1 FMF

iTrader: (1)

Regarding fuel economy or gas mileage, what will happen if you do the following on a modern (year 2000 and later) electronic fuel injection engine?
You can also mention the affects on torque or horsepower and general overall engine performance if you like. Engine is not modded, would be stock from the factory. Engine may or may not have a turbo, would it matter?


a) a new paper air filter in place of a dirty paper air filter;

b) a new K&N or similiar high-flow or high performance foam air filter in place of a dirty paper filter;

c) a high dollar uber-fancy cold air kit in place of the stock setup, everything in front of the throttle body;

d) changing out anything or everything from, but NOT including, the cylinder heads. So that could mean a new intake and/or new throttle body.

e) you take a piece of cardboard and block off half the air filter, so it's only half the square area that's available for air to flow through. How will that affect highway gas mileage?

engineermike

If improving the air filter helped out gas mileage, then so would opening the throttle more.

DanO

Ding Ding!

with modern fuel injected vehicles, the air fuel ratio is closed loop during part throttle. Part throttle means the throttle is causing a huge restriction. Having a slight restriction across the air filter is nothing compared to the huge restriction across the throttle plate.

now if the argument was that a less restrictive filter allowed for higher maximum power, i could agree.. but fuel economy.. nope!

InchUp

iTrader: (2)

The throttle blade itself is a huge restriction at part throttle compared to an air filter, but take the throttle body out of the picture and there's still a slight vacuum created behind the filter isn't there?

I'm not sure exactly how much restriction a filter creates but nonetheless isn't there at least something measurable? If I understand that correctly, a clean filter (or at least one with less restriction) should still lessen the amount of work the engine has to do to pull in air passed the throttle body. Am I right or wrong in that assumption? I'm not saying the difference will be significant, I'm only trying to acknowledge that a restriction exists when comparing a new filter to the same dirty one.

engineermike

Yes, a dirty air filter is more restrictive than a clean one. However, neither come even close to the restriction imposed by the throttle blades at cruise. If you take the throttle out of the equation, then you're making full power too.

Take an extreme case, for example. Let's say your air filter is nearly plugged completely. It's plugged so bad that the throttle has to be 60% open just to maintain cruise speed. Well, guess what...the manifold pressure is the same as with a clean filter and 15% throttle. It is of no consequence where the restriction is at part throttle, as long as its there. So, the fuel mileage will be the same too.

Mike

InchUp

iTrader: (2)

Ahh yes, I was overlooking the fact that if you take the throttle body out of the equation you go to full power anyways. I suppose then having a clean filter just gives you that marginal amount of throttle back. In your extreme example the dirty air filter required 60% pedal usage just to maintain cruise, whereas the clean filter gave back 45%. Of course this is just an example.

Perhaps a clean filter would return 1% throttle back, I'm not sure, just thinking out loud again.

lxarlo

So then the theory of a clean vs. dirty air filter and its change on fuel economy would be more accurate of a diesel powered vehicle as they do not have throttle plates? I guess it would play a part in what type of turbo vs NA diesel?

engineermike

You are absolutely correct. One of the main fuel efficiency advantages a diesel has over gas is that the piston isn't working against a vacuum on the intake stroke.

Add a restrictive air filter to a diesel and now the piston has to work against a small amount of negative pressure on the intake stroke, thus reducing efficiency.

Conversely, a properly sized turbocharger can apply pressure to the piston on the intake stroke and, as long as it doesn't impose more exhaust pressure than intake, increase fuel efficiency.

chrs1313

iTrader: (24)

Have to disagree both on my 02 z28 and my 03 impala...when i added my huge custom ram air to my car i gained about 2mpg on the highway at cruise speed...verified it about 3 times when i used to go to college a SIUC and live near Chicago...300 mile trip...every time gained about 2mpg over not having the ram air effect, if i remember right it was showing around 4" H20 at 75mph after the air filter inside the lid...

i also added a 4in dryer hose to my 03 impala due to trying to gain mpg because i have a 140 mile round trip to work every day ...that also gained around 1mpg...

even though the throttle body creates a huge restriction small changes to that huge restriction account for a big change in flow...

just recently i did a test a work with a small hole cant remember off the top of my head around 1/8" dia in a plate that was inside of a 1.5" pipe...one side of the plate was under 20 psi pressure and other side was open to atmosphere... this gave a certain flow, around 20 CFM...

now the interesting part when just 4" H20 (~.15psi) of vacuum was added instead of just atmosphere the flow jumped to around 23-24CFM that is 15-20% increase in flow...when the ratio of the vacuum .15psi to the huge pressure 20psi...is only 0.75%...

In the end ram air does work and will help MPG...with less restriction in the intake track (i.e. being a dirty air filter) the car will not have to work as hard to get its air...

DanO

ok..... do you understand what a throttle is? (hint, its a purposely designed air restriction device) We are talking fuel economy at part throttle, i.e. when the throttle is imposing large amout of restriction to the intake. a slight decrease in restriction upstream has no effect.. especially at the cruise rpm flow levels.. restriction is very low through the air filter.

DanO

Actually.. ANY exhaust restriction is a decrease in fuel efficiency.. this is often overlooked by many people when talking turbos.. its not FREE power! lol...

ask any oem what exhaust restriction (especially high levels like turbo engines) does to the combustion event?

all im trying to say, is that sometimes everything needs to be broken down in to great detail to get the full picture..

engineermike

chrs13, DanO is right.

Think about the way a speed-density ECU works. "Load" is synonymous with "manifold pressure". So, at cruise, the power required to move the car is the same, regardless of what happens ahead of the throttle body. Since the power is the same, the load is the same, which means the manifold pressure is the same. So, the mpg's stay the same too. Measure your cruising MAP with a CAI versus stock and you'll see the same reading. This manifold pressure is the net combined effect of the air filter, MAF, etc, and throttle body. If a better air intake actually raised pressure/flow in the plenum, then you'd be making more power and going faster plus using more fuel to go along with the added air. Reduce throttle to slow down and you're back where you started.

I have my doubts about the mpg and flow readings. People generally change their driving style when trying out a new device because they want to give it a fair chance. Instead, they gave it an unfair advantage. As for the airflow through an orifice, there's no way you can convince me that a 1% increase in dP caused a 20% increase in flow. It's more likely to gain you .4%. Again, I doubt the measurement methods.

engineermike

IF you manage to create more manifold pressure (remember, we're talking diesels here) than exhaust pressure, then the net effect of the two combined will be an increase in fuel mileage. You're essentially just gathering energy from the exhaust heat and applying it to the piston on the intake stroke. If you could get the intake pressure without adding exhaust pressure, you'd have the best of all worlds, but unfortunately, the energy has to come from somewhere.

chrs1313

iTrader: (24)

well we all have our beliefs then but i have see improvements of three occasions...the true test get a leaf blower and hook it up to your intake and let it run wide open just idling in the driveway, with equal amount of gas the test with the blower will keep the car running longer, better mpg...i would be willing to bet on it...

and also my methods of driving i know how to keep a control best as possible in a test...that is why the cruise was used and i made sure it wasnt a fluke by doing multiple tests on the same interstate...

engineermike

Is this a joke? Seriously.

You DO realize that this isn't about "beliefs". This is about Physics and the basic operating principles of IC engines. Do you care to explain how increasing air flow increases fuel mileage, even though you have to throttle back to achieve the original air flow anyway? If the key to better fuel mileage were increasing air flow, then why not simply open the throttle more? Same with the leaf blower experiment... just open the throttle blades some. I think we all know what would happen then, and it's not better mileage.

Jassick

iTrader: (1)

Could it be that when throttling back you're at a lower rpm, thus the engine is not only turning slower, but a less strenuous load. Wouldn't that result in better mpg?

engineermike

rpm is dependent on gear ratio and speed. Assuming the gear ratio and speed remain the same, the rpm would also.

Mattsz28

iTrader: (1)

^^Thread could have ended right there.


In any system were "work" is to be completed with an IC motor, the motor will require x amount of fuel for y amount of air. Allowing the motor to pull "more" air at "less" throttle still requires an equal increase the the fuel your going to burn.

1 FMF

iTrader: (1)

for an equal amount of gas if you could somehow manage that, forcing air into the engine with the leaf blower would make the engine run longer (for a given amount of fuel) and thus you could conclude an increase in fuel economy. So in that specific scenario ok you are correct.

However, the real reason the engine runs longer which you don't mention is because the of the air supplied by the leaf blower. That volume (or mass) of air at whatever cfm is free energy provided to the engine that you don't account for. And you don't get that cfm and pressure of air for free, it takes energy to make that either from fuel or electrical power consumed by the leaf blower. When you factor that energy usage in, your whole system is less efficient and using more [fuel] for the same energy output because both are less than 100% efficient.

but my whole issue with the claims of increased fuel economy based on increased or less restrictive airflow, is what about the computer monitoring air-fuel ratio ??? !!!

That's why i specifically stated EFI in the beginning, doesn't the computer maintain an optimum air fuel ratio? So if you hook up your leaf blower and force extra air into the engine, or add your less restrictive air filter or your cold air kit lets in colder air, the computer would/should see that via the mass air flow (MAF) sensor and if more air goes in (colder = denser = more mass) then more fuel has to be added to maintain the same air fuel ratio does it not? If that does not happen, then are you not running the engine lean? ...which okay that would mean increased economy but running leaner is (a) bad and (b) does not happen.

I am surprised nobody mentioned the computer maintaining A/F ratio,
the reason I post this was an argument I had where friend claimed increased mpg from either adding a filter or cold air kit (which was advertised in magazine) to his ride which also had a turbo (speed3). So I said B.S. and if his measurement of a few more mpg were actually correct that it had to be coming from something else based on A/F logic.

So is my A/F logic correct?

Do I dare ask what if the filter or cold air kit was over a carburetor?

engineermike

Good question, but it depends on where the bowl vents are referenced, fuel circuits in the carb, etc.