Ah... I see. Not just a K&N flat panel drop in, but a K&N FIPK of some sort?

And is the snorkel kit you've been considering already designed to integrate with a stock air box? I could see that as being one of the reasons you might be considering going back to stock. That, and the stock filter FILTERS (verb) better than the K&N. And the stock box itself is likely cheaper... heck, people are giving them away. Even in this thread!

But there is a reason why the stock box is being given away. The filter works better as a filter than most, but the box... especially the pre-2001 box, not so much.

Unless you are FORDing rivers with your Ex, and are worried about a bow wave of water flooding your grill up to the Ford logo, I can't see that using the stock box just because it is free and integrates well with a snorkel is worth the careful maintenance routine you might have to maintain with it.

The AIS mentioned in this thread will likely filter more dust, and have more dirt holding capacity, then a snorkel set up attached to a stock air box.

 

Someone explain to me the difference in my stock air system on my 2003 and the AIS system, They look exactly the same, but I do understand that the filters are different.

I use the oiled K&N filter in my stock box and my tube is spot clean going back to the turbo. Filter minder never moves and I clean and re-oil about every 10-15K miles.

I admit that I don't drive in dusty conditions, but what am I not really understanding? What would I benefit switching to an AIS or another system from my stock box?

 

Pop open the boot on the turbo inlet. If it's sparkly-clean in there with a little oil, you're doing fine. If there's any dirt... it's time to re-think your choice. The AIS is pricey up-front, and the cartridge cost causes cardiacs... but they last a very long time. I drive my truck 20,000 miles per year in a dusty environment, and I replaced the cartridge once a year on a chipped truck (30 PSI WOT). They can go significantly longer with the stock boost... and they are possibly the cleanest filter on the market - which is of paramount importance on our rigs. [LINK]

 

Tact... when you say your stock 2003 airbox and the AIS system "look exactly the same"... are we talking about the same thing?? To me, they look very different. You asked someone to explain... Okay, but be careful what you wish for! Starting from the top and working downward:

Airbox Lid:

Stock: Rectangular perimeter, hard corners, 2 spring latches, 3 finger tabs
AIS: Oval perimeter, radiused corners, 4 spring latches, no finger tabs

Why these differences matter::

The rectangular corners of the stock aibox lid are not as impervious to heat deformation leading to sealing surface distortion, latch to latch, as the radiused corners are of the AIS. Like the inherent strength of a Roman Arch supporting an aqueduct, the radius wrap of the AIS lid is mechanically superior at maintaining a more constant clamping force at the midpoint of the perimeter edges that bridge between the two spring latches on either sides.

Here is illustrated evidence of a filter seal leak, around midpoint between latches, on the rectangular lid of a 2001.5 to 2003 stock airbox:



In the photo above we are looking at the CLEAN sides of the inboard longitudinal edge (parallel to the vehicle axis) of two different Motorcraft replacement filters for the stock airbox. The bottom 2" unscreened filter is a 1680, and the top 3" screened filter is a 1710. Look at the dark spots on the crowns of the perimeter seal. That'd be the tell tale tracks of oil impregnated dirt intrusion. The fact that we can follow these tracks intruding over the top hump of the perimeter seal proves the one of the short comings of the stock air box lid that the AIS lid has addressed. More proofs to follow.

The 4 spring latches of the AIS offer superior "spring loaded" filter seal compression that is distributed more evenly throughout the perimeter of the filter gasket, as opposed to the one sided "squeeze to fit" stock airbox finger tabs on the outboard side with only two spring latches inboard.

The 3 finger tabs on the stock airbox were redesigned to be stronger in 2001.5. Prior to this redesign, these tabs would easily break, or more often than not the receivers for the tabs, also weaker in the late 99 and 2000 air box design, would either break.. or, the lid would too easily be snapped down on the inboard end while the fingers rode high on top of their receivers.

I'm sorry that I don't have digital photos available to illustrate this. They are all on film, and I've given away all the older air boxes I had from testing, so I can't reshoot the details. But this note is made here for the original poster: If you still insist on a stock airbox, be certain and obtain one that has a number 1 or higher in the first digit of the part number. Any part beginning with F, X, or Y should be considered as one of the evolving older stock designs, that may not have the subtle improvements needed to make the box more reliable.

Leaving the finger tabs aside and returning to the spring latches on the latest and last of the stock airboxes, what is wrong with this picture below?



This is a "latched" 2001.5 to 2003 stock rectangular air box lid. But it is missing something. Have a closer look:



A thin little bar of plastic is all that the spring clamp had to act against. Since there is only 1 spring clamp per side on the stock box, AND since the finger tabs are fixed, inducing a "rolling compression" on the filter gasket that requires more force from the spring clamp to snap the lid down inboard, that little bar was not quite sturdy enough to do the job for a hurried, minimum wage oil and lube jockey to be fooling with.

In the AIS photo below, compare and contrast the robust amount of lip material that simultaneously serves as an integrated cleat for the spring clamps to pull against to compress the filter gasket and retain the lid tight on the AIS:



There is no little bitty bar to snap off in the AIS, like the stock air box. That is a nice thick meaty lip to grip. There are also two ramps on the bottom to keep the spring clip from slipping off.

In the photo above, you can also see the radii of the lid that curves immediately away from the spring latch on one side toward the spring latch on the opposite side. The stock airbox by contrast makes a hard right hand corner, a bit further inboard from this latch, which is an inherently weaker way of applying even clamping force to the perimeter seal at the corners and at midpoints furthest from either latch.

There is more differences to observe in the photo above. I've run out of photo hosting space, so this close up will have to do. Notice how much higher the perimeter lip of the lid is over the snorkel inlet (look in lower left of photo above, where you can barely make out the top of the snorkel inlet, that is out of focus in the lower background).

This is a subtle but SIGNIFICANT improvement over all the previous stock designs. Here's why: The lid to the AIS does not ever get hung up on anything other than the sealing surface itself during the process of getting clamped down.

Let's take a closer look at the top of the snorkel inlet to the stock air box:



When Ford improved the stock airbox mid 2001, they added two vertical guidance tabs, pictured above and below, to help folks make sure to pull the lid toward the fender so that the finger tabs get inserted under their receivers. These vertical guidance tabs were a good idea, because without them, such as in the first design after the early 99 version was revamped entirely, the lid might get snapped down without being correctly aligned with the filter seal because the finger tabs were not engaged all the way into their receivers.

But these new guidance tabs themselves created a different problem:



The lid could get snapped down on top of them. Does this happen because the service tech or the owner is an idiot or careless? Not always. The problem stems from the profile of the perimeter seal of the filter, which allows the lid to "ride high" over and above the top of the vertical guidance tabs until the lid is snapped down tight. The durometer of the Motorcraft factory replacement filters is calibrated to have a certain amount of compression to make up for variances in the filter seal that are in no small part due to the distances between the spring clamps, the "rolling compression" induced by the fixed height finger tabs having to be inserted first, the rectangular shape, and the heat cycling of the plastic.

The lid "rides high" not due to the absolute profile height of the filter gasket, but more due to the fact that the 3 fingers to the lid have to be inserted into the outboard receivers first, and when done so with a filter in place, the lid is forced to ride high at an angle on the inboard end until the spring clamps are snapped down.

When the clamps are snapped down on top of the guidance tabs, two problems ensue:

1. There is an inconsistent compression of the filter gasket, leading to leaks on the clean side (top) of the seal

2. The lid takes on a new heat set shape after being softened and baked into this position, and then cured when cooled overnight. Enough heating and cooling cycles in this mismounted strain gives the lid a new memory, and it never seals right, because there is no constraining jig in use on the truck to make it conform back into the shape it was when new.

This is what a banana shaped lid looks like. This is at rest without a filter installed:




See how the left hand (inboard) side has subtle banana curve upward? And notice how close the snorkel inlet is to the lip edge of the lid? The AIS avoids these potential problems with a perimeter sealing lip that is far above and away from any interference in lid closure. This is a really important serviceability difference between the AIS and the stock box.

Ok, that's probably enough detail for the time being in examining the externally visible differences in just the airbox LID alone, never mind the rest of the air box, the internal air flow differences, or the filter media itself.

 

Anyone using the K&N flat panel with the newer style of stock airbox should make certain that they also have a newer style of K&N flat panel. The reason why gets back to the airbox lid again...

In the pre-2001.5 stock airbox lid, the center of a flat panel filter that was loaded to restriction could pull upward into the lid under suction. This upward pulling and flat panel distortion under load caused problems, like

1. Filter media tearout, disintegration, and injestion.
2. Perimeter seal pull out and clean side infiltration.

The fix for this was multipronged:

First, the filters were fitted with stiffening screens. Believe it or not, the earlier filters didn't have them! Both the Motorcraft 1675 for the e99 box and the Motorcraft 1680 for the 99.5 up box were flippy flappy paper. While later versions of these thin panel filters came with screens, the originals didn't have them. Second, the panel filters were made thicker and stiffer. This was also to increase dirt holding capacity.



On the left is a 2" Motorcraft 1680 without a screen, and on the right is a 3" Motorcraft 1750 with a stiffening screen. The similar Motorcraft 1710, which was the intermediate part number between the above two, also had a stiffening screen, and was 3" deep.

Compare the flexibility of a Motorcraft 1675 air filter from an early 99 box below:


PHOTO CREDIT: Solarflex from FTE/TDS

The filter shown above is somewhat fanned radially outward, and yet pleat tip blocking is still quite evident. Imagine how blocked up this panel filter is when seated flat. Now imagine the perimeter being held in place, and suction pulling the center of the filter upward, in a concave shape rather than the convex shape as being held by hand above. The flat panel filter is least constrained from suction in the center, so this lifting action when installed restricts this filter further by squeezing these dirtladen tips together, blocking the surface area of the pleat valleys, and opening vulnerabilities at the perimeter seal.

Ford addressed this by a hidden change in the lid design. A stand off post was integrated into the mold in the middle of the underside of the lid, to keep the air filter tame and in place:



Now here, because of this new post, is where issues with some of the earlier K&N drop in panels comes into play:



The damage shown to the K&N screen was from just one installation inside of a stock airbox on a flow bench for testing purposes only. Not a single mile was driven on this filter, and it was never installed in a truck. So this damage isn't even from road vibration. It is simply a 5mm post clearance issue. On this particular flat panel drop in replacement K&N, the filter media is positioned too high.

The difference might be seen in a comparable flat panel oil gauze filter for the same application:



In the K&N on the left, the pleat crowns are positioned higher relative to the perimeter seal than in the S&B filter on the right, which are lower seated.

Now, I could get into a huge amount of detail about the merits and pitfalls of these different types of filters, but that subject has been discussed to death in just about every automotive forum in existence. There are a lot of opinions out there, made without test data to back them up. When I tested dozens of filters and airboxes for this application, on a flow bench, indoors in a lab like setting with all other variables being equal, I found that this brand new K&N installed in a stock airbox flowed between 814 (snorkel variation) to 854 cfm at 25" water column restriction static flow delta P in steady state. By comparision, even a stock Motorcraft 1680 flowed an 824 cfm averaged over 30 test cycles under the same conditions. So we won't go there.

But when it comes to comparing AIR BOXES, rather than just air filter media, the focus needs to shift to the mechanical design attributes that a lot of folks don't give much thought to, like the profile, hardness, ductility, durability, and other characteristics of the perimeter seal of any aftermarket filter, and how well it integrates with the lip flange and lid design.

K&N later did redesign (after these tests were made) their flat panel drop in, so make sure, if you go this route, that the later design that clears this post is being used. I still see old stock K&N being sold on eBay that I am quite sure will collide with this post.

 

Excuse the filibuster here, but I wanted to get back to the OP's question... "which air filter is best" for the stock air box. That question cannot be rightfully answered without questioning the stock air box itself. Nevermind flow characteristics on a flow bench... let's take a close look at some tell tale evidence of flow characteristics inside the airbox itself:



Above we have a Motorcraft 1710 on the left and a Motorcraft 1750 on the right. Both filters are for this same application, and require an updated stock airbox with a 1" deeper snorkel inlet baffle plate premolded into the lower half of the airbox to protect the lead pleat of these filters, which themselves are 1" deeper than the Motorcraft 1680 that they superceded.

When viewed side by side, these lightly used filters, each with approximately 5,000 street miles on them before being rotated 180 degrees for another 5,000 street miles, has a very tell tale L shaped air flow signature left behind.

This flow signature is exactly the same on both filters (and would show the same if the filters were rotated 180 degrees and rephotographed). Notice the consistency in the inconsistent way the filters are being loaded? Why is air drawn in unevenly, biasing more into only 65% of the filter? What is it about the internal characteristics of the stock airbox that repeatedly creates this lopsided flow signature?

Here's another example:



See the same air flow signature (that L shaped dirty vs clean witness mark)?

This is also a Motorcraft 1710 that I believe to be an earlier production version from an alternative vendor using twin rows of glue beads to keep the pleats separated from each other so as to prevent pleat tip damming. These glue beads broke apart. The pleat count and pleat construction is also different on these black gasketed filters.

I recommend sticking with the orange gasketed Motorcraft 1750 filter for those utilizing the stock airbox. Wix developed it. In fact Wix first had to develop the machinery and production process to produce the fan folded pleats with pillow like embossments that protude out from either side of the pleats in order to keep the pleats separated without use of glue beads.

Have a closer look at the end result of this patented pleating process that was initially developed to address needs motivated by the 99-03 7.3L:



These protrusions embossed into the filter media keep the more of the surfaces of the pleat valleys open and available to draw filtered air through, and also increases the service interval by having more dirt holding capacity before restriction.

But even with all that said, to not also consider the AIS, which offers mechanical design improvements that far and away exceed the factory air box, as well as a filter media design by Donaldson that offers an incredible amount of dirt holding capacity in a much smaller, fully enclosed packaging envelope when compared to larger cylindrical pleated paper filters (like the 6637)... might be ignoring one of the "best" stock like air filters available.

The defense rests, your honor. (Or is it the prosecution?)

 

This is all very infomative and makes me glad I run an AIS. Nice clean and quiet. It flows well for the amount of filtration it provides. And if you shop around you can get filters for half the list price. I have new filters on the shelf.

 

Those 3 posts should be made into a sticky! Great explanation!! Reps sent!

 

Yes, this is excellent, and reps from me also.

 

Excellent write-up and explanation.

Looks like I'll be shopping for an AIS as soon as I can recoup after Christmas.

 

i can get a stock box for $25 but thinking now that the ais with the foam element will do better in the dust after doing more research

thanks again for the input

 

Your 314th, 315th and 316th posts where better than most of my 1-10,000 posts

 

These posts make me want to run out and get AIS for mine.

Tim M

 

Tugly you could do well auctioning yours off right now. LOL

Now who has the best price on them.

 

I was going to ask him how much he wanted for his.