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Air Box Modification

This tech article was originally posted at It is primarily intended for 1997-2003 Ford trucks but most likely also applies to other Ford vehicles.

Performed on a 1998 Ford F150 4x4 with 4.6L V-8.  5.4L V-8 and 4.2L V-6 trucks have identical airboxes.

Follow these instructions at your own risk.

Tools needed for the job:

Note: if your vehicle is leased then you may want to follow the instructions shown at F150Online as their procedure is reversible. I wasn't concerned in saving the original airbox.

Update: Finally posted the pics after having the digital camera back. As you can see in the pics below, the airbox over the filter still remains below the filter and about an inch wide where it clamps the filter to the MAF sensor. It is essentially the same as removing the whole filter box but you keep the support beneath the filter this way and don't need to make anything to replace the missing material of the outer filter box in the clamp.

Open filter

Air box mod

If you aren't concerned with saving the factory airbox or have already bought one from a wrecker so that you can reverse the changes, then follow my instructions.

  1. loosen the large band type clamp that holds the airbox together.
  2. loosen the hose clamp that holds the intake hose onto the airbox/maf.
  3. disconnect any electrical connectors such as the MAF as you'll need to pull up on the whole airbox assembly to get it out of the rubber grommets it is held down by.
  4. pull the snorkel/airbox cover from the inside driver's side fender.
  5. mark where you will cut the filter cover (attached to the snorkel into the fender) making sure to leave the two little stub legs that hold the air filter to the engine bay.
  6. cut the filter cover starting from the top about 1 inch from the edge which mated with the MAF so that there is plenty of plastic "lip" material left to hold the K&N filter. I used a hacksaw cutting from the top until a few inches from the bottom. Then I used a jigsaw to cut the rest as you have to cut some corners so you don't cut the two little feet off on the bottom.
  7. Use a file or coarse sandpaper to smooth off the edges of the plastic where you cut along.
  8. Now put everything back together - don't forget to clean your filter if you haven't done so for a while and re-oil it.

You're done!

Cut lines shown

Red arrow shows cut by hacksaw from top down 2/3. Green arrows show cut with jigsaw or coping saw to cut around stub legs and leave them on. DO NOT cut off the support legs on the bottom!

Direction of arrow

Mount tabs

Green arrow shows support legs under filter.

Cut filter plastic shown

Red arrows show hacksaw cut from top of filter cover, then cut remaining plastic with a jigsaw or coping saw at the base of the filter cover around the two stub legs.

I noticed a bit quicker throttle response. Some have said it adds a bit of intake noise but doesn't appear to on my vehicle. Whatever it added sounds good to me.

This modification does not change your requirements for emissions testing or engine performance in winter.

You may find lower performance in extremely hot weather and towing when under hood temperatures rise (less dense hot air). This is an extreme case and generally won't be a problem when your vehicle is moving.

This article is reposted with permission from Peter Ferlow of