considerably better.
A friend of mine installed a K & N filter in his 1990 F350 diesel and got lower mpg, he contacted the local Ford dealer and was told not to use K & N filters.
Question: Will my mpg improve over time or is it normal to get lower mpg using K & N filters.
Would love to hear from other diesel truck owners that have used K & N air filters and learn of their experiences.


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Old 05-15-2008, 08:49 AM   #2
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I had a 1997 Ford F250 7.3 I put a K&N on it and it was a great improvement. I now have a K&N on it and my mileage droped 2 to 3 mpg but I also put a cap on the truck at the same time so I am not sure which one dropped the mpg. I have never heard of a K&N filter causing lower mpg. As far as your Ford dealer that dosen't suprise me as they don't make any money when people buy K&N filters.


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Old 05-15-2008, 09:59 AM   #3
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The K & N is designed to allow more air into the engine, which by SOP seems to have better throttle response, but clearly there is a gain in HP by using these types of filters. As the old saying goes, performance costs, so if in fact these do increase HP, I could see it taking an MPG hit. There is a million mile warranty on the filter element, but to be honest, less filtering would mean to me, more junk getting into the engine. For a gasser after about 150k, it's pretty moot, time for an overhaul if you want the exact output it had when it was newer. Diesels can last 3x that amount....

I have one and once you get use to it, it's hard to go back, but at $4 and $5 a gallon, I could learn and learn fast.
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Old 05-15-2008, 10:11 AM   #4
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That is suprising. Usually K&N "improves" air flow. They do this by having a higher pore size. Why this lowers fuel milage? It may upset the balance that the enginers programed into the computer?
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Old 05-15-2008, 12:02 PM   #5
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We've found two problems with K&N type air filters.
The first is over-oiling the element. This has been found to foul the Mass Airflow Sensor. Yes, even diesels have them now. A buggered up MAS can throw off the mixture, injection timing and boost pressures.
The second generally doesn't relate to our tow vehicles but involves the use of cold air intakes. The "tuners" mount the intake tube with a bare filter element mounted low behind the bumper. Works great a sucking puddles up into the engine on rainy days.

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Old 05-15-2008, 02:30 PM   #6
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I’m with Tom N on this one, I stopped fooling around with after market stuff a long time ago. Most of it is a ripoff, offers no measurable gains, typically sacrifice one area of performance for “gains” in another, and will void your warranty under many circumstances. If nothing else, you tend to put your foot into more after making mods, either to “verify” your investment (“ooohhh feels fast!”), or because it “sounds cool” (exhaust mods), so forget about mpg gains. These vehicles are so highly engineered now there are no mods needed. If 365 hp and 660 lb/ft of torque are not sufficient, you need to get a dragster. If mpg is your concern, slow down.

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Old 05-15-2008, 03:15 PM   #7
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There have been many tests performed on air filter elements. Suprise! The best overall filter element is the stock AC Delco.
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Old 05-15-2008, 03:41 PM   #8
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I used a K&N filter on my 97 Sub for years while on the east coast, not as much dirt as we get credit for. On a trip out west someone told me that was not a good idea as the K&N with it's larger air flow also allowed larger particals to pass through. Sure enough after laeving Utah I looked in the air tube, after the filter, and found the walls covered with fine sand. Thats when I bought a stock filter.

As far as air flow. If you are running a diesel I assume you have a boost and EGT gages, if not you should have them installed. Watch the gages while pulling a load with the stock filter than watch it with any after market filter. If you are seeing the same boost levels the engine is getting the same amount of air. Unless you see a significant decrease in boost levels and an increase in the EGT there is no advantage. Surely no advantage if you are in a dusty area as the piston rings will ware out early from the sand that passes through the K&N. K&Ns are great at the drag strip where you are turning 7,000 RPMs and intend to tear down the engine after each run.

I believe the MAP sensor is looking at the quality of the air, humidity and barometric pressure, rather than the volume of air. Those plus throttle position and air intake temperature combine to define the fuel ratio and thus yor MPG
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Old 05-15-2008, 05:11 PM   #9
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Originally Posted by HowieE
I believe the MAP sensor is looking at the quality of the air, humidity and barometric pressure, rather than the volume of air. Those plus throttle position and air intake temperature combine to define the fuel ratio and thus yor MPG
MAP sensor is a Manifold Absolute Pressure sensor. It senses the amount of vacuum (or pressure) inside the intake manifold, either by direct sampling (mounted directly on the manifold) or by remote mounting with a vacuum hose running to a port in the manifold. It also extrapolates the barometric pressure based on RPM, engine load, and volume of air sensed by the MAF (Mass Air Flow) sensor. The Mass Air Flow sensor uses a heated element to sense the volume of air that passes by it. The more air, the cooler the sensor. The temperature difference is measured using the Air Charge Tempurature sensor mounted upstream of the MAF sensor.
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Old 05-21-2008, 10:40 AM   #10
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Having spent almost 40 years in the high performance aftermarket industry, I have to take issue with BillTex on the subject of aftermarket performance parts. Installing a K&N filter is fine. However, installing a K&N filter charger with a cat back exhaust system will provide maximum power and economy. Never forget, an engine is nothing more than an 8 cylinder air pump. Allow more air in via a K&N filter and you have to let more air out via a better exhaust system. There are plenty of quality high performance parts out there for your truck/SUV that will produce power and improve mileage when they're correctly installed. I have K&N filters and/or K&N filter chargers on all my vehicles as well as Petronix ignition systems on my collector cars and hot rods. On the later model model stuff, I installed low restriction mufflers and larger diameter exhaust pipes. BTW, I've installed a number of cold air intakes with the scoops mounted under the front bumper and never had a problem with water getting into the plenums. Seems to me that some of these conditions mentioned above might stem from bench racing after having been over served at the beer tent or user error.
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Old 05-21-2008, 05:22 PM   #11
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No bench racing. Only 36 years as a dealership tech. We had two engines this spring alone with bent rods from water ingestion. One we fixed the other was towed out. Both with puddle sucker cold air intakes. User error, maybe, but look back on some of the peculiar situations you’ve found yourself in while on the road.
The claims of added horsepower by just bolting on a few parts are IMHO bogus. I’m looking at a can of Mass Airflow Cleaner sitting on my box right now that claims "Proven to Gain 4-10 Horsepower at the Wheels!*". (Note the asterisk.) Any gains that may be possible by bolt-ons to a modern car or truck cannot be achieved without reprogramming the software map of the PCM to take advantage of the modification. Now you’ve opened yet another can of worms.
In the turbo diesels we’re talking about, power comes from balancing injection quantity and boost pressure. Neither will be measurably affected by just an air filter.
Free flow cats and fatter exhaust pipes can be argued to add some horsepower but the result on a gas engine is usually a loss of the low-end torque needed when towing.
SEMA and the auto manufactures are often at odds over what is best for any particular vehicle. I think the point should be if you’re a couple hundred miles from home and have a problem you would be better off having the TV as close to stock as possible.
You want OEM parts or a generic equivalent available ASAP and don’t want to risk confounding a mechanic who may have never seen some of these modifications.
Like HowiE said. We’re not at the track. We’re just a bunch people looking to spend what leisure time we can get by going camping. I'll keep things simple.

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Old 05-21-2008, 07:42 PM   #12
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I have heard that the stock ACDelco air filters can't be beat for a balance of air flow and filtering for the newer Chevy and GM diesel trucks. On my truck, O7 3500, I left the air filter stock and invested in an after market free flow muffler with larger pipes. I don't have gauges, but using this combination should keep the EGT's low while towing.
Also the K&N filters can be a problem for the MAP when the owners re-oil and spray too much oil on them and as the air gets sucked past the sensor it oil coats the MAP in short order causing problems.
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Old 05-21-2008, 09:29 PM   #13
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Two problems with K&N:

- The filter oil contaminates the Mass Air Flow (MAF) sensor, causing it to read incorrectly and mess up the mixture. Eventually the "check engine light" comes on.

- The higher airflow -> less filtering. The contaminated air can gradually wear out the engine, although at a very slow rate.
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Old 05-21-2008, 10:00 PM   #14
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More flow has to have bigger holes and bigger holes will let more dust in.
This dude took the time to prove it; Filtration Testing for Amsoil, K&N, Napa, Jackson Racing, Baldwin, and Mazda air filters on a Miata

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