A new air filter will increase fuel economy. Myth?

OK. This may or may not be a controversial topic, but I wanted to put it out there for discussion. I will state that a restricted air filter will not reduce fuel economy on a modern, fuel injected vehicle. I will state that installing a high flow filter mod will not improve fuel economy. I will state that a filter has to be SO restricted on a modern vehicle to have any effect on fuel economy that the vehicle will be in the shop for performance concerns before that happens.

Agree or disagree? More importantly I want to know why you believe what you do. Do you have any actual data to show that you can back up your position. Data must be controlled enough to rule out other variables.

Flame on

I do not intend for this thread to be a fight, but a discussion. I will post my reasons for my statements soon. Let's all be cool and see what the discussion is. I put this in "technical chat" on purpose.

 

When I installed a K&N factory replacement filter and noticed about a .5mpg gain according to my scangauge compared to what I was averaging beforehand. I would do the same 500 mile round trip highway trip repeatedly every couple weeks, so after several times I could see the trend.

 

A restricted filter may not do much to reduce efficiency directly in terms of AFR, but you're likely to hit the pedal a little harder while driving with it to compensate driving feel with a restricted system. Also intake restrictions can create a "negative boost" effect which makes the engine do a little extra work to suck air in compared to a clean or high-flow filter.

In my personal experience the difference in throttle response is much more pronounced than mpg differences. I still run a K&N drop-in though; even if it doesn't save gas, it'll eventually save money on replacement filters.

 

My un-scientific experience is as follows:

Several months after buying my truck I put on larger tires at which point I started having trouble staying up to highway speeds in 5th gear. After changing to a K&N drop in, the problem went away and gas mileage went back up a little.

 

So you notice performance issues before a fuel economy complaint. Does that sum it up? Chris, are you a tech? I notice that a lot of your posts tend to have a technical flair. Another question. Is the Moderator thing a new gig for you? I did not notice it at first. If so, congrats.

Are you talking about the mpg calculator in the unit? How does that compare with actual math on miles and gallons. I do not have one of those units and wonder what data it used for calculations.

So it sounds like you are talking about pumping losses in the engine here. That is one area of loss if the engine is fighting to breath and would seem to make sense. The other point you make is that it will not likely effect AFR. That I think is the main point. The MAF will still see the correct amount of air flowing by so the base fuel calculation will still be accurate.

The throttle response/performance increase is what I see as well.

Not to be a stickler, but how much? I have this saying that "good is not a specification" so I usually like numbers. Again, not trying to be a tool, just liking numbers.

I have to take a trip today, so time is limited. I will post a summary of a more modern test that was conducted. I think you will find it interesting.

Thanks

 

Went by about 1mpg.

 

Ok, here is what I was thinking. The marketing that we see these days about air filters that increase fuel economy is based on a study from the 1970's that was done in France. Think about how cars were built back then. That's right, they had carburetors! Now a restricted air filter on a carb vehicle will act much the same way as driving around with the choke on. The restriction will cause it to create more of a low pressure area in the venturi and increase fuel flow. Those systems were set and could not adapt to current conditions.

Now let us fast forward to modern cars. They are fuel injected. Not only that, but the system is a feedback system that is constantly recalculating the correct fuel rate for delivery to the engine. The ECU (engine control unit) calculates the actual air flow or load on the engine and injects the correct amount of fuel to keep the engine running a optimal mixture rates. Even with a restricted filter, the MAF (mass air flow sensor) will see the correct amount of air flowing by and the base fuel calculation will be accurate. If it is not a MAF vehicle, the MAP (manifold absolute pressure sensor) will see the actual air pressure in the intake that will fill the cylinder with air. Again, the base fuel calculation will be accurate or close to it. Even if we are off a little, the O2 (oxygen sensors) or AFR (air fuel ratio sensors) will see the over or under fuel condition and the computer will trim to optimal.

What this means is that the load on the engine (driver demand) is still being read and compensated for so the fuel control is accurate.

There was an actual test of this done in a lab in Colorado that is used to certify emission control levels on cars in the US. The lab is able to capture all the gasses that come out of the tailpipe during a simulated run. Since matter is not created or destroyed during combustion, by measuring these gasses a calculation of what was burned can be done very accurately. The test ran a vehicle for a base line with a new air filter and the fuel economy was calculated. The filter was removed and one quarter of the surface area was duct taped over. The test was run again and the economy calculated. It was the same. This continued until the filter was 75-80% taped over. On that last test the fuel economy actually went UP a small amount. The very experienced dyno test driver said that the vehicle was running very poorly and he had to be gentle on the pedal to get the car to run through the drive trace. (essentially this was a right foot mod!) The increase (slight) in economy could be attributed to the driving style, but the real point is that there was a performance loss noticed before any decrease in fuel economy was measured.

A little about the two gentlemen that ran these tests. One is a PHD that can talk combustion chemistry at the "bend your brain level" and the other is a well known shop owner that also does technical training.

Thought you might find this interesting.

 

I put in an AFE pro dry. It lugs better at low/manouvering speeds, seemed to do better on hills in 5th, but ill still downshift as needed (cyl btw) , and my mpg seemed to drop. I'd say I agree with o.P. on mpg gain/loss.

My expectation for the filter was, a hopeful mpg gain of +1mpg or so, and maybe power gains. I'm not dissapointed in the filter, but might go back to the factory filter and see if i get some mpg back.