Air Filter Restriction, effect on economy.

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https://www.fueleconomy.gov/feg/pdfs/Air_Filter_Effects_02_26_2009.pdf

Time to ditch the K and N, and forget about the airfilter until the restriction gauge tells you to look at it (actually, a torn filter won't trip the restriction gauge, so you still need to look).

Quote:
The goal of this study was to explore the effects of a clogged air filter on the fuel economy of vehicles operating over prescribed test cycles. Three newer vehicles (a 2007 Buick Lucerne, a 2006 Dodge Charger, and a 2003 Toyota Camry) and an older carbureted vehicle were tested.

Results show that clogging the air filter has no significant effect on the fuel economy of the newer vehicles (all fuel injected with closed-loop control and one equipped with MDS). The engine control systems were able to maintain the desired AFR regardless of intake restrictions, and therefore fuel consumption was not increased. The carbureted engine did show a decrease in fuel economy with increasing restriction. However, the level of restriction required to cause a substantial (10–15%) decrease in fuel economy (such as that cited in the literature3,4) was so severe that the vehicle was almost undrivable. Acceleration performance on all vehicles was improved with a clean air filter.

Once it was determined how severe the restriction had to be to affect the carbureted vehicle fuel economy, the 2007 Buick Lucerne was retested in a similar manner. We were not able to achieve the level of restriction that was achieved with the 1972 Pontiac with the Lucerne. The Lucerne’s air filter box would not hold the filter in place under such severe conditions. (It is believed that this testing exceeded the design limits of the air box.) Tests were conducted at a lower restriction level (although still considerably more severe than the initial clogged filter testing), allowing the air filter to stay seated in the air box, and no significant change was observed in the Lucerne’s fuel economy or the AFR over the HFET cycle.

Closed-loop control in modern fuel injected vehicle applications is sophisticated enough to keep a clogged air filter from affecting the vehicle fuel economy. However for older, open-loop, carbureted vehicles, a clogged air filter can affect the fuel economy. For the vehicle tested, the fuel economy with a new air filter improved as much as 14% over that with a severely clogged filter (in which the filter was so clogged that drivability was impacted). Under a more typical state of clog, the improvement with a new filter ranged from 2 to 6%
 
I went 73,000 miles on the last air filter in my car. Although fuel consumption was not affected by this long change interval, I believe two other things were.

With a dirty filter, the engine started to ingest oil from the "fresh air breather" of the PCV system. With the throttle opened wide, not enough air was available from the filter and it drew more air from the breather attached to the top of the valve cover. This oil coated the throttle plate and settled in the intake manifold. Replacing the filter cleared most of this up.

Secondly, the engine seemingly made more torque with a fresh filter. I am not saying this is true but it acted this way. When I came to a slight hill, a greater throttle opening was required to maintain speed with a dirty filter. This caused the transmission to downshift more than I would like. A clean filter allowed more air to enter the engine [producing more power] without opening the throttle enough to call for a down shift.
 
That would make sense. The system measures the amount of "clean air" and adjusts the fuel mixture accordingly. While your fuel economy wasn't overly hurt, with a decrease in air flow, your computer also gave it a decrease in fuel to correct the mixture, and thus less air+less fuel= less power
 
Originally Posted By: Jetronic
I'd love to see this test with a turbocharged engine...


My STI has a road-vacuum airbox inlet on it. It always has bugs, small stones, grass clippings, leaves, and any other random detritus from the road embedded in the filter. I can tell when the filter starts to get full, my boost threshold gets higher and higher in engine RPM. I swapped the filter out and it felt like a different car. I noticed absolutely no difference in fuel economy from a nearly clogged filter to a brand new one. I know it's not scientific, but in my application clogged filter = torque loss (only)
This car has a pretty sophisticated EFI system with a wide-band 02 sensor as well as the standard issue MAF system. I think it's pretty hard to trick them into running poorly anymore.
 
I have a vnt turbo, and I'm afraid the turbo will just make more revs trying to get upto the desired pressure reducing usefull life and increasing exhaust manifold pressure.

EGR will also be more effective, introducing extra soot into the intake and ultimately increasing the soot in the dpf aswell resulting in more dpf regens and extra wear on the engine.

Less performance/worse mileage would be the least of my worries....
 
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