I recently had a jump in fuel economy, and I'm trying to figure it out.
The vehicle is a 2008 Duramax, and I wasn't getting any better than 12.5 mpg (which was disappointing).
I recently changed the air filter, and the fuel economy jumped to 15.5 mpg. An oil change later, it bumped to 16.2 mpg.
The old air filter wasn't horrible. Visibly dirty, but not horrible.
Can an air filter make that big a difference?
Yes. I've had noticeable bumps in economy from a air filter change. Doubly true when going from a nasty paper filter to a new K&N.
I thought I got a bump in fuel economy in new wheel bearings in my old Miata.
I consistently got a 3mpg bump when i changed the oil in my protege5. Air filters never seemed to change anything. I think that may be a product of filter surface area to engine demand. You have a smaller ratio of "extra" with the duramax, where the protege shares a similar surface size being used by a 2.0
My old Land Rover topped out at around 60mph. I replaced the oil bath filter for a K&N and could easily do 75mph even with the Range Rover diffs. Probably extremes in filtering and not necessarily mpg-related, but the air filter had an impact and I'd expect mpg to be improved because the engine isn't working as hard as it was.
Diesels and carbed gassers will lose mpg from a dirty air filter. On a fuel injected gas engine, a dirty filter will restrict maximum power output but it won't do a damn thing to mpg.
rslifkin wrote: Diesels and carbed gassers will lose mpg from a dirty air filter. On a fuel injected gas engine, a dirty filter will restrict maximum power output but it won't do a damn thing to mpg.
SWMBOs focus has one of the lifetime filters. I pulled it out, shook the leaves and water out, blew it out and threw it back in. The average mpg on the dash went from 26 to 31 and stayed there. It pulls harder too. There could have been something else going on but this is what happened to me. I've heard of better intakes increasing mpgs on all sorts of cars as well. What's your reasoning?
In reply to spandak:
On an EFI gas engine, a restricted filter will reduce max airflow (reducing max power). However, the computer has feedback on how much air it's getting (or at least manifold pressure) as well as fuel mix (from the O2 sensor). So it shouldn't end up running richer or anything and should burn about the same amount of fuel to output a given amount of power.
A carb-ed gas engine or a diesel that doesn't strictly limit fueling based on manifold pressure will end up running rich at times due to the restricted filter and will both make less power and burn more fuel (and in the diesel's case, it can get smoky).
On an EFI car, a change in MPG due to an intake is usually due to different airflow causing the MAF to read differently or a change in air temp (hotter air will slightly reduce pumping losses and can increase mpg, so some of those short ram intakes that suck hot under-hood air will help mpg a bit).
Alls I know is that way back in the day I had a Ford Explorer for a company car. Had it up in the Mesabi Iron Range. V8 with 3.73 gears. It ran like a dog when I got it from its former caretaker. I pulled the air filter and it was a solid black brick of cake. Our corporate credit cards at the time were to be used solely for travel and living expenses. I called the boss to see if I could use it at Advance Auto for a new air filter. He was like "...well you should be careful about that...". Screw it. I tossed the blasted thing in a dumpster and drove home. Holy Mother of Moo Moo. For an SUV that thing would sink you back in the seat. Gas mileage? Pft never checked it to begin with since it didn't come out of my pocket. Ran that stupid thing filterless for a year until I transferred out to a desk job closer to home.
20% increase from a filter seems unlikely.
In reply to rslifkin:
Speaking of pumping losses on gas engines, air filters create pumping losses as well. The more restrictive the filter is, or becomes, the more pumping losses would occur as a result.
Driven5 wrote: In reply to rslifkin: Speaking of pumping losses on gas engines, air filters create pumping losses as well. The more restrictive the filter is, or becomes, the more pumping losses would occur as a result.
You'd think that, but you end up opening the throttle further to get the same airflow / power, so the total restriction stays the same at a given power output. You just end up with more restriction from one source and less from another. For a given engine, pumping losses are pretty constant at a given AFR, RPM, air intake temperature and power output combo.
Of course, total restriction in the system at WOT will be higher, so max power output will be limited.
z31maniac wrote: 20% increase from a filter seems unlikely.
That's what I was thinking.
I would think with todays stuff, you are going to see an increase not in steady driving, but in acceleration. Yes, the computers will keep the engine from running rich, but with lower power from a lack of air, you are going to be accelerating LONGER to get to a set speed. This might not matter on the highway, but in stop and go traffic, it could add up
In reply to mad_machine:
That's possible. Although considering how slowly most people accelerate relative to what the car can do, I doubt it's significant (they'll just put their foot down slightly further to maintain the normal snail-pace).
My car went from 18mpg to 20mpg average over a 2500 mile period with changing too a slightly larger and clean filter. i think the quality of filter might have some effect as well.
rslifkin wrote: In reply to mad_machine: That's possible. Although considering how slowly most people accelerate relative to what the car can do, I doubt it's significant (they'll just put their foot down slightly further to maintain the normal snail-pace).
Around here they still do the stop light dragstrip. Light turns green and they floor it!
In reply to mad_machine:
Aw man. I wish people drove like that around here. Here, they crawl off the line, crawl through turns, etc. I've had times where someone in front of me has crawled up to speed so slowly that I can't even steadily keep my foot on the throttle until 20+ mph in the Jeep because "throttle just barely cracked" is still faster than they're accelerating 
Will a clogged filter work just the opposite of a ram air or turbo?
Sucking through a straw will create higher vacuum and less dense air, that will act like thin mountain air.
Using a CAI and ram system offers some increase in performance, for the inverse of thin mountain or hot air as it offers more dense air.
I totally believe that a filter change can improve MPG.
SVreX wrote: The vehicle is a 2008 Duramax, and I wasn't getting any better than 12.5 mpg (which was disappointing).
I'm curious, is that about average for those trucks? Or even the 15-16mpg you see w/ the new filter? That seems absolutely abysmal, but I stopped paying attention to any of the new stuff once I bought my diesel ('01 Dodge/Cummins). I average something like 19mpg overall, and that usually includes a fair amount of stop and go.
In reply to NorseDave:
It's lower than what I heard others get before buying it. I was expecting 17-18.
I had no trouble getting 16-17mpg from my 2005 Duramax, and I drove it like it was a company truck.
I had an AFE intake and after I cleaned my filter it was blowing smoke like a mofo. I had to clean the MAF and drive it for 50 miles or so before it was back to normal.
If your filter was severely dirty, maybe it is affecting your MAF sensor or fuel trims?
I found there to be much more variance in the quality of diesel than gasoline too.
rslifkin wrote: In reply to mad_machine: Aw man. I wish people drove like that around here. Here, they crawl off the line, crawl through turns, etc. I've had times where someone in front of me has crawled up to speed so slowly that I can't even steadily keep my foot on the throttle until 20+ mph in the Jeep because "throttle just barely cracked" is still faster than they're accelerating
oh they still slow down to a crawl for the corners, but they tend to rocket off as soon as the road straightens. Nothing more annoying for somebody who likes to keep momentum up
SVreX wrote: Can an air filter make that big a difference?
On a Diesel? Absolutely.
On a gasoline engine car? Nope.
Tyler H wrote: I had an AFE intake and after I cleaned my filter it was blowing smoke like a mofo. I had to clean the MAF and drive it for 50 miles or so before it was back to normal.
Is the AFE filter the one that looks like some weird mesh instead of a pleated filter?
I've seen those have hard failures. They suck themselves in so they look like a jack-o-lantern on November 10th. It always happens in the spring, so what I think is happening is, when it is coldish, very humid, and the dewpoint is near freezing, the humidity freezes in the pressure drop across the filter, which makes a restriction, which makes it freeze harder, and pretty soon the intake is constipated with ice and the filter gets sucked shut.
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