MPG "myths"

[KrisZ]

I also think that people change the air filter too often. I think the article is right, they didn't say not to change the filter. Usually the first question is "did you change the air filter". If the filter is still within the manufacturer recommended interval why would you change it?
I check my filter regurarly (about twice a year) my car is 2 years old and still on the original filter, no change in the fuel mileage, no change in performance. I pulled it out after winter, it looked good, I vacuumed the leaves out and it went back in. Now I don't live in a dusty area, but a lot of people that live and drive in similar conditions change their filter every year or sooner. Changing the filter sooner then recommended is actually worse than keeping it a little bit longer than the recommendation. The reason is that a brand new filter has a worse filtering capability then the one that is slightly dirty.

 

[meep]

I absolutely agree with Test_Drive and XS650 in *modern* FI cars. The MAP and MAF systems read the air flow or plenum pressures as well as O2 sensor, and balance the A/F ratio regardless of *where* the throttling comes from. Plus, the restricted filter is much more noticeable under acceleration than cruise.

If this were not the case, all cars driven at higher elevations would get cr@ppy mpg.

If I recall, some of the first FI systems might bias more towards the internal tables than system feedback, so this might not be the case with them, but I'm not certain.

Now, fewer restrictions on exhaust will undoubtedly raise mpg.

M

 

[Redlightning]

I for one am taking no chances on the lower octane gas in my '01 Lightning. This is one example of a modern vehicle that doesn't have a knock sensor,and this truck is very gas sensitive. At the first sign of boost on sub-par gas i get a light spark knock so I keep a couple cans of octane boost on hand just in case.

 

[Drew99GT]

My theory is, when your air filter is gummed up, you in fact do have to get into the throttle more to maintain the same amount of performance. ie, the engine is under more load.

Try running a marathon with a dust mask on and tell me if if you feel more spent than without it (ie, you burned more calories).

I live in a very dust place and by 15,000 miles, air filters are quite loaded. Replacing them always brings the fuel mileage back of, along with throttle response and power. If they are really loaded with dust, it makes a pretty substantial difference.

Throttle does matter. More throttle, all else being equal, means more engine load. Think about it in a larger case: Trying to cruise down the interstate with an air intake opening 1/4 the size it is now. You're gonna have it floored to do 70 mph. It's gonna use a ton of gas compared to normal because the engine will be under much more load.

 

[WishIhadatruck]

Originally Posted By: Drew99GT
Throttle does matter. More throttle, all else being equal, means more engine load. Think about it in a larger case: Trying to cruise down the interstate with an air intake opening 1/4 the size it is now. You're gonna have it floored to do 70 mph. It's gonna use a ton of gas compared to normal because the engine will be under much more load.


The load is no higher. You still only need to put out enough power to maintain 70 mph and that power requirement won't change with a smaller intake. As long as the engine can compensate and maintain the same Air/Fuel ratio then you will open the throttle to get enough air to make the power and use the same amount of fuel. The clogged filter or your example of a small intake opening will be part of the throttle that already exists in your intake.

 

[Bamaro]

If you read each item closely, I find nothing wrong with them.

 

[XS650]

Drew, your analogies don't fit the real world.

 

[punisher]

Originally Posted By: Bamaro
If you read each item closely, I find nothing wrong with them.


Truth.

Quote from article that sums up fuel saving devices:

"Likewise, car companies are already spending big bucks to increase fuel mileage. If General Motors could make its cars go significantly [censored] on a gallon simply by putting a device into the fuel line, don't think for a second it wouldn't be doing that. GM's car sales would go through the roof.

"There are a number of these gas-saving devices that are generally useless," says Champion.

But drivers who try them will swear they work. In reality, it's probably an automotive placebo effect, says Reed. Buy one of these devices or additives, and you're like to pay extreme attention to your fuel economy and how you drive."

 

[rpn453]

Originally Posted By: Drew99GT
Try running a marathon with a dust mask on and tell me if if you feel more spent than without it (ie, you burned more calories).


It won't make any difference as long as I'm still breathing in the same amount of air.

 

[OVERKILL]

Originally Posted By: meep
I absolutely agree with Test_Drive and XS650 in *modern* FI cars. The MAP and MAF systems read the air flow or plenum pressures as well as O2 sensor, and balance the A/F ratio regardless of *where* the throttling comes from. Plus, the restricted filter is much more noticeable under acceleration than cruise.

If this were not the case, all cars driven at higher elevations would get cr@ppy mpg.

If I recall, some of the first FI systems might bias more towards the internal tables than system feedback, so this might not be the case with them, but I'm not certain.

Now, fewer restrictions on exhaust will undoubtedly raise mpg.

M


Base-table is typically used for WOT (which can be TPS position over 3/4 throttle for example). Ammendments to base-table are done from the CL tables over time as the ECM "learns" the engine.

So, if you have a car that goes wicked-rich at WOT either due to say high-side-of-spec fuel pressure or restricted breathing, as long as the deviation is not outside the ECM's means of correction, then it will eventually tune it out using the feedback system.

MAF systems are superior to SD systems in this regard, as they will "learn" the airflow characteristics of the engine, and adjust for them MUCH quicker than SD, which relies simply on estimates and calculations based on manifold pressure.

If you drive aggressively, then a plugged filter CAN, and likely WILL affect gas mileage. If you drive like an old lady and the car never sees a throttle point where the pressure-drop over the filter is relevant due to throttle position; IE, the blade is never open enough to reveal the restriction in the element, then yes, it will have little to no bearing on economy.

 

[Black Bart]

Originally Posted By: OVERK1LL
Originally Posted By: meep
I absolutely agree with Test_Drive and XS650 in *modern* FI cars. The MAP and MAF systems read the air flow or plenum pressures as well as O2 sensor, and balance the A/F ratio regardless of *where* the throttling comes from. Plus, the restricted filter is much more noticeable under acceleration than cruise.

If this were not the case, all cars driven at higher elevations would get cr@ppy mpg.

If I recall, some of the first FI systems might bias more towards the internal tables than system feedback, so this might not be the case with them, but I'm not certain.

Now, fewer restrictions on exhaust will undoubtedly raise mpg.

M


Base-table is typically used for WOT (which can be TPS position over 3/4 throttle for example). Ammendments to base-table are done from the CL tables over time as the ECM "learns" the engine.

So, if you have a car that goes wicked-rich at WOT either due to say high-side-of-spec fuel pressure or restricted breathing, as long as the deviation is not outside the ECM's means of correction, then it will eventually tune it out using the feedback system.

MAF systems are superior to SD systems in this regard, as they will "learn" the airflow characteristics of the engine, and adjust for them MUCH quicker than SD, which relies simply on estimates and calculations based on manifold pressure.

If you drive aggressively, then a plugged filter CAN, and likely WILL affect gas mileage. If you drive like an old lady and the car never sees a throttle point where the pressure-drop over the filter is relevant due to throttle position; IE, the blade is never open enough to reveal the restriction in the element, then yes, it will have little to no bearing on economy.
Congradulations OVERKILL it is good to see that someone on this forum understands how a FI engine works.
After reading this thread it is clear that some don't have a clue.

 

[BrianWC]

I've been planning on trying 87 octane and running off boost as much as possible to see if I can maintain my current mpg figures. So far, dropping my interstate speed 8 mph and staying off boost (still using premium) has gained me 5-6 mpg. My last tank I got 31.5 and I'm on track for around 33 mpg so far this tank. I would be around 35 but had to use the a/c a lot today.

 

[JHZR2]

regarding the dirty filter - one of the main inefficiencies in an IC engine is "pumping losses", right? i.e. the loss due to having to move air.

Nothing is free. A higher pressure drop across the air filter means more "suction", i.e. work must be done to move the air across the media. Therefore, efficiency is effected.

However, given the fuel mapping, I suppose there is some level of restriction that the ECU is optimized to. The OE wants a good "filter cake" if they want longevity... there is a point where the pressure drop knees and skyrockets, below that, it is splitting hairs.

That said, I agree with Overkill - if the throttle is never opened enough for it to matter - then the effect may never be seen. An extremely crude analog of this might be putting a coffee filter over the tap. If the tap only drips, water will pass through at about the same rate that it shows up. If you turn the tap on, youll either flood the filter or break it (or the tap will spurt out the sides) because the pressure drop is too high for the required flow.


JMH

 

[Drew99GT]

Originally Posted By: XS650
Drew, your analogies don't fit the real world.


And you live in an alternate universe.

 

[Drew99GT]

Originally Posted By: WishIhadatruck
Originally Posted By: Drew99GT
Throttle does matter. More throttle, all else being equal, means more engine load. Think about it in a larger case: Trying to cruise down the interstate with an air intake opening 1/4 the size it is now. You're gonna have it floored to do 70 mph. It's gonna use a ton of gas compared to normal because the engine will be under much more load.


The load is no higher. You still only need to put out enough power to maintain 70 mph and that power requirement won't change with a smaller intake. As long as the engine can compensate and maintain the same Air/Fuel ratio then you will open the throttle to get enough air to make the power and use the same amount of fuel. The clogged filter or your example of a small intake opening will be part of the throttle that already exists in your intake.

DUH, and to maintain that same air fuel ratio, if you're not getting as much air as before, you must open the throttle more. More throttle = more load + input from the TPS = more fuel usage. ***, is this rocket science? Why don't y'all go out and throw a bunch of dirt in your air filter and run your engines for 1K. Report back with your fuel mileage.

Better yet, put your money where your mouth is and don't change your air filter. After all, it's become common folk lore round here that a dirty air filter filters better...

 

[XS650]

Originally Posted By: Drew99GT
Originally Posted By: XS650
Drew, your analogies don't fit the real world.


And you live in an alternate universe.


Obviously not the one you live in.

 

[-Clayton-]

If throttle position doesn't match up with the incoming air I'm sure mileage may suffer.

 

[rpn453]

Drew99GT lives in a universe where a restriction on the intake results in the engine running rich because the engine controls are incapable of adjusting air/fuel ratio to compensate.

XS650 lives in a universe where the engine controls accurately regulate air/fuel ratio and are capable of doing so even with a restriction on the intake.

I'd really like to see a scientific study to determine which universe I live in. I suspect it falls somewhere in-between, and depends on how dirty the filter is. Until then, I'll just go 30K miles on each filter like my manual recommmends. They don't look too dirty by that time and I don't think they're affecting fuel economy yet.

 

[Duffman77]

Originally Posted By: OVERK1LL
Originally Posted By: meep
I absolutely agree with Test_Drive and XS650 in *modern* FI cars. The MAP and MAF systems read the air flow or plenum pressures as well as O2 sensor, and balance the A/F ratio regardless of *where* the throttling comes from. Plus, the restricted filter is much more noticeable under acceleration than cruise.

If this were not the case, all cars driven at higher elevations would get cr@ppy mpg.

If I recall, some of the first FI systems might bias more towards the internal tables than system feedback, so this might not be the case with them, but I'm not certain.

Now, fewer restrictions on exhaust will undoubtedly raise mpg.

M


Base-table is typically used for WOT (which can be TPS position over 3/4 throttle for example). Ammendments to base-table are done from the CL tables over time as the ECM "learns" the engine.

So, if you have a car that goes wicked-rich at WOT either due to say high-side-of-spec fuel pressure or restricted breathing, as long as the deviation is not outside the ECM's means of correction, then it will eventually tune it out using the feedback system.

MAF systems are superior to SD systems in this regard, as they will "learn" the airflow characteristics of the engine, and adjust for them MUCH quicker than SD, which relies simply on estimates and calculations based on manifold pressure.

If you drive aggressively, then a plugged filter CAN, and likely WILL affect gas mileage. If you drive like an old lady and the car never sees a throttle point where the pressure-drop over the filter is relevant due to throttle position; IE, the blade is never open enough to reveal the restriction in the element, then yes, it will have little to no bearing on economy.

This is the correct explanation, when your car is in open loop mode it is running rich and at this point the engine wants as much air as it can get.

 

[Gary Allan]

Originally Posted By: XS650
Originally Posted By: Drew99GT
Total horse [censored]. Dirty air filter isn't bad for fuel mileage? Whiskey Tango Foxtrot...


How would it be bad for mileage, assuming it isn't so plugged that the fuel injection system can't maintain the proper mixture?


I'm with XS650 here. If most of you were old enough, you would remember when they STOPPED using poor fuel economy as a sales point for air filters (they've since brought it back - but it's more like "90% of wear occurs at startup" type thing). It's when they went to in internally (inside the filtered air area) vented fuel bowl. Before that, if your air filter caused you to draw neg 5" W.C. ..the externally vented fuel bowl attempted to balance to atmosphere.


Now I'd say that it's reduced to fractional hp losses due to increased effort to draw the air in (adding a couple "W.C.)