This thread was originally posted by Soccerbrace on the ClubXterra forum. All credit goes to him.

It will take you a while to read this but it can get done in much less than an hour. Don't be afraid to try this yourself. You can and will succeed! So stop trippin boo.
First, I have OWNED a K & N FIPK so I am not just relaying information. I want to tell you guys how to (IMHO-in my honest opinion) get the same performance of an after market intake using your stock intake system.
Start by cleaning your MAF (Mass air flow) sensor. I included a video. This guy on the vid is not me so I am not claiming to be the guy on here. Nor is that an Xterra, but the concept is nearly the same. Bear with the guy, he is about in depth as I am with the instructions. The MAF cleaner claims to give 4-6 hp at the wheels. :?
Video- http://videos.streetfire.net/video/4...6f015c3dac.htm
Here
(circled) is the MAF sensor. Pull it with a couple of philipps head screws.

Now remove your resonators and plug the existing holes.
Here are a few videos of vehicles with their resonators removed. Do not worry, we will sound throaty not ricey.Promise!:P
http://www.youtube.com/results?searc...&search=Search
My favorite is this before and after:
http://www.youtube.com/watch?v=9tVAZeEgFVg
You will see the term "T-Pipe" used quite frequently throughout this post. Here it is. It is located between the left front headlight and the air box.

You must buy the following test plugs. You can find these in the plumbing section of Home Depot. I only have 2 because I JUST installed a snorkel. You have to get 2 plugs -- one 2" plug and one 1.5" plug. MAKE SURE YOU GET THE GALVANIZED METAL PLUGS!

Start by popping your hood. This is what you will see. (duh) The circled area is where you will be working.

In the end, you should have these 3 thingamajigs out.

From left to right, there is the baffle (located in between the inner and outer fender). In the center is the resonator attached to the main intake hose and mounted on the wheel well, and on the far right is the resonator mounted to the top of the radiator from the T-Pipe.
First, remove the resonator attached to the main intake hose. It is outlined in the picture below.

We will start by loosening the hose clamp circled in yellow by using a Phillips-head screwdriver.

Loosening the hose clamp:

Next we will LOOSEN the lower resonator screw. Use a 10mm wrench, or a nut driver, or a stubby screwdriver such as the one pictured here:

You will need to get to the screw in the following picture. It is located underneath the brake booster on the wheel well.

This is a picture of the screw's location (after the entire resonator was removed) within a larger view of the engine.

The screw and resonator should look like this once you have loosened the screw and freed the resonator.

Next you will need to disconnect the two electrical plugs close to the inner fender to give you the working room you will need to remove the resonator. Don't forget to plug them back in when you are finished.

There is a black plastic clip holding a zip-tied wire loom to the resonator. You will take a pair of needle-nose pliers to pinch the bottom of the clip and remove the wiring from the resonator as shown.

Now, you will remove the remove the upper mounting screw for this resonator.

Once this screw is removed, simply pull the resonator out of the engine compartment as shown below.

Resonator removed. At this point, go ahead and reconnect those two electrical plugs that you almost forgot to remove.

Make sure you also remove the hose clamp as shown below.

Now you will place the 2" PLUG in the hole on the intake hose as shown below. To do this, simply place the plug all the way into the hole until the top metal is flush with the hose and tighten the wing nut. DO NOT TIGHTEN EXCESSIVELY. There need only be a snug fit, too tight and it will pop back out.

THIS CONCLUDES THE REMOVAL OF THE FIRST RESONATOR. CONGRATULATIONS, 1/3 OF THE WAY DONE!!!
Next we'll tackle the radiator-mounted resonator. It is outlined in yellow below. The inset on the bottom left is provided to give you an idea as to it's location within the larger context of the engine compartment.

First, remove the screw that fastens the resonator to the radiator:

Next, simply pull the resonator (which is loosely connected to the "T-Pipe" at the other end) out of the "T-Pipe" and then out of the engine compartment.
Next, you will remove the T-Pipe. First, remove the T-Pipe from the air box as shown:

Before going any further, remove the air inlet temperature sensor plug and then remove the sensor itself from the T-Pipe. Once it is removed, you will see a blue-painted element. DO NOT TOUCH THE BLUE ELEMENT! Treat the sensor with care and lay it aside in a safe place. The sensor's location is pictured below.

Next, twist the circled opening on the T-Pipe (that you just removed from the air box) towards the sky so that you can see directly inside the T-Pipe from a bird's point of view.

This next part is going to require a little bit of elbow grease on your part. Grasp the end of the T-Pipe farthest from the fender and pull towards the sky, then towards the fender so that you will twist the 90-degree bend in the T-pipe out of the inner fender wall as shown. The T-Pipe will not be inclined to cooperate. You're probably going to have to wrestle it out of the engine compartment.

After you've pulled the T-Pipe completely out of the engine compartment as shown below, you'll need to make one small cut.

Find the smallest opening in the T-Pipe where the radiator resonator was connected. You are going to trim it at the ridge indicated in yellow in the picture below.

You can trim it using a common steak knife, as shown below:

This is what the pipe should look like after the cut. You will only be cutting about 1cm off the pipe.

Next, you will remove the nut shown below. After it is removed, push the stud all the way through the inner fender wall so all that you see is hole. This nut attaches to the baffle that you will be removing in the next step. This is the easiest time to do this. I highly recommend a 3/8" driver with a 10mm deep socket for this task because of the limited space available to remove the nut. PUSH THIS STUD COMPLETELY THROUGH THE HOLE IN THE INNER FENDER WALL SO THAT IT FALLS INTO THE WHEEL WELL!

Finally, plug the end you just cut with your 1.5" plug, just like you installed the previous plug. Then, reinstall the T-Pipe back through the fender wall and reconnect it to the air box. This is also kind of a PITA
THIS CONCLUDES THE REMOVAL OF THE SECOND RESONATOR
Finally, you will remove the baffle, located in between the inner and outer fender walls. For reference, the baffle is shown below; however know that YOU DO NOT HAVE TO REMOVE THE FENDER FOR THIS JOB. This picture was taken during a different project, and is shown merely for reference.

First, you will need to remove the four screws holding up the fender trim, as shown below. Use a stubby screwdriver for this

Now that the fender trim is removed, start pulling down and pushing in one the outer fender lining to create space.

If the clips become completely detached, reattach them in the following manner



Pull down the outer fender liner, as shown below.

Look up into the fender and make sure the front end of the baffle is detached from the inner fender wall. Reach up to the top of the baffle on the aft end and yank down. YANK ON IT!!!! Put your back into it. It is only held on with double-sided tape. You should see something resembling the picture below.

Crank your wheels hard right.Once you have detached both ends of the baffle, and after much coaxing, your fender will give birth to a brand new baby baffle. simply slide it out of of the wheel well as shown below.

Now, reattach the fender lining and the fender trim. Alright! You are done!
YOU HAVE SUCCESSFULLY REMOVED ALL THREE INTAKE RESONATORS. CONGRATULATIONS!!!

I am only going to recommend the K & N drop in filter. From my experience this filter has been absolutely wonderful. I always feel a HP gain and it is guaranteed to do so. I am not mistaking this for throatiness either. It does give the exhaust a deeper,meaner sound which you will dig. Also, there is an increase in gas mileage that has been noticed in both my vehicles. It has been tried and true numerous times. One of the best things about it, is that there is a million mile warranty. This filter is completely re-usable and I would never go back to a stock filter after this and I just put one in my MR2. I paid around $50 for it. Google it and price match it . Copy and paste this part number.
K & N air filter (V6 Xterra VG33 engine): 33-2031-2

Be aware 2" body lift owners, if you have not replaced your air filter since the body lift, you are in for a treat. It is a pain in the butt to get it to seal properly. Andy (WhiteX2000) suggested that I take an exacto knife and cut the "lip" off the filter to allow proper fitment.
But James...Will the oil from the K & N get on my MAF sensor and rend it inoperable!? No it won't
http://www.youtube.com/watch?v=UjwBiPm2W5Q
http://www.youtube.com/watch?v=_SPd1...related&search=
http://www.youtube.com/watch?v=HrU-s...related&search=
http://www.youtube.com/watch?v=Ml8OI...related&search=

Alright my friends! Happy modding!
DISCLAIMER, ADDING A K & N AIR FILTER WILL NOT VOID YOUR WARRANTY. I DON'T THINK I CAN SAY THE SAME ABOUT ALL THIS STUFF