Air Box Conversion


We wanted (ok…I wanted) more power out of our 4 banger TJ. So I started looking at the intake system and figured an FIPK would be a good way to let the motor breathe better. I was going to be adding a 4.0 TB as well as a TB spacer anyway, and then a more open exhaust. So the FIPK seemed like the easiest starting point. But when I found out that they cost almost Cdn $400 I laughed. There’s an easier way. Which is what this is all about.

The Jeep this was done on was a 98 TJ, 2.5 L, 5 spd, with around 80 to 90,000 miles on it.

The plan was to make my own FIPK so I looked around for a suitable K&N filter, but when I finally found one it cost almost Cdn $100!!!!! No thanks! I ended up going with an identical filter from APC (American Products Co ) that I stumbled on while looking around in the auto parts store. Looks exactly like the K&N I was after, and was half the price. I went so far as to call APC and talked to one of their reps and even though they didn’t say it was an actual K&N, they told me its re-useable and to use the K&N kit to clean and re-oil it. We’ll see. So with filter in hand I was ready to start my own intake.

Here’s a material list of what I used.

(1) APC Intimidator series Air filter
(1) Universal Crankcase vent
(1) Rubber Straight coupler for ABS tubing (with 2 clamps) from Home Depot
(1) ABS 90 degree fitting
(1) ABS 45 degree fitting
(1) ABS tube; 3ft long x 2inch dia

Before I started on the new intake tube I took off the stock TJ airbox assy and air tube (man does that free up a lot of space). Just for fun I put the filter right onto the TB and drove it around all day! I could sure hear the air being sucked into the TB! Kinda cool, but got old real quick.

After looking at the engine compartment for a while I realized there wasn’t a lot of room to run the air tube straight forward like I wanted to. Probably not a good idea to run it directly over the exhaust manifold anyway. There’s too much stuff on the drivers side of the compartment. Between the radiator, shroud, washer fluid reservoir, power steering fluid reservoir and the coolant over flow bottle, there’s little to no air movement in there. I thought of moving the stuff, but it’s just too much of a pain/hassle! So I had to settle for angling the air tube diagonally across the top of the motor.

Putting the new air-tube together was pretty easy. I put the straight adapter onto the TB (nice snug fit) and pressed the 90 degree fitting into the other end of the adapter. Then added the straight 3 foot section to the other end of the 90 degree fitting, and figured out how much to cut off the length to position the filter. It turned out that the length of the tube ended up around 24 inches. At this point I could have just clamped the filter on the end of the tube, but I wanted to take advantage of the open ended design of this filter. Because this filter doesn’t have a cap on the front of the filter element, it lets air in from the front as well as from the sides. The open end would allow air to go straight through the filter and down the tube. Unfortunately, the way the tube crossed over the motor, the filter would have ended up more or less sideways to the air flow. This is where the 45 degree fitting comes in. Attach the 45 degree fitting to the end of the tube and attach the filter to the other end. The 45 degree fitting more or less corrects the direction of the filter.

Once I had everything in place I used duct tape to hold the pieces together so they wouldn’t vibrate loose. The duct tape was a temporary measure because I wanted to try some other positions for the filter, so I wanted to be able to take it apart. Once it was done and finalized, I planned to go to an exhaust shop and have them bend me a one piece tube out of 2 ½ inch exhaust pipe.

I still had to deal with the breather hose. Because I didn’t want to drill a hole in the tube and figure out how to hold the hose in place, I replaced the breather tube with a universal crankcase vent filter (also from APC).

After doing all this, I’m actually going to re-install the stock air filter box in its original spot. After I had this all done I realized that the filter box can be used to store things like spare spark plugs, serpentine belt, tire repair kit etc!! Just have to block off any open holes where water could get in.

After driving around with it for a while, there definitely is a difference. I’ve really noticed a difference when passing other cars! The jp seems to have more “get up and go” than before (especially in the 2000 to 3000 RPM range), it doesn’t lose as much speed on the hills, and as an added bonus, the RPMs are down a bit! Keep in mind this is all very unscientific. I have no actual dyno numbers to back it up. Just seat of the pants feel. The way I figure it, the setup can’t hurt, and if it helps it’s a bonus.

Update: It’s now been about a year since I first did this mod and the original write-up for a fellow jeeper that was interested. I pulled the filter to clean it and while it was off, I looked down the PVC tube. What I seen concerns me a little. There was a fine layer of dust inside the tube. Now if that’s what’s in the tube, I gotta wonder what got into the motor. Two things can be done here. Add a pre-filter on the element, or go back to the original airbox set-up.

Personally I’m leaning more towards going back to the stock airbox set-up (somewhat modified). First of all, after seeing the dust in the tube, I’m not so sure these filters are worth it, and secondly, I’m worried about getting water in the motor. There is no way to drain off any water that may get into the tube without reducing the effectiveness of the air tube itself. Sooo…back to the original airbox type design.

To each his/her own. If you try this and it works for you, great!!! It’s worth a try, it does feel like it makes a difference. If you’re a commuter and a mall cruiser, hey this will work for you. If you’re into rock-crawling more than flying through water, then maybe the pre-filter idea would solve the dust issue. If you’re into the water end of the fun, maybe try fabbing a splash guard. Whatever you do, It’s cheap enough to try it and you can have some fun as well.