This is my interpretation of a cold airbox. Cost~$10
Cold airbox prevents underhood hot air from making intake air excessively warm--cooler air=more horsepower, better throttle response, (maybe) better gas mileage. For more info about cold air, see this link, this link, and the place where I got the idea, this link
EDIT: Marcus has added his interpretation of a cold airbox to his "Go Jeep" website: Marcus' airbox insulation (scroll down a little, past the manifold heat shield).
EDIT2: I should add that some of the tape is coming off my airbox--the 'shielding' isn't exactly permanent, the way I've done it. Marcus' version is faster, easier, and simpler to maintain, I suspect.
-one roll of noninsulated thermal tape (thin strip-aluminum with sticky backing) from Home Depot [heating/air cond (HVAC) section of the store]
-one roll of insulated thermal tape from Home Depot (aluminum foil backed with foam) [same section of the store]
-smallish (10" x 6") piece of (flexible) windshield sunshade/exhaust wrap/heat shielding
-3 medium-length zip ties
-tin snips to cut the aluminum tape
You need not use both kinds of thermal tape. The foam-backed stuff insulates better, but the aluminum tends to detach itself from the foam. I used the regular aluminum tape to reinforce the insulated stuff.
1) remove airbox, clean it thoroughly, cut pieces of aluminum tape to fit shape of airbox (time consuming--probably 1 hr.), then reinstall airbox
2) remove & clean air hose between throttlebody & airbox, then reinstall
3) wrap piece of sunshade/exhaust wrap/heat shielding around the air tube, & secure with zip ties
I didn't use tape on the air tube b/c tape would not stick to it. The wrapping I used was some stuff I saw fall off another guy's exhaust on the street one day (I tried to catch up to him, but he got on the highway, so I went back & picked up the exhaust wrap, & I still have some of it today ). Some guys use a cut-up sunshade to cover the whole airbox, but I thought it would get in the way of the clips that hold the lid on, & the little CCV tube, so I used tape, which can be cut to a more precise shape--but it takes much longer to do. Pics:
This was probably the second or third mod I did after I bought my XJ, & it is the one that has produced the most perceptible result. Here is the website where I first got the idea (pics on this site--I'll post my own pics later): http://www.angelfire.com/my/fan/manifold.html.
Cost~$25 or a little less (the way I did it).
-roll of thermal shielding: Thermo-Tec, part number #THE-13575 from SummitRacing ($17.88 at time of this writing, excluding shipping costs). Or, you could fab a shield out of sheetmetal if you have some, like GoJeep did. I prefer the flexible stuff, because you can just jam it in under the manifold--with the sheetmetal, you have to remove the airbox & possibly some other parts just to get it in there.
-metal strapping kit [see picture] (in the exhaust section at AutoZone & other stores)--maybe $3.00? Or, go the route that Dino took (which will double the cost of this mod), & buy Thermo-Tec's strapping kit from Summit Racing ($18.88 at time of this writing).
-one or two screws with washer & nut (should come with the metal strapping kit if you get the same one I did [see picture])
-tin snips (to cut the metal strapping to length)
1) follow the link to GoJeep's intake shield to see the shape & dimensions you need to cut the heat shield to (in millimeters) [I'll update this with the figure in inches later]
2) shove the heatshield in underneath the air intake manifold--if you cut it right, it will probably stay there without any support, but
3) to secure it, you should strap it to the intake manifold by means of the metal strapping--run the strap underneath the manifold, cut it to the right length with tin snips, & use the nut & screw to secure the ends of the strap.
Small extinguishers are cheap ($10) & will come with a mounting bracket. You have to decide where to place it though.
Other parts/supplies: 1 screw, 1 nut (no larger than #10 x 1/2"--I bought a $0.99 packet of  #10-32 x 1/2" screws & nuts f/Home Depot), and a screwdriver. If you are not mounting it where I did, you'll want beefier screws, but the bracket on my extinguisher wouldn't have taken anything larger than a #10 anyway.
Generally speaking, you want to be able to get to the extinguisher quickly (obviously), so take that into consideration when you're looking for places to mount it. But you also have to mount it securely--the bracket that comes with it won't keep it in place if you get into a car accident!!... unless you mount it under a seat, in which case you might get away with just using the bracket, though a zip tie around the bottom of the extinguisher wouldn't hurt (bracket usually holds the extinguisher from the top, but not from the bottom--when you need it, yank it from the top & it will slip out of the zip tie at the bottom).
I couldn't do this b/c of my leather seats (they're shaped differently than the cloth seats--no room under there). I didn't want it in the cargo area, lest it break loose & smash some thing or person. So, I put it under the rear seat, which, handily, has a cross brace with mounting holes already available. This is under the rear seat, driver's side (the tire jack takes up room on the passenger side) & you can see the holes in the cross brace, which I used to mount the extinguisher:
Here it is installed (the thing on top is my WalMart 0/2-guage jumper cables, so you can see there's room for more):
Talk about a projectile (sp).....That mag light it not that secured and sitting heavy side forward... if you get in a front in collison that will be a 3D missle instead of a 3D Mag......
JMHO Good Luck Happy Jeepin!
All you do is buy a larger filter with the right threading to match the OEM filter. The NAPA filter part number is 1515. Mobil1 filter part number M1-301. The Bosch filter number is 3500. Not sure about other filter brands, but see the linked article for more info.
Doesn't cost any more than the usual filter. Falls into the "why not?" category.
Haven't done this one yet, myself, so this is all borrowed.
The stock XJ wiring harness is quite inefficient. Owners report 2 volts less power at the lights than what should be there. Also, owners who upgrade to Hella, IPF, or Cibie H4 headlight housings/bulbs are sometimes disappointed, if they haven't also upgraded their wiring harness. The stock wiring just doesn't pump enough juice, and may even fail when used with high-powered bulbs. Purchasing a harness can be expensive, so several forum members/users have made their own--usually for around $50.00.
in no particular order...
1) DefaZ's diagram, writeup, and genesis (see p. 2 for pic of relay mounting location). I've linked the diagram separately because the link in the original writeup is now broken. I've also attached it to the bottom of this post.
2) Patrick Norton's pair of writeups (with pics) on the harness and headlight upgrade: page 1, & page 2.