Clean vs. Dirty Air Filter MPG.

Discussion in 'Fuel Economy' started by Pirate, Jun 25, 2013.

  1. worthywads

    worthywads Don't Feel Like Satan, I am to AAA

    How are you accurately measuring that? As I already posted there is a good study that shows pretty conclusively that even a near totally clogged air cleaner does not reduce mpg, therefore a higher flowing air filter can't help either.

    I'll post again.
  2. RedylC94

    RedylC94 Well-Known Member

    Modern cars with functioning closed-loop fuel systems (i.e., since the early 80s) automatically compensate for the restriction of a partially clogged filter to maintain proper fuel-air ratio. Power will be reduced, but that's another issue.
    Even my GLC, which slightly predates such systems, shows no noticeable improvement in power or gas mileage when I've replaced air filters at over 4 times the recommended 30,000 miles.
  3. worthywads

    worthywads Don't Feel Like Satan, I am to AAA

    Been there, done that, know better.
  4. The Doctor

    The Doctor Old Dude, New Car

    10-4 on that!

    I don't get it! Why some people seem to get off, on posting false info and then trying to justify it, with some Expert posting from somewhere on the internet.

    But I won't get sucked into this senseless argument.

    And, I calculate my MPG with entries into a great little Excel Spreadsheet, that calculates the average MPG for the last tank of gas used, and even calculates the cost per mile. It's also a great place to record every penny I spend on my car and the date I spent it. Then I save all my paper receipts in a file folder.

    I also use my Scan Gauge II, to tell me if I'm pressing the GO Pedal too hard.

    Thanks Wayne, for a great forum. I love my Scan Gauge II !

  5. RedylC94

    RedylC94 Well-Known Member

    No, you don't know, because you don't know the extent to which flow was restricted. That's what matters, and it depends not only the distance traveled, but also on the size of the filter relative to (average) air flow rate, and on the (average) concentration of dirt in the air. Of those three factors, the only one you know is the distance. Have you measured pressure drop across a filter?

    I was using a filter designed for a much larger engine.

    Filters ideally should be changed when they reach a certain pressure drop at max power, not according at some fixed mileage. To make things easy for us though, manufacturers assume average conditions and specify a mileage, with a footnote to change more often in dusty conditions.
  6. worthywads

    worthywads Don't Feel Like Satan, I am to AAA

    Must not have made myself clear, I was saying I tried changing my air filter more often years ago, saw no change in mpg and now know better than to waste my money changing filters more often.

    I decided to go to a 60K change interval and at 40K on current filter I see no difference.

    Now there is also a precise study that confirms what I found out, that air filter doesn't matter, especially considering I rarely even exceed 2000 rpm. and never find myself at maximum power.
  7. RedylC94

    RedylC94 Well-Known Member

    Worthywads, thanks a lot for the clarification. I'm relieved to learn you agree, after all, at least as I understand your latest post.

    If we efficient drivers use less fuel per mile in our cars than the manufacturer assumes, we also consume less air, and therefore collect less dirt in our filters. That means less restriction than the manufacturer allowed at the recommended change interval, in addition to the valid point you made about our tendency to avoid using maximum power.
  8. phoebeisis

    phoebeisis Well-Known Member

    Worthy- wording was unworthy!!

    Just couldn't resist that pitiful pun!

    I wasn't sure what you meant either-since you two seemed to be agreeing.

    The " fancy filter CAI improved my FE 15%" idea will never die since it "seems" intuitive less restriction means better FE-MAKES SENSE??
    And explaining that the total resistance -ISN'T changed-usually doesn't sink in.
    Even in the old days-with carbs-it didn't work -your foot just opened the throttle plate a bit more to get the same charge in-
    Yeah-this idea will never die-not while folks make money selling fancy filters and noisy CAIs
  9. RedylC94

    RedylC94 Well-Known Member

    Thanks, Charlie.

    Actually, "it" did work "in the old days" with non-feedback (pre-1981, about) carburetors, because, even assuming the same air flow rate and same overall restriction, high resistance of a filter would decrease pressure above the throttle plate, thereby increasing fuel flow. In other words, a badly clogged filter would cause the fuel-air ratio to veer rich, and that's probably the particle of truth behind the persistent misleading claims for trick filters, or for needlessly frequent replacement of standard filters. Even for cars of that vintage, in normal driving the fuel consumption difference between a new filter and one with a few miles isn't necessarily significant.
  10. phoebeisis

    phoebeisis Well-Known Member

    You have a point there.
    Not sure just how the decreased flow velocity (from increased cross section) would effect the fuel flow where the jet actually opened into the carb venturi-can't remember exactly where those jets "entered" the venturesome had a couple of openings

    But you might be right-in old carb days-bit of truth to it since they were jetted with a clean filter in mind-no one is going to re-jet for a dirty filter.
    On the other hand-the dirty filter might have a governing effect-NASCAR LIKE

    you are probably right-grain of truth and the aftermarket folks have run with it despite it being untrue for 30 years.
  11. RedylC94

    RedylC94 Well-Known Member

    Somewhat so, in that both would limit air flow and hence power. However, the clogged filter is upstream of fuel entry, like a choke. Restricted air+unrestricted gasoline=much CO and unburned hydrocarbons!
  12. phoebeisis

    phoebeisis Well-Known Member

    but slightly different-
    with a choke the throttle plate is nearly closed
    dirty filter is like a choke plate-but throttle plate can be open-
    meaning decreased airflow velocity-which will slightly increase absolute pressure at the jet opening into venturi-relatively decreasing fuel flow

    But I gotta generally agree with you -a dirty filter on old style carb-might richen mixture
    depends a bit on just where the jets enter the venturi-relative to throttle plate

    Carbs were so much more complicated than EFI with mass air sensors and O2 sensors-amazing cars ever ran with those things!
    But you might be right-dirty filter-slighten richen mixture

    The pressure at the cross section right at the jet opening-will be more bigger-with a dirty filter-so flow velocity will be less-meaning pressure difference will be less-lessening fuel flow-
    pressure drop right downstream of the dirty filter will never change for a given CFM-since cross section never changes- near the venturi-different story

    But I think you are probably right-slightly rich dirty filter
    Can't resist arguing of course!
    PS Double amazed those carbed cars ran sooo well considering.
    PPS better argument for you-is we were told that at altitude sea level jetted cars ran rich-
    and I certainly KNOW from experience-Denver and west-they got HORRIBLE FE.
    So opening the throttle plate more sure must have richened it more.

    Wondering if it is more a pilot jet vs main jet "thing"-MJ circuit becomes relatively more important-and on MC- the needle has much smaller cross section-so it plugs the MJ circuit less as it opens more-
    can't remember HOW Holley handled that problem
    MC had an actual needle-can't say I remember the equivalent in car carbs-but there MUST have been

    The needle setup- would pretty much negate my entire argument-since it effectively makes the main jet circuit WIDER- as you open the throttle-forgot about that-it is how MC carbs dealt with increasing cross section under throttle plate-which would normally decrease fuel flow-car carbs MUST have had something similar?? Can't remember what it was???-there sure wasn't a needle hanging in the carb throat??

    So yes-you are right
    What did car carbs have-that was the equivalent of the needle plugging the MJ circuit?
    been 1978 since I fooled with a car carb-maybe 2007 since I fooled with MC carb

    Thinking out loud-delaying looking for sub teaching job-SS-62-I can be picky now

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