engineermike

There has been some discussion about gas mileage versus the type of air filter you use. I would like to explain, using science, why it does not:

Some may argue that increasing airflow increases fuel flow requirements, therefore mileage will go down. I would like to point out that the increase in airflow, whatever tiny amount it may be, only happens at full throttle. The vast majority of folks do not drive around at full throttle, especially those concerned with fuel mileage. If it were true that a simple increase in full throttle air flow potential resulted in worse mileage, then the 2013 360 hp 5.0 would not be rated 2 mpg higher than the 2003 260 hp 5.4. But it is, even though the 2 mpg higher 5.0 will consume nearly 40% more fuel at full throttle.

Secondly, a little deeper into the speed-density theory of operation...speed-density isn't just an ECM or a method of controlling fuel injection. It is a theory of operation. The theory is that the air flow (and, thus, power) of a 4 stroke cycle gas engine is a function of the engine speed (rpm) and the density of the air in the plenum (a function of air pressure and temperature). The pressure (controlling density, leading to airflow, creating power) is controlled by none other than your right foot. Looking from the other end, please understand that, for any given acceleration rate or speed, the hp required to propel a given vehicle does not change, regardless of the type of engine. In other words, if it takes 50 hp to push your truck a constant 60 mph, it will require 50 hp regardless of whether the engine is capable of 51 hp or 400. So, if we put 2 and 2 together, the driver regulates speed by regulating power, which is determined by manifold pressure, which is controlled by your right foot.

THEREFORE, any given constant speed is achieved at one distinct manifold pressure, assuming rpm and temperature stay the same. A high flow air filter does none-other-than reduce restriction, which increases downstream pressure. IF this increase in pressure were to increase manifold pressure, THEN the vehicle would accelerate and the driver would react by closing the throttle, thereby restoring the exact same manifold pressure as with the standard air filter. This means the engine itself sees absolutely no effect at all, so long as the driver is regulating power with the pedal.

I hope someone finds this helpful.

And before anyone asks, yes, I've run half a dozen different air filters and intake setups on as many vehicles, and never saw a difference in mileage. However, I would also argue that these types of mileage tests are highly susceptible to variability.

Mike

Roughneck12

Makes total sense, just like most mechanical things. If you understand how things work, there is a logical progression. Same goes for any mechanical problem.

white_dc

iTrader: (12)

Hate to ask a dumb question, but does that mean the increase in manifold pressure actually means it makes more power at WOT?

engineermike

YES! Increases in manifold pressure increase power. That IS the speed density theory, in a nut shell.

springer64

"I would like to point out that the increase in airflow, whatever tiny amount it may be, only happens at full throttle. The vast majority of folks do not drive around at full throttle,"

this is the main part that i think many guys just don't grasp. not to say there isn't confusion about plus or minus fuel mileage...

the throttle blade is the restriction at part throttle.

i see many street vehicles with huge aftermarket airboxes and k&n, airaid, etc. i shake my head and move on. just a waste on the street.

engineermike

Bingo! Any reduction in restriction in the intake tract will simply be offset by closing the throttle blade the required amount to achieve the very same power level desired by the driver. Net change = 0.

wolff809

IF this increase in pressure were to increase manifold pressure, THEN the vehicle would accelerate and the driver would react by closing the throttle, thereby restoring the exact same manifold pressure as with the standard air filter. This means the engine itself sees absolutely no effect at all, so long as the driver is regulating power with the pedal.

Mike[/QUOTE]

Sorry if this seems dumb but wouldn't the increase in pressure like you said and would cause the truck to accelerate. When you backed off the gas to maintain the same manifold pressure wouldn't you be running at a lower RPM which would increase mileage?

I have no idea if this is true or not just a thought that ran through my head when I read that part of your post.

engineermike

The rpm is determined by the gear ratio, tire size, and vehicle speed...assuming the torque converter is locked up. On the other end, the power being produced is the same in both cases. Hope this helps.

Mattm400

Thank you Mike!! That other thread about this was getting out of hand, you explained that very well, so, in some cases, I figure increased airflow at any given moment even when throttle is not fully open can indeed help in acceleration and throttle response. Although it may not be too noticeable depending on engine & setup etc..

jarrett

Ok, the science is good and I understand what your saying. Mike look at my thread up my thread I started today. Tested stock vs CAI. Can you explain the cause and effect of it. I'm not trying to argue in any way. I would like to think the CAI by KN would be better but ill sell it if it not worth it, tuned or not.