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Old 03-29-2007, 06:40 PM   #1
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Best type of Air Cleaner

what is the best air filter to get for mpg?
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Old 03-29-2007, 06:53 PM   #2
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Originally Posted by Traugbot View Post
what is the best air filter to get for mpg?
A clean one Even dirty - if you're driving for max mpg - you'll never outflow your filter. (I'm sure there's some extreme exceptions on a few vehicles though).
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Old 03-29-2007, 08:50 PM   #3
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ive heard k&n is good but im done messing with my air flow after wut my intake did to fe
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Old 03-29-2007, 10:01 PM   #4
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Originally Posted by theclencher View Post
I just was involved in a big debate on K&N's vs. paper filters on another forum. It's the same old thing: due to sloppy test methodology some will claim they help FE and some will claim they don't. Any potential difference is going to be very minute and hard to measure. The best info I came across was this: http://www.hotrod.com/techarticles/70738 /index.html
One thing that I have never seen tested about these filters.... How well do they filter? It's funny -- Better Flow! they sell... But better filter?

Do some searching around BITOG if you haven't already.... For the most part - the trend with washable filters happens to be higher silica numbers in the oil analysis.... You know... dirt. It's much harder to test oil with decent accuracy due to the time variable - but it's a powerful statement when most have high numbers.
http://www.bobistheoilguy.com

Doh! I have a bad habit of not reading the last sentence of everything....
Quote:
Also it would have been nice to have had the elements tested for filtration effectiveness too. Somebody out there must have done it and if I find it I'll share.
See above :P
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Old 03-30-2007, 06:24 AM   #5
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So here is my thing on air filters on fuel-injected gasoline powered cars. Follow my reasoning, if you will.
How do we control the power output of our vehicles? By increasing and decreasing the amount of restriction we put in the system ahead of the intake valves. If you want more power, you open the throttle. If you want less, you close it. Everyone agree so far?

So when we look at the steady state operation of an engine at cruising speed on a level road, we see that in order to make the car cruise at a faster or slower speed, we decrease or increase the manifold vacuum.

When you are cruising along, the engine does not care if the restriction comes from the throttle plate or the air filter or the HeMan action figure your son jammed in the air intake.

If you remove the HeMan action figure, then to maintain the same cruising speed, you have to close the throttle a little more. If you put in a K&N filter vs the stock filter, then you will have to close the throttle yet a little more.

Therefore, it is physically impossible for an air filter with lower pressure drop to increase gas mileage. It can increase POWER at WOT, but it CANNOT increase gas mileage while cruising.

Carbureted engines and diesels play by different rules and may see different results.
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Old 03-30-2007, 07:41 AM   #6
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Newbie Chiming In

To fall back to my simple physics education:

Any work an engine must do to "pull" air into the engine is work generated by gasoline consumption and thus would affect mileage. The work required is a function of the force that must be applied to pull the air in to the cylinder over the distance the force must be applied. The distance is the piston stroke. The force increases with respect to the pressure differential between the outside air and the cylinder during the intake stroke.

Stopping here for a sanity check... if there is zero pressure differential (and zero air friction, and we ignore the work to move the piston itself), the work needed is zero. Conversely, if the pressure differential is infinite, the force and thus the work are also infinite. Sanity checks. Also, if the pressure differential is positive, the net work is negative, and work would be done "on" the engine (Ram air for free, turbo or supercharging at the expense of robbing energy from the engine to start with). Sanity still checks.

Now, what affects the pressure differential? Many things along the flow of the intake, but we can examine the section immediately before and after the intake valve in isolation and to get the same answer for the engine work, regardless of what in front upstream is affecting the airflow, because the work, again, is affected by the pressure differential inside the cylinder versus outside the intake valve..... the magnitude of the pressure differential, not what generated it up stream.

So here is the logic, and I believe can be proven with the simple thermodynamics (extended from the simple pressure volume heat equation).

The pressure outside the car is at atmospheric pressure: roughly 14 lbs per sq inch. If no restriction between the oustide air and the outside of the intake valve, the pressure differential during the intake stroke would also be 14 lbs per sq inch. If however, an obstruction is introduced upstream, the pressure differential would be less (exactly minus the differential across the upstream obstruction). I repeatedly see that in the case of the fancy air filters, the pressure differential across the filter is indeed less than the papers (in fact, if you read the HotRod article previously, you'd note that in the test, they needed at least 2 inches of pressure differential across the filter for the test to be valid, and to achieve this, they actually taped off half of the filter area for some of the filters..... hmmm). So the net is that if the pressure differentiial is less across the intake valve in the air filter versus, say no obstruction whatsoever case, the engine must indeed do more work to pull the air in ( proportional to the delta in the pressure differential, whether linearly or the square or some other realtionship I am not sure).

Oh, this does assume that the same volume of air enters the cylinder in each case, which can't be true, so no firm conclusion can be made using the previous argument unless one also introduces that affect. But it is clear the the engine must do less work in the higher pressure delta case (the extreme being a super charger so strong so as to run the engine itself). Less work means less engergy expended, QED. However, the relationship between less energy expended on intake, and resulting less energy produce during combustion (the less air volume case), does affect overall engine power efficiency, but, using the argument of conservation of energy, using more energy during the intake must yield less energy down stream... thus the less differential pressure, less marginal energy output case must be more less efficient than the high differential case.

ie. The less restrictive upstream intake, the less work the engine must do, thus the more efficient the engine becomes.

Actually, I think you can make this same argument using a main bearing friction argument. The more work the the piston has to do to pull the air in, the more friction on the bearings, and thus more work, and thus less efficiency.
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Old 03-30-2007, 08:21 AM   #7
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Not really, on a lot of levels. In the case of a diesel with an unthrottled intake, I think you are correct. But on the gasoline engine, we use the throttle to control the pressure upstream of the intake valve. Anything besides the throttle that creates more or less resistance just means we have to adjust the throttle to get to the same pressure upstream of the pressure intake.
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Old 03-30-2007, 08:23 AM   #8
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for me honda cb125 motorcycle I have both a foam filter, a K&N filter, and a paper air filter, the K&N filter and the foam filter give me the same top speed when tested A-B-A-B on the same road same day with digital speedometor hooked up, thus that gives me the imprssion that foam, and K&N have about the same air flow, now rebuilt the carburator then ran the K&N for about a year, then did some carburator work, and the amount of dust, dirt, and crud that had passed thru that K&N "filter" was amazing, it was filthy, I now do my best to only run foam air filters, and when I do carburator work it's spotless.
My civic vx came with a foam air filter when I got it, it has 236,400 miles on it and perfect compression, and give the foam air filter credit for that, and if you have an engine with good compression, you are going to get better mileage, a poor air filter is going to let more dirt in, your engine is going to wear faster, and your mileage is going to drop.
I would like to try a Nano Fiber air filter, but last I checked I couldn't get one for my car.

from what I can tell the only reason a higher flow filter would give better mileage is if you have a carburator, insted of fuel injection, because carburators rely on restriction to help suck fuel in, less restriction, and you have less fuel going in, so it really is running leaner.
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Old 03-30-2007, 08:46 AM   #9
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An Exeriment to Determine

I think all the experiments with one filter or the other are showing differences that are less than the margin of error in most cases, therefore it is tough to draw any conclusions. In practice, I think I agree with the throttle argument.... the throttle might be so restrictive as to make any upstream restrictions simply noise in the equations.

How about this: Compare the worst case, best case, and intermediate cases.

Worst Case: Restrict your intake with tape or whatever significantly to the point where the engine barely gets enough air to run at highway speeds. Check your mileage.

Best Case: Pull the air filter and all the crap in front of the intake. Check your mileage again.

Intermediate Case: The default.... any air filter, standard intake. Check your mileage.

Perhaps.... perhaps, this experiment would yield results greater than the margin of error and some rudimentary conclusion could be drawn. These would at least show the extreme pressure differential cases (my argument is not about volume of air flow, although this is certainly related. It is about more or less engine work related to the intake stroke "pull" effort).
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Old 03-30-2007, 09:16 AM   #10
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Quote:
Originally Posted by lca13 View Post
I think all the experiments with one filter or the other are showing differences that are less than the margin of error in most cases, therefore it is tough to draw any conclusions.
I think you're right there. But I think you can make a conclusion from that: at fractional throttle openings, filter type has little impact on fuel consumption.

Quote:
Worst Case: Restrict your intake with tape or whatever significantly to the point where the engine barely gets enough air to run at highway speeds. Check your mileage.
The potential snag there is it may cause the throttle to be open more (or fluctuate more widely), and the ECU may respond to sensor input and change other variables. EG if TPS is connected to timing advance, this would throw off fuel consumption in unintended ways.

Quote:
How about this: Compare the worst case, best case, and intermediate cases.
Something like this?



(from: http://metrompg.com/posts/air-filter-part-2.htm)

As you suspected, the difference between filter types (including NO filter) is so close to the margin of error as to be inconsequential.

See also this thread:

http://www.gassavers.org/showthread....ghlight=filter

Before I made an effor to start learning about this stuff, I was sucked in by the marketing and by my own faulty logic and spent a bundle on a K&N filter. Even after all this time, it's still sitting in my intake.

But after reading Ryland's words just now, I believe I will remove it and see if I can recoup some of my money on eBay.

PS - welcome to the site. If you feel like it, post a 'hello' in the
Introduce Yourself forum.
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