Seems that when most folks play around with the idea of replacing their stock air filter, what comes to mind is something like a pleated gauze drop-in tray filter, or something more exotic that involves removing the stock airbox and using a pleated gauze cone-type of filter.

I have an incessant need to tinker with things, so I played around with the airbox on my XJ300, and this is the result. I have to hand it to Jag, they made removal of the stock airbox one of the simplest procedures I have ever seen: remove the hose at the side, unsnap the neck at the sensor, remove the one mounting bolt next to the fenderwell, tug up gently on the two rubber mounts towards the front of the airbox, and viola, it's done, and in one piece. I opened up the box and looked at the tray filter, and it was a typical Fram paper filter, and it looked like it was at about half it's useful life. I was going to replace it with a K&N pleated gauze filter, but looked around my spare parts stock, and found a Uni dual foam oiled foam filter, and it fit perfectly inside the sensor neck.

To me, the Uni foam filters have some advantages over an oiled gauze filter like a K&N. They're much lighter, so just pressing the rubber neck into the sensor neck gives a good tight fit, and I'm not worried that it's going to fall out. I might look into putting a light spring or two on there just in case, but I don't really think it's going to need it. Also, the Uni filters are about half the price of a K&N, are washable like a K&N, are good for as many miles, and can fit into some tighter spaces since they're not as rigid as most cone filters.

As for the hose that was attached to the airbox, I solved that by buying a valve cover breather filter that would fit a 3/8" - 1/2" fitting, bought some fuel injection hose (15mm OD with a 9mm ID opening), and had a 3/8" hose barb in my parts bin. I cut about a 5" section of hose, put one end of the hose barb in, put the valve cover breather filter on the other end of the hose barb, sprayed some Armor All on the hose, then stuffed it inside the hose that was attached to the airbox, all the way down so the filter was flush with the top of the hose. Everything fit snug, so no need for clamps or zip ties or anything like that. I check all the fluid levels every time I get gas (except for coolant, since it's under pressure), so I'll check the Uni filter and this filter too, to make sure they're still nice and snug. So far, so good, but I'll put some road test miles on this setup soon, and report back with any issues. But I'm sure any connection problems can be easily solved with some RTV sealant and a hose clamp or two.

The air inlet opening to the airbox is pretty small, so with the removal of the airbox, that opens up all kinds of space for this Uni filter to take in air. I've done this on numerous other vehicles I've owned, and the typical result is a little louder noise on the intake side, and the seat of the pants dyno says a little better response (and increase in mpg). It's easily undone for vehicle inspections and such, so I don't see any real harm with this kind of modification.