MPG "myths"

[Drew99GT]

Throttle position equals engine load - all else being equal.

I'll say it again. You fellas who don't think a clogged air filter lowers your fuel mileage - don't change it for 50,000 miles. Track your mileage.

I live in an area with a lot of dust. I regularly drive through dust storms. I track my fuel mileage and the condition of my air filter. A visibly dirty air filter = less fuel mileage and engine performance.

This is actually laughable at this point...

 

[Drew99GT]

Originally Posted By: 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.


You live in a universe where your head is up your [censored]. Perhaps if you slow down and don't drive as fast, then fuel mileage will remain the same with a dirty air filter.

 

[Gary Allan]

YMMV, Drew. I think the main thrust is that you're not going to run rich due to a clogged air filter. It will probably reduce power levels ..and trap times on more "free flowing" air filters have shown this. However given the VME of most engines, the most an air filter can alter is the engine's perception of a higher altitude ..or so I reason.

Our Wrangler Wix air filter is rated @ 210CFM (they don't give a W.C. at that volume). I think a 4.0 liter engine @ 100% VME is good for 400CFM @ 6000 rpm (it may be 5000 rpm).

I guess a W.C. gauge would show the difference in "draw" between a clean and not so clean filter.

 

[glennc]

I think people are not realizing that there are two separate issues being discussed. You could separate them into volumetric efficiency and thermodynamic efficiency. It is correct that modern fuel injection systems will compensate for the lowered amount of air, so the mixture will be burned appropriately and thermodynamic efficiency will not be affected.

However, an engine's overall efficiency depends on both this, and volumetric efficiency - pumping efficiency. A restriction of any amount in flow into or out of the engine will affect volumetric efficiency and therefore overall efficiency.

My argument when it comes to air filters is actually that this effect is not important until the filter gets a lot dirtier than most compulsive-maintenance types think. There is an insignificant diminishment in efficiency, and a significant increase in filtering effectiveness, that will eventually evolve into significant diminishments in efficiency - probably sometime after the manufacturer's replacement interval is exceeded.

However, to say that there is NO decrease in efficiency as the filter ages is wrong. It ignores the basic physics of pumping a gas. To put it in the simplest possible terms, if it is even slightly harder to draw in intake air, the pistons, during the intake stroke, must do more work. Remember from high school physics, work = force x distance. They travel the same distance, but must exert more force because there is a slightly greater vacuum against which they are working. This small increase in work required is present on every intake stroke. Work is the same thing as energy. The engine expends more energy to draw in air - regardless of how perfectly the air is then mixed with fuel - and that increase in energy requires that slightly more fuel be burned.

Altitude is yet another, separate issue. Cars get better mileage at altitude not because the engine is more efficient but because it must do less work - there is less air to push out of the way.

 

[Lumberg]

I do know that if one is in decel (zero throttle pressure and engine below 1200 RPMS) most post-1994 cars cut fuel deliver to zero.

So some people who turn off their engines while decelerating are making things worse.

But if you know you're stopping for a light that will be red even once you roll to a stop, then maybe.

 

[Drew99GT]

All I know, is I've driven through 2 haboob type dust storms in the last 2 months. I mean, you literally could not see your hood. Had to slow to a crawl on a US highway.

My air filter already had about 10,000 miles on it - I checked it recently and it was covered, absolutely covered with dust and tumbled weed debris.

A new air filter renewed power and fuel economy as measured by the tried and true odometer mileage/gallons used. Simple as that. And it works that way everytime.

 

[Audi Junkie]

Drew, come on...you know you are in the minority with that dust condition where you be at.

 

[Kestas]

I keep reading threads above which mention that dirty air filters result in overly-rich mixtures. I'm surprised no one has mentioned O2 sensors and their function.

If air is restricted by a dirty air filter, the throttle opens a bit more to compensate for it. Extra fuel is NOT dumped in... the O2 sensors sees to that and makes adjustments so the air/fuel mixture is at the proper ratio. The end result is the same volume of air and the same volume of fuel for the engine... no loss in mpg.

Drew, I too would change the air filter if it was absolutely covered in dust and debris. I don't think anyone would argue that point. Let's stick to the milder cases where you can still see light through the filter element at 50K.

 

[blackcherry06]

I'm gathering from the logic in this thread that the only thing that would be affected by a clogged filter is maximum power output. Correct?
Air restriction at full throttle = less than optimum amount of fuel injected = less dense mixture = less power. It does kinda make sense if you think about it.

The ECU's main function is to keep air/fuel mixture at optimum. Less air detected by MAF = less fuel. Running rich is not a condition allowed by the ECU. But, it is allowed by a carburetor. My conclusion is that air restriction likely reduces MPG in a carbureted car, but a F.I. car is a different ballgame.

 

[KrisZ]

Heavy duty trucks have a device that tells you when to change the air filter. From what I saw on my fater's in low Volvo there is a vacuum guage attached to the filter housing with a sort of float that goes down as the filter gets dirtier (more vacuum in the housing?). When the float reaches red mark you know its time to change the filter.
Why can't regular cars have something like that? The small device doesn't seem to be expensive and you reset it each time you change the filter, that would take a lot of guess work out.

 

[Black Bart]

Originally Posted By: glennc
I think people are not realizing that there are two separate issues being discussed. You could separate them into volumetric efficiency and thermodynamic efficiency. It is correct that modern fuel injection systems will compensate for the lowered amount of air, so the mixture will be burned appropriately and thermodynamic efficiency will not be affected.

However, an engine's overall efficiency depends on both this, and volumetric efficiency - pumping efficiency. A restriction of any amount in flow into or out of the engine will affect volumetric efficiency and therefore overall efficiency.

My argument when it comes to air filters is actually that this effect is not important until the filter gets a lot dirtier than most compulsive-maintenance types think. There is an insignificant diminishment in efficiency, and a significant increase in filtering effectiveness, that will eventually evolve into significant diminishments in efficiency - probably sometime after the manufacturer's replacement interval is exceeded.

However, to say that there is NO decrease in efficiency as the filter ages is wrong. It ignores the basic physics of pumping a gas. To put it in the simplest possible terms, if it is even slightly harder to draw in intake air, the pistons, during the intake stroke, must do more work. Remember from high school physics, work = force x distance. They travel the same distance, but must exert more force because there is a slightly greater vacuum against which they are working. This small increase in work required is present on every intake stroke. Work is the same thing as energy. The engine expends more energy to draw in air - regardless of how perfectly the air is then mixed with fuel - and that increase in energy requires that slightly more fuel be burned.

Altitude is yet another, separate issue. Cars get better mileage at altitude not because the engine is more efficient but because it must do less work - there is less air to push out of the way.
Glen all of what you say is true but you are overlooking one thing.

If you are cruising at 60 mph you will have the throttle open a small amount and pumping lose will be high.
The throttle controls the pumping lose not the filter.

If the filter restricts the air and you open the throttle more then the filter becomes the pumping lose.

If the filter causes a restriction the filter would have to be very dirty to not allow 60 MPH.

If the filter is dirty it will affect WOT but it would need to be very dirty to affect normal driving.

Those who argue that the manual states a certain mileage to change need to understand that is a general recommendation some will need to change more often and others can run much longer it depends on the conditions you are driving in.

Their would be no set mileage but common sense will tell you when to change by looking at the filter.

Bottom line is if a filter affects pumping lose and therefore fuel mileage at normal driving it will be visibly very dirty

 

[meep]

Originally Posted By: blackcherry06
I'm gathering from the logic in this thread that the only thing that would be affected by a clogged filter is maximum power output. Correct?


Yes.

OK - Granted I neglected the changeover from feedback to table-based under WOT conditions as determined by TPS; was trying to keep it simple. However this is where 99GT has some legitimate point, albeit for the wrong reasoning IMO.

IF The filter is dramatically clogged, he has to dip deeper into the pedal to compensate. IF he's going past that point, then he'll tip into richer mix and lesser mpg. That can explain why he's seeing what he's seeing.

Let's for a moment ignore open loop operation.

Throttle does not equal power!!!! It's a valve that restricts. Period. Clogged filter restricts. Period. Same thing. ECU determines actual flow (MAF) or resultant intake pressure (MAP) and sends fuel accordingly.

Pumping losses - is true but often misunderstood.
Exhaust restrictions impact mpg and peak output.
Intake restrictions impact peak output, but
due to reasons above not necessarily mpg.

I will admit that the integration of TPS input, from what I have seen, appears inconsistent. Remember the old pump-jet carbs? Some FI systems seem to mimick its operation. some don't. TPS movement may impact mix more in some systems than others. TPS absolutely impacts AT shift points and shift behavior (if 99GT has an AT and it's running in a lower gear due to higher throttle position even tho engine is basically unloaded since restriction is in filter vice throttle then this would also explain his filter/mpg observation) as well as emissions (IAC position on decel, etc) but by and large TPS is not the determinate "Throttle = Power" input.

Let's keep this healthy and not a bash session. These systems are complex and unless you're working on them frequently we all have something to learn.

Mike

 

[JHZR2]

Originally Posted By: Drew99GT
After all, it's become common folk lore round here that a dirty air filter filters better...


I dont necessarily disagree with you, and Im not going to argue on most of your points, as I dont have first-hand experience in the super-dusty areas. However the one quoted above is not folk lore.

However, it is common filtration knowledge that is used all the time, which indicates that a filter cake must be built up prior to a "filter" working properly... it is the cake that does the actual "filtration", not the filter itself!

As an aexample, in our lab we filter fine particles and wash them with water. We use a filter media that is designed to stop the nominal particle size correctly. however, the first few introductions of particles that we pour onto the filter paper end up with a lot of pass-through. Until a cake builds up, the filtration efficiency is poor or sub-optimal, and lets more junk through. At some point, a cake builds up, and filtration is excellent. Beyond some point, the cake gets too large for its own good, pressure drop builds, and odd flow conditions begin to occur.

So there is an element of wanting a dirty filter, but not too dirty, to optimize filtration.

JMH

 

[Drew99GT]

Maybe my head is up my own [censored] (been told that many times), but here are some threads to ponder. Terry Dyson seems to think replacing a dirt air filter will increase, or perhaps, re-establish fuel mileage.

http://www.bobistheoilguy.com/forums/ubb...true#Post946816

Here's two quotes from a member here who's been in the commercial HVAC business for 29 years and has been around a filter or two.

"This #@$%! about a filter being more efficient when it gets dirty. A molocule of air will not pass through a molocule of dirt, so it goes around. When it can't find a place to go around, it pushes the dirt through. Why people try to see how long they can make a cheap filter last, instead of protecting an expensive piece of equipment is beyond me, but I make a lot of money off of folks like that. Same theory applies to the filters in your car."

"I said this before in another thread. In 29 years in the A/C business, I've made a good living from folks who don't change their filters often enough. As long as the air can pass around the dirt to get through, it will . When it can't, the dirt gets pushed through. Using dirty filters in expensive equipment, is always the wrong thing to do. Try to find any equipment maker, wheather it be car engines, A/C units, or any other air breather, that recommends using dirty filters.
GSD"

 

[Drew99GT]

Now that I remember it, Terry Dysaon has extensive analysis on the Honda's that produce major fuel dilution with a dirty filter, specifically the Odyssey - and subsequent decrease in fuel economy from a rich condition. Heck, I think Pablo even had the issue in a used oil analysis.

I'll try and find some of those used oil analysis reports. Man, I wish Mr. Dyson was till around.

 

[Drew99GT]

https://bobistheoilguy.com/forums/posts/756285/

Darn, that's the only one I could find, but I know there were a couple other late model Honda reports with high fuel dilution, and Terry had come to the conclusion that replacing the air filter at a maximum of 10,000 miles lessened the overly rich condition on them.

 

[Black Bart]

Let's not get too hung up on the TPS position
While at cruising speed or mild acceleration the ecm looks at the TPS MAP or MAF and engine coolant to determine the pulse width of the fuel injectors but the O2 sensor has the final say.

The O2 monitors the A/F ratio and adjust the fuel to maintain stoichiometric ratio.
At WOT the ecm ignores the O2 and reads the fuel tables unless you have a stand alone wide band fuel management system like I use on my supercharged Corvette and it uses the wide band O2 to control the A/F ratio 100% of the time.

 

[Drew99GT]

I'll take Terry Dyson's perspective over theory about how fuel injection works. He's built and tested many engines, and done extensive oil analysis testing - that's what he does for a living. He repeatedly sees analysis with more fuel dilution. Tells clients to change their dirty air filter. Fuel dilution drops, engine performance and fuel mileage comes back.

 

[Black Bart]

Drew99GT The link you posted Terry states that a Fram air filter is less restrictive than a Wix or Honda OEM..

He does state that when the filter becomes clogged from tree pollen the computer can not compansate.

So he is saying what most of us are saying that within limits the computer can compensate.
Anyone with an IQ higher than outside temperature knows that a clogged filter needs to be replaced.
What I posted was not theory about how FI works it is fact but since you have your mind made up you don't want to be confused with facts.

 

[Drew99GT]

Originally Posted By: Black Bart

Drew99GT The link you posted Terry states that a Fram air filter is less restrictive than a Wix or Honda OEM..

He does state that when the filter becomes clogged from tree pollen the computer can not compansate.

So he is saying what most of us are saying that within limits the computer can compensate.
Anyone with an IQ higher than outside temperature knows that a clogged filter needs to be replaced.
What I posted was not theory about how FI works it is fact but since you have your mind made up you don't want to be confused with facts.


This is also what he stated:

"I have been preaching this for 30+ years and base it on UOA testing. Air filter IS THE MAIN ingredient to better performance and MPG for most engines,spark or diesel. Those that take the change interval for air filter to factory recommended durations will MOST of the time find the leaks in the system and suck dust and or needlessly increase specific fuel consumption.

Had a major racing engine builder argue with me about semantics on this point when our common racing engine customer was running high levels of fuel in the oil and he poopooed my observations because ECU and injection systems correct for restricted air flow. ....Poppycock......"

This, from perhaps the most trusted and experienced member on the topic of engines and lubrication we've had at BITOG. This stuff is what he does for a living.