CJ7 Defroster Duct Modification

A lot of you guys may have never experienced the back side of a CJ dash. If you have, you probably chuckled if you saw my recent post (what have you done to your CJ today) about me adding two gauges above the factory pair. Why? Because there is NO room back there thanks to the OEM plastic defroster duct. "You'll find out..." people probably muttered. Well, I did find out. When I first test-fitted the new dash I hadn't put the heater box stuff all in yet. I didn't find any interference problems other than a little with the parking brake bracket and the magnets on the new speakers. You can see the notch I carved to address that.




After trimming that everything fit fine, so I turned my attention back to the heater box stuff and installed everything. That's when I did the ol' forehead slap and saw there was zero possibility of those two new gauges fitting in there. Hell, even the stuff on the back of the radio wouldn't fit quite right. At first I was going to just remove the offending gauges and rearrange a few things, but then I decided that it was stupid to be so restricted. So, I created my own duct system, and in doing so opened up a lot more room behind the dash.

I found that standard 2" x 2" thin-wall square tubing fit perfectly in the channel below the windshield ducts. After taking out the nice, smooth plastic OEM duct (wind tunnel-tested I'm sure), I measured the span.




After settling on 13 1/2" as my new length, I clamped it up and traced the ducts and screw holes.




Another trip to the hardware store for a couple of hole saws, (Why is it you never seem to have the size you need?) and bored the ends of the duct openings.




The metal cutoff saw helped me magically transform circles into ovals.




Instead of downward, toward the defroster duct in the heater box, I decided to make my plenum box exit out the ends. After all, the the channel runs the whole length of the windshield cowl, and that would give me a lot of hose-routing options. I stopped at O'Reilly's and picked up a section of 2" tailpipe and ordered some defroster hose. After going home and cutting all my parts I got the notice my new defroster hose was in. When I got back home with the new hose I realized my mistake: The new Dorman defroster hose was the exact same size as the 2-inch tailpipe I had previously bought and already sawed holes for. I had naively assumed it would be flexible vinyl crap like dryer hose, and hoped for hose flex, but this (Dorman # 708-083) was pretty quality stuff and didn't have much (if any) side flex to it. Back to the store I went--this time for 1 7/8" tailpipe. It was a perfect fit!




That's when I found the smaller, 1 7/8" pipe fit too loose to weld. I got out one my trusty (but seldom-used) exhaust pipe shaper tool and my favorite Ford hammer. I was able to put nice outward bends to fit the square tubing, as well as putting inward shapes on the hose ends to make them slide on easier.




Here's the finished plenum. Eh, I've done worse.




The lower duct was harder to build because all I had was 22ga. to work with (easy to cut and shape--harder to weld).




I shaped the strip that would become the lower duct outlet using a big socket mounted in my bench vise, fitting it to the outlet lip of my spare heater box as I went.




I used the same process on the pipe snorkel pieces as I did before--putting inner and outer lips on them the best I could. I'll tell you, welding that thin crap to the pieces of exhaust pipe is a pain. I decided to do lots of tacks and finish it up with seam sealer before I ran out of places to burn-through...




I left a small lip around it so I could "pot" the whole area with the seam sealer.




A little primer and paint, and we're ready for installation!




I left the hose in one piece like this at first because I'm always paranoid about having one side too short. I used stout zip ties to attach the hose to the snorkels. (Having a zip tie gun means you can get a good squeeze on 'em.) I was kind of surprised at the fit of the lower portion when I put in on. It was such a nice fit I didn't even need to drill and screw.




Lotsa room back there now!