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Does The Air Filter Diameter Matter?

blueflyer

Well Known Member
Does the diameter of the air filter matter? For an 0-320 Vans calls for the K&N E-3260 that has an inside diameter of 5 inches. For the 0-360, Vans calls for the K&N E-3450 which has an inside diameter of 6.25. So, I tend to think diameter matters, but why? What would happen if I used an K&N E-3120 that has an inside diameter of about 4 inches on my 0-360?
 
air flow is all that matters and that is determined by total filter surface area (ht, dia, pleat count and pleat depth) Go to the K&N site and get the CFM rating for the two filters. Then go to a web based calculator to figure out the air flow needed for a 360 cubic foot engine (typical VE) running at 3000 RPM. Add some fudge for partial filter blockage.

My guess is that the smaller filter ID will not clear the attach bolts on the larger servo and why it is spec'ed for the 360. The 360 version is the same as the 540, so it flows plenty.

Larry
 
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Had the opportunity to speak with an Engineer from K&N after his presentation.

"Partial filter blockage" is actually a design goal with air filters.

The filter design engineer relies on "partial blockage" as its used to reach a lower particle diameter restriction. It's termed "cake"

If actually designed to meet minimal particle size from the get-go, the mean time between servicing for the K&N or, with other types of media, the useful life of the filter would be very short.
 
air flow is all that matters and that is determined by total filter area (ht, dia, pleat count and pleat depth)

Well, yes and no. Air flow though any media is determined by how well air passes through it. You could use a window screen for your filter media and it would flow air just fine. But we also consider the particulate size that we want to exclude from passing through the media.

As our window screen (or whatever media we choose) starts to catch particulates (dirt), the openings will get smaller, and we start to filter smaller particles. Adding pleats to the filter gives more surface area so the filter can hold more dirt. All the pleats do is allow you to go longer between service or replacement. Adding or reducing physical size only determines how often you need to service the filter.

-Marc
 
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...Adding pleats to the filter gives more surface area so the filter can hold more dirt. All the pleats do is allow you to go longer between service or replacement. Adding or reducing physical size only determines how often you need to service the filter.

Partially true, in that a small filter reaches an unacceptable lever of dirt restriction before a large one. However, the small area filter will be more restrictive even when clean.
 
Well, yes and no. Air flow though any media is determined by how well air passes through it. You could use a window screen for your filter media and it would flow air just fine. But we also consider the particulate size that we want to exclude from passing through the media.

As our window screen (or whatever media we choose) starts to catch particulates (dirt), the openings will get smaller, and we start to filter smaller particles. Adding pleats to the filter gives more surface area so the filter can hold more dirt. All the pleats do is allow you to go longer between service or replacement. Adding or reducing physical size only determines how often you need to service the filter.

-Marc

For any given media type specification, filter material surface area determines total flow rate, excluding the effects of particulate build up. Yes, different medium will flow different amounts of air for a given size. However, the OP was asking specifically about K&N air filters, therefore, we can assume a common media. Given the common media, air flow differences across different K&N filters will be based upon filter material surface area. If the OP was asking about using screen door material, I would have mentioned that filter area alone was not applicable. BTW, they measure the area of the filter material flat, as all of it filters and passes air. Therefore, you cannot compare diameter alone, without knowing the pleat sizes. Pleats are not just for collecting debris, they are used to increase filter material surface area without increasing total filter package size.

All of this is kind of moot, as K&N lists a flow rate in CFM for each of it's filters and I assume that flow rate factors in a defined amount of restriction based upon debris collection, but not sure. So, it's pretty simple to determine which K&N filter is acceptable for a given engine displacement/RPM.

Larry
 
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What Larry said

As most process parameters tend to be limited by the operating conditions, the same is true for both filters and heat exchangers.

Surface area, surface area, surface area.
 
In my go-faster notes I have calculated that going to a larger (o-360) air filter on my o-320 would gain 0.15" of MP. I also have in my notes that Don Rivera considers the RU-1470 air filter to be a 100 to 150 HP filter, but that filter is 31% more area than the stock Van's o-320 filter.

I don't recall how I did the calculation, or where I read about the RU-1470.

But, if correct, the larger filter would gain almost 1 HP. Low on my priority list given the cost of a whole new airbox, and that the o-360 airbox interferes with the nose gear leg.

Clearly going to a smaller filter would increase MP drop and reduce power.
 
K&N FAQ's

Still looking for CFM chart for the filters. I found this on K&N's site. :

HOW DO I FIND/CALCULATE THE PROPER FILTER SIZE FOR MY CUSTOM APPLICATION?

SHAPE

When fitting a conventional round filter on top of your engine-such as a carburetor, central fuel injector, or throttle body fuel injector-a large diameter, short filter will provide more airflow than a small diameter, tall filter. For example, a 10-inch diameter, 2-inch tall filter will provide more airflow than a 5-inch diameter, 4-inch tall filter. Where space permits, the height of the filter should be between 1/5 and 1/4 of its diameter.
The shape of the filter is less important if the application calls for a remote mounted filter, which includes many late-model fuel-injected vehicles.

SIZE

Use the formula below to compute the minimum size filter required for your particular application. The usable portion of the filter-the effective filtering area-is calculated by multiplying the diameter of the filter times pi (3.1416), multiplying by the height of the air filter in inches, and then subtracting .75-inch. We subtract .75-inch to compensate for the rubber seals on each end of the element and the filter material near them, since very little air flows through this area.

Keep in mind, these formulas are the minimum size requirement. To provide an even greater volume of air to the engine, install the largest filter that will fit in the space allotted.

formula1.PNG For an 0-360, you have Effective Filtering Area=360*3000(for a small cushion)/20839=51.83 square inches

If you are sizing a round filter, use the following formula to determine the height of the filter.

formula2.PNG Then, Filter Height=51.83/(7.75*3.14) +.75 = 2.87 inches. The 7.75" diameter is for the E-3450 Vans uses which is only 2.5" tall...so my math must be wrong somewhere.
 
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View attachment 23731 Then, Filter Height=51.83/(7.75*3.14) +.75 = 2.87 inches. The 7.75" diameter is for the E-3450 Vans uses which is only 2.5" tall...so my math must be wrong somewhere.

The usable portion of the filter-the effective filtering area-is calculated by multiplying the diameter of the filter times pi (3.1416), multiplying by the height of the air filter in inches, and then subtracting .75-inch.

following the formula posted, with my logical edits and with a 7.75" diameter and 2.5" height.

(7.75 * 3.1414 * (2.5 - .75) = 42.6 effective area

The words say to do the following, but it doesn't follow engineering principles or logic

(7.75 * 3.1414 * 2.5) - .75 = 60.11 effective area.

because the .75" of rubber takes away from the filter area, it should be subtracted from the filter height BEFORE it is multiplied by the circumference to get an actual effective area.

found this on the web:

"Pleated K&N filter material will flow 6.03 cfm of air per square inch. " Assuming this is clean. It will flow less dirty.

Following that, 42.6 * 6.03 = 257 CFM.

(360*2800/3456) * .8 (estimated VE) = 233 CFM required for a 360 CID engine turning 2.8k.

Clearly they do flow testing https://www.knfilters.com/testmethod.htm. Unsure why they no longer publish the flow data by filter; They used to.




Larry
 
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that does make more sense, although those numbers I used are straight off the K&N website. Guess I will not try and reinvent the wheel here and stick with what Vans calls out. I appreciate the education
 
Van's filter too small? Per K&N?

The calculations could be correct - I seem to remember someone (DanH?) determining that he should use a lot larger air filter as part of his custom snorkel filter design.

Unfortunately there's really no space to put a larger filter in for us updraft / cowl inlet folks without major engineering. Maybe 1/4" more height, but K&N doesn't make a filter of the same diameter that's just a little taller.

And - if you go look at a Cessna 172 air filter square inches... I wouldn't be surprised if Van's filter size is bigger that what Cessna used!
 
A K&N E-3322 is 2 1/2 inch tall and will fit an O320 without cutting the notch.
A K&N E-3300 is 1/2 inch taller and will fit an O320 without cutting the notch. The air box will need to be deeper. Will hit gear leg.

K&N will flow more air, but it will also flow way more dirt...... https://www.nicoclub.com/archives/kn-vs-oem-filter.html

Exactly. It's been stated that a larger airfilter will "flow more air", but your engine will only use what it uses. To the OP's request about whether it matters,
I maintain that both airfilters (larger and smaller) will already flow more air than your engine will use. So that leaves us with servicing frequency as the last factor that might affect our choice. Certainly a larger filter will hold more dirt, so the servicing frequency will be extended over the smaller filter, everything else being equal.

-Marc
 
Exactly. It's been stated that a larger airfilter will "flow more air", but your engine will only use what it uses. To the OP's request about whether it matters,
I maintain that both airfilters (larger and smaller) will already flow more air than your engine will use. So that leaves us with servicing frequency as the last factor that might affect our choice. Certainly a larger filter will hold more dirt, so the servicing frequency will be extended over the smaller filter, everything else being equal.

-Marc

Yes/No. Most any applied device can reach a target flowrate given the proper motivation. All other things being equal, more surface area will have lower losses. That is the part that matters here.

All that said, the resulting performance deltas will probably be imperceivable unless the original product was truly miss sized; doubtful.

Questioning things, overthinking issues, etc. It's what VAF'rs do. I'm as guilty as anyone.
 
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Air Flow <> MAP Drop

I guarantee any tiny air filter will flow a massive amount of air when the air is propelled by a cannon. :)

As I alluded to earlier, the larger filter area will have less MP drop, given an identical material, identical amount of oil, and similarly 'dirty'. MP drop is lost power. Typical Cessna intakes have around a 1" MP drop, which directly amounts to 3% power loss at sea level (full power, take-off). Van's inlet system's ram air does better.

Good tip on the 1/2" larger filter, but not sure that there is 1/2" clearance between my airbox and lower cowl. Thanks for that part number.
 
A K&N E-3322 is 2 1/2 inch tall and will fit an O320 without cutting the notch.
A K&N E-3300 is 1/2 inch taller and will fit an O320 without cutting the notch. The air box will need to be deeper. Will hit gear leg.

K&N will flow more air, but it will also flow way more dirt...... https://www.nicoclub.com/archives/kn-vs-oem-filter.html

I believe both of these have less filter area as the filter thickness is less and so the pleats are shallower.

The trick on K&N specs is to look at their "oiling requirement".
The Vans spec filter is 16 mL of oil,
while the larger 3300 filter above is 13 mL of oil,
and the 3322 is 11 mL of oil.
More oil means there's more filter area.

I'm bummed :-(
 
K&N filters are terrible at filtering - this has been known for decades.

I used a K&N on my motocross bike for one race in the 70s, and I had to rebuild the engine due to the amount of dirt that got into it. The good part is that it allowed me to meet an amazing machinist, who gave me a great job, taught me to fly in his tripacer, and introduced me to homebuilding with his KR-2. So I'm really happy K&N filters exist! :)
 
Filter data confusion

I have done surface area calcs including pleat depth . And following guidelines for cfm requirements for an IO360, I built a custom FAB to match.

The FAB didn’t work out well for the SamJames cowl look I wanted, so I went back to the K&N cone filter supplied with my AFP injection system. I called Don to discuss the fact the calcs on the cone filter capacity were low compared to RPM/displacement recommendations.

Don advised that that same filter is used for 540’s with acceptable results. So go figure. Maybe sometimes we overthink these things ? 😏
 
The original topic was filter size. I think the "K&N is a good/bad filter" discussion should move to another thread.
 
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