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Old 10-12-2011, 11:04 AM   #1
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K&N Filter and Mileage

Hey All!

I must be the worst back yard mechanic in the world! I can't figure out why, on a modern engine, a dirty air filter should affect mpg. Conversely, why should a K&N filter improve mpg?

Don't modern engines control fuel/air mixture? If a filter passes more air at a given throttle setting, doesn't fuel flow increase. How does more gas, which results in faster acceleration, improve mileage? From what I can see, it's more likely that a dirty filter will improve mpg.

The only way I can see a K&N filter improving mpg on a modern computer controlled engine is if it somehow 'fools' the computer. Does the MAFS get fooled? Maybe the engines are set up to run at a air/fuel range instead of a hard target. Maybe, then, a change in air flow through a filter, can let the engine run richer or leaner?
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Old 10-14-2011, 02:04 PM   #2
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Re: K&N Filter and Mileage

You are correct in that modern engines monitor the amount of incoming air via a MAP and/or a MAF sensor to meter the correct amount of fuel for the existing running, WOT, etc. So, in most cases a high-flow filter such as from K&N won't result in much improvement.

Driveability and lower long-term maintenance costs are more the reason I use them. I drop them in and forget them now because where I live dust is not a major issue (though spring pollen is!). But when I lived in Brownsville, TX, I did have to clean them regularly as my truck would become more and more sluggish as the filter loaded up. As you note, the fuel/air ratio won't change, but the engine's ability to suck in air and consume fuel to make power when desired is affected...what I term driveability.

In some extreme cases such as where it is dry and dusty, the filter can become so loaded up that the air flow is low enough to be "off the map" of the engine's ECM programming and forces the engine to run in what is called "open loop" mode where those sensors are ignored and the fuel/air mix is a pre-defined map mainly based on the engine RPM. The open loop condition is usually an increasingly fuel-rich condition to maintain power and driveablility at the expense of economy and the emissions equipment longevity (lots of excess fuel dumped into the cat converter results in very high temps, evenutally melting the ceramic honeycomb).

Hope this helps!
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