Mirror of: http://www.concentric.net/~jman/jeepmods.html

Removal Instructions

This modification is for the 4.0 liter 6 cylinder engine in the Jeep Wrangler (YJ). The pictures were taken of a rather muddy 1994 model, but I know it can be done on model years '91-'95. While you're at it, you might want to clean the throttle body. That is located at the other end of the 'hose' (as described below). FYI, this modification will not work in a '97 or newer Wrangler (TJ), that vehicle has a different air box. Also, I have some information on my gas mileage before and after, you can see that below.

New Info!

I've been told that this mod can be done on the '91 thru '95 2.5 liter 4 cylinder engines! I don't know if it can be done on later years, but check out your 4 clyinder and let me know if you can do the mod on your vehicle.
#1 - Airbox with location of restricter
1) Open the hood and locate the air filter box. The restricter you want to remove is inside the top piece.

#2 - loosen hose clamp
2) loosen the clamp for the hose with a regular (flat end) screwdriver

#3 - Restricter lip
3) rotate the hose to one side, you can now see the lip of the restricter

4a) With the same regular screw driver, pry out restricter
#4b - it's tight, but will come out
4b) It is snug and you may want to use a second screw driver on the opposite side to help pry it out.

#5 - replace hose and secure clamp
5) put the hose back on the air box and squeeze shut the clamp

Your done, enjoy the extra oomph!
restricter view #1restricter view #2
this is the piece that was removed.

Gas Mileage

I've been keeping track of my gas mileage since I purchased The Jeep in July of '97. I do 80% highway, 20% city driving and I have a lead foot. My average gas mileage up to the modification was 18.43, with a high at 20.09 and low at 15.99. My first reading with the restricter taken off is 18.60. 

The Story

(Originally posted 5/27/98 in rec.autos.makers.jeep+willys. The below is posed with permission from it's author, Thomas Hereford)
I have a brother-in-law who is a big wig at Chrysler and well read in
whats what. This guy is a wealth of info.

He told at a party Sunday that the reason the YJ's had 10 less
horsepower that the XJ's is because there was a big to-do about not
wanting the populus to be worried about safety in having too much power.
The decrease in HP was accomplished by putting a tapered horn inside
the  air hose where it attaches to the air filter box to decrease the
air flow the the engine thus decreasing HP/performance.

Sure enough, I got home, removed the hose and there it was.  I yanked
that bad boy out and now I'm doing 0-60 in 5 seconds...well, maybe not
that fast....kinda like being able to run faster with new tennis shoes I
guess...well you know....
            (__)      (  Thomas Hereford, Plymouth, MI
            (oo)      (  therefor@pobox.com
   /---------\/   --< (  Product Data Management Administrator,
  /|        ||          (  Simpson Industries, Inc.
* ||-------||
  ~~       ~~

Questions? Jason Smith