AIS filter
K&N drop-in filters are no good with the stock air box. Some have reported dusting of the turbo running the K&N drop-in filter with the stock air box.
If you have the Ford AIS filter like the one in the link, then just get an AIS replacement filter for it. See here: Riffraff Diesel: Motorcraft AIS Replacement Filter
I have the 6637 on my truck with zero problems. And they are cheaper to replace. There is nothing wrong with the AIS setup though. The lid on the stock air boxes tend to warp and cause unfiltered air to come through.
Just a little extra info for you. Hope this helps.
I've bench tested a lot of air-boxes and air filters, but I've never heard of a K&N drop in to replace the element in the oval shaped AIS air box.
I have heard of, and have tested, the rectangular shaped K&N and equivalent S&B drop in panel filters for two different versions of the stock air box. You heard right, avoid the stock box AND the K&N.
If you really have the oval shaped AIS, then your very best option also happens to be the cheapest. The Motorcraft FA1757 is $67.00 online, or $72.00 at Clay's.
By the time you're done doing a Tymar / 6637, you'd be looking at about $80.00, depending on where you sourced your plumbing. It could easily shoot north of $100, and you would lose some of the benefits people pay $200.00 for when they upgrade to the AIS.
Now, if you have a stock air box (filter is rectangular), then you have a choice to make. But if you already have the AIS box, there is really no better choice than to buy the filter designed for it.









