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The Amazing Shrinking Air Filters

So for warranty purposes, does anyone know what cars this filter will fit? It's showing cheaper to buy it locally than at the aircraft suppliers (as usual) and it'll be easier to get them to warranty it at the local store if needed. Currently $46.99
 
One thing I've never seen posted here is why not build internal filter supports on the upper and lower sides of the FAB? Those would effectively hold the filter in its proper dimension. Back in the olden days of carbureted cars which used this same configuration of filter, the filter housings were always designed with a lip of some sort to hold the filter on its ID, top and bottom.

You could use "skin stiffener" stock as used in control surface construction-- that 5/8 x 5/8 x .025 90 deg angle supplied in kits. In the non-prepunched kits, that material came in 6' or so lengths. Could call Van's to buy some.

Cut notches spaced about an inch apart to allow the angle to bend enough to make the oval shape. It doesn't need to be perfectly contoured to the filter shape; the resulting flats from the notches 1" apart would be fine. It would be a good idea to drill small holes at the ends of the notches to prevent cracking. Make 2 and rivet those to the top and bottom of the housing.

You could shorten the angles a bit so as to not block the filter media, but on K&N's there's at least a 1/2" of non-flowing area top and bottom anyway.
 
I've decided to replace the filter at every other annual. Some shrinkage, yes, but the hard, fossilized seals are of more concern.

I lined the bottom of the airbox with .063 sheet, ProSealed in place. The aluminum gets a little fretted over time, but should last the life of the aircraft.
 
I called K&N, based on this feedback from forum and they mentioned the filter is not warrantied due to not being used " as designed" as the E-3450 is not meant to be used as a aircraft filter.

Next time don't tell them it's used in an aircraft. Tell them you use it in your RV. :)
 
That's my plan but I don't know what vehicle this filter will fit, do you know?

It doesn't matter. Say it's for a right-hand-drive Japanese import, and go online and find some unusual car and use that as a reference. Their lookup computer won't find it, they'll have to take your word for it.
 
One thing I've never seen posted here is why not build internal filter supports on the upper and lower sides of the FAB? Those would effectively hold the filter in its proper dimension. Back in the olden days of carbureted cars which used this same configuration of filter, the filter housings were always designed with a lip of some sort to hold the filter on its ID, top and bottom.

You could use "skin stiffener" stock as used in control surface construction-- that 5/8 x 5/8 x .025 90 deg angle supplied in kits. In the non-prepunched kits, that material came in 6' or so lengths. Could call Van's to buy some.

Cut notches spaced about an inch apart to allow the angle to bend enough to make the oval shape. It doesn't need to be perfectly contoured to the filter shape; the resulting flats from the notches 1" apart would be fine. It would be a good idea to drill small holes at the ends of the notches to prevent cracking. Make 2 and rivet those to the top and bottom of the housing.

You could shorten the angles a bit so as to not block the filter media, but on K&N's there's at least a 1/2" of non-flowing area top and bottom anyway.

If something similar to the above is attempted, be absolutely positive none of the added structure can come loose due to wear or vibration....there is nothing to keep anything inside the filter element from being sucked into the carb.

Personally...I wouldn't add that mod.
 
Shrinks Worse Than My 401(k)

I love VAF! Been flying our O-320 carbureted RV-6 for about 8 years on the same filter, unknown how long this filter was there before we got the airplane. It's been modified with the cutout at the bottom and someone did a nice job of molding red RTV around the accelerator pump. Serviced exactly as K&N says at each yearly condition looksee. I've always needed to do tricks with safety wire, scribe and majick words to get all the bolts back in. Never thought to question it, but it seems to have gotten worse every year and I finally got on the internet to see what the deal is. The rubber seals top and bottom are harder than Chinese arithmetic. I am not convinced that the cotton element is shrinking and reducing the diameter/circumference of the filter. I'm a LOT more suspicious of those rubber seals. I have another filter, still in the box and the plastic wrapper, age unknown. Thanks to all you folks posting here, I know what to look for during the current CI.
If it needs that gob o' sealant again, I'll use ProSeal. The RTV seems to have done OK, but I don't like that stuff around fuel or oil.
 
Shrinks Worse Than My 401(k)

I love VAF! Been flying our O-320 carbureted RV-6 for about 8 years on the same filter, unknown how long this filter was there before we got the airplane. It's been modified with the cutout at the bottom and someone did a nice job of molding red RTV around the accelerator pump. Serviced exactly as K&N says at each yearly condition looksee. I've always needed to do tricks with safety wire, scribe and majick words to get all the bolts back in. Never thought to question it, but it seems to have gotten worse every year and I finally got on the internet to see what the deal is. The rubber seals top and bottom are harder than Chinese arithmetic. I am not convinced that the cotton element is shrinking and reducing the diameter/circumference of the filter. I'm a LOT more suspicious of those rubber seals. I have another filter, still in the box and the plastic wrapper, age unknown. Thanks to all you folks posting here, I know what to look for during the current CI.
If it needs that gob o' sealant again, I'll use ProSeal. The RTV seems to have done OK, but I don't like that stuff around fuel or oil.
 
..someone did a nice job of molding red RTV around the accelerator pump.......I'll use ProSeal. The RTV seems to have done OK, but I don't like that stuff around fuel or oil.

Please watch the EAA webinars done by Vic Syracuse because he talks about the shrinking K&N filter and to never use RTV in that carb area and why.
 
Yup!

Carlos, thanks for bird-dogging Vic?s webinar for me. I?ve been reading his stuff in Kitplanes and he?s a great guy, very knowledgeable and willing to share what he knows. I?m a long-time A&P and among other things, I?ve been maintaining, preening and modifying our RV-6 and helping friends with their airplanes, including RVs for some years. I have seen some pretty incredible things on airplanes, too, and not just experimentals. The webinar was a great brush-up. As I said before, I?m not using RTV on the next air cleaner, for the same reasons Vic pointed out.
 
If it needs that gob o' sealant again, I'll use ProSeal. The RTV seems to have done OK, but I don't like that stuff around fuel or oil.

The RTV you're talking about is at the top of the airbox, so unless there's a big problem with an oil leak or fuel leak, it should not experience more than a misting of anything that is going to degrade it.

But if you want to use proseal, it's probably a superior product for the application.
 
Carlos, thanks for bird-dogging Vic?s webinar for me. I?ve been reading his stuff in Kitplanes and he?s a great guy, very knowledgeable and willing to share what he knows. I?m a long-time A&P and among other things, I?ve been maintaining, preening and modifying our RV-6 and helping friends with their airplanes, including RVs for some years. I have seen some pretty incredible things on airplanes, too, and not just experimentals. The webinar was a great brush-up. As I said before, I?m not using RTV on the next air cleaner, for the same reasons Vic pointed out.

I bought my plane built 10 years ago and I found a few things on mine to fix thanks to his videos.
 
Square K&N ?

I have the square K&N filter (not installed yet) for the horitontal induction snorkel.


Does this square filter shrink like the round ones?
 
I have the square K&N filter (not installed yet) for the horitontal induction snorkel.


Does this square filter shrink like the round ones?

Don?t know but the video does mention those square filters too so I?d take a look. If I were home I?d watch it and tell you but I?m on the road.
 
The air filters made of cotton shrink just like our clothing, especially when exposed to thermal cycles, 100LL and engine oil...oil draining down intake tubes and over priming.

This particular filter is undersized for the air volume passing through it for any of our engines of 360 cu in or greater turning 2700 rpm. Add to this the ram effect at 160-180 kias and it is easy to see why they shrink.

K&N does not warranty filters used for off road use.

I bonded a .032" aluminum plate to the fiberglass bottom to prevent the filter from wearing through and to provide better attachment for the alternate air door.

+1 on the size issue. The 3450 is too small and is poorly supported in the stock installation.

I see an intermittent (automotive) vs continuous (aircraft) duty cycle issue. I've always intended to use a 50% larger and more robustly constructed filter when i get to that point in the build.
 
I have the square K&N filter (not installed yet) for the horitontal induction snorkel.


Does this square filter shrink like the round ones?

Mine's 15+ years old and hasn't shrunk enough to tell yet. I clean it every 5 years or so. K&N's work better with some dirt on them.
 
Just addressing a couple of comments/questions:

I haven't seen any shrinkage on the square filters used in the snorkel inlets. I think it is due to the fact that they are installed in a position more like that in an automobile---in the cooler area of the engine, and not subjected to leaking avgas and other fluids.

Second, I don't know about the comments that the air filter is not the correct size. I vaguely remember doing some analysis a few years back and that size of filter does support the airflow needed for the engines in our RV's.

A quick test is to open the alternate air door during full throttle. If the engine surges, then the air filter is a likely constraint, but I have only seen that on some of the earlier Rod Bower ram air systems, where the filter was remotely located.

I believe the primary cause of the shrinking is the exposure to the higher temps , especially after shutdown, and the avgas that drips down on them, from carburetors when pumping the throttle prior to engine start, leakdown from injection units after shutdown, and from overpriming prior to engine starts.

I do think a aluminum sheet in the bottom half of the FAB is a good idea, as we replair quite a few FAB's where the air filter has eaten away at the bottom of the fab from movement.

Vic
 
I tripped over this issue yesterday. I'd removed the airbox/filter a few days ago for a carb replacement, and when I went to reinstall the filter, it was too small to easily reinstall. I'm sure I could have cussed it into place, but it just wasn't worth it - a new filter will make cleaning/reinstallation easy for the next several years.

A point of minor frustration was that the local auto supply stores used to carry the right replacement filter. They don't anymore, so I had to order one instead.
 
Just addressing a couple of comments/questions:



I believe the primary cause of the shrinking is the exposure to the higher temps , especially after shutdown, and the avgas that drips down on them, from carburetors when pumping the throttle prior to engine start, leakdown from injection units after shutdown, and from overpriming prior to engine starts.


Vic

I agree with the above. I have been flying my RV-4 for 20 years and have struggled with shrinking air filter and cracking air bracket. I solved both problems as follows:

I redesigned the air bracket by remolding with fiberglass. I have no metal in the bracket other than nutplates and filter retainers. The input opening to the air induction is round not oval. The opening is only about 2" diameter and I mathematically calculated the inlet flow far in excess of the air requirements.

Nothing on the bracket touches anything on the cowling and does not require any support brackets. A fabricated gasket was made from wet suit material of which I recently replaced. I have flown it for several years and several hundred hours and never have had a single problem. Dave Anders helped me with the design of the inlet taper degree which gives me about 1/2" more manifold pressure than the original bracket. It is a lot stronger and lighter than the original Vans designed bracket.

I am careful about over priming so I don't get any fuel in the air-filter assembly. I drilled a small hole in the bottom aft part of the air filter bracket that drains fuel or any liquid that may get into the filter box area. I don't clean the filter unless I can not see light through it. It stays pretty clean most of the time. I do knock out debris like bugs etc. It does shrink a bit but I simply stretch it back out by grabbing the sides opposite of each other and it returns to the normal dimension.

On inspection, I check the inlet to the engine on the fuel servo and it is always sparkly clean so I figure I must be doing the correct thing as far as filtering the air is concerned.

Hope this helps someone
 
Without searching back through the thread... Will K&N still replace a shrunken filter? They used to at one point, although you couldn't tell them you were using it on an airplane...
 
Without searching back through the thread... Will K&N still replace a shrunken filter? They used to at one point, although you couldn't tell them you were using it on an airplane...

We all have our comfort levels in terms of ethics...

Perhaps at some point, K&N will simply stop making the sizes that we use on our planes if they are replacing enough of them for use against their indications. I guess it depends on how many other applications those sizes are used for.

For me - I replace mine every 3 to 5 years - by purchasing one.
 
I just ordered one and paid for it. My ethics have a certain comfort level, too. It would be nice if K&N detailed their warranty info on the box, but I already know what those requirements are. Besides, I got something over 6 years and 600 hours or so from the last one. In the airplane world, that?s a howling bargain for a $46 air cleaner. No regrets.
 
For you RV-10 guys wanting a bigger filter, one option is to replace the standard filter with a taller one, the K&N E-1000. It is taller but same outside diameter.

You take the standard filter box and invert the top. Note the aluminum plate on the bottom of the box with the brackets. This was done to help prevent the filter from going conical.

This was for an RV-10 with a James Cowl and it worked very well. I assume it will work for a stock cowl but don?t know.

Carl

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I tripped over this issue yesterday. I'd removed the airbox/filter a few days ago for a carb replacement, and when I went to reinstall the filter, it was too small to easily reinstall. I'm sure I could have cussed it into place, but it just wasn't worth it - a new filter will make cleaning/reinstallation easy for the next several years.

A point of minor frustration was that the local auto supply stores used to carry the right replacement filter. They don't anymore, so I had to order one instead.
I found the E03450 K&N filter at Autozone for $47 I rodered it today and it will arrive at my store in 2 days!
 
What is the purpose of the small hole?

My filter box which looks the same (ref post #80 this thread) also has a small 1/8 inch hole at the bottom rear of the box located inside the filtered area. What purpose does that small hole serve? I can see one outside the filtered zone for water to escape but the small hole inside the filtered area seems like a source for unfiltered air to enter?
 
My filter box which looks the same (ref post #80 this thread) also has a small 1/8 inch hole at the bottom rear of the box located inside the filtered area. What purpose does that small hole serve? I can see one outside the filtered zone for water to escape but the small hole inside the filtered area seems like a source for unfiltered air to enter?

Excess fuel drain.
 
Okay. Thanks. I have a purge valve so that shouldn't be a problem? I guess the small amount of unfiltered air that enters is not a detrimental factor? Probably beats a small explosion?

If you over prime fuel will run down into the airbox so it needs a place to drain. Normally a #30 hole is what I use, not enough air thru that size to make a difference.
 
If you over prime fuel will run down into the airbox so it needs a place to drain. Normally a #30 hole is what I use, not enough air thru that size to make a difference.

There is also occasionally oil/gas goo that can accumulate in the bottom of the FAB, but inside the filter. I find that if I fly at low MAP and low altitudes, I may get a few drops of goo dripping from the hole Walt is describing. I suspect a film of oil is drawn from the valve guides into the induction tubes, where the next prime washes it down.
 
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