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April 2007 4x4 Tech Questions - Tech Line

Willie Worthy | Writer
Posted April 1, 2007

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Question: I own and wheel an '02 Jeep WJ Grand Cherokee and have issues with the NP242 Selec-Trac transfer case. Here's the lowdown: 5-inch lift, 33-inch M/Ts, locker front and rear, engine mods, lots of custom metal work, 4.0L engine and 42RE transmission.

My transfer case is giving up on me, and I was wondering if it was possible to take out the NP242 and replace it with a NP231? The reason I was thinking of doing this is the availability of the 4:1 kit for the NP231 and the reliability of the NP231 and to keep the weight down (compared to an Atlas II). What are your thoughts on this? Will it work?
Mike Sutherlin
Orlando, FL

Answer: I spent a lot of time checking out this swap and wasn't able to find much out about it, mainly because most people want to swap their 231 out for a 242 due to the advantages it has. In fact I even want to swap my 249 out of my own '96 Grand for the 242. My guess is that it should be a direct bolt-up since your WJ has the 4.0L motor. If it was the V-motor, then yes, you might have to disassemble both cases and swap the input gear from the 242 into the 231.

Yes, the 231 offers you the advantage of going to a 4:1 low range, but I think that the advantage of the ability to select so many combinations of drive outweigh the swap. Tom Wood's Driveshafts (www.4x4shaft.com) has a tailshaft conversion kit for the 242.

Question: My '01 Super Duty has a V-10, auto trans, and 3.73:1 axles. Since I put 285/75/R16s tires on, it runs 2,000 rpm at about 65 mph. I can currently squeak by with 15 mpg with a super-light foot. I have read in your magazine that if you go to a bigger tire, you should change your gears. If I don't do any crazy four-wheeling or tow anything worth mentioning, is it really necessary to go to lower gears? (Can you guys remember when you didn't think twice about mpg?)
Kenneth Branscum
FOB Summerall
Bayji, Iraq

Answer: Ford built the Super Duty with the idea that everyone who buys it will eventually tow or haul a heavy load, and they don't want them to be disappointed with the truck's ability to do so. However, they also wanted the truck to maintain the maximum fuel economy as well as minimum emissions, so the gear ratio is a compromise.

When one installs larger-diameter tires on a vehicle, it effectively raises the overall gear ratio. This, in turn, can sometimes put the engine out of the proper powerband that the engineers designed it to work in, especially when the truck is loaded. This gives the impression that the engine is not putting out as much power as it used to. The power output of the engine has not changed-you're just not operating the engine at the same rpm as before, and power output is in many ways related to engine rpm. That's what they make transmissions for.

If you're happy with the way your truck performs, and you are willing to grab a lower gear when hauling a heavy load, go a bit slower up a hill, and fuel mileage has increased, then there is not a problem. A decrease in fuel mileage is a pretty good sign that a lower gear is needed as it's an indicator that the motor is working harder than it was designed to do.

Something to keep in mind is that usually, when taller tires are installed, the tires are also wider, which means more rolling resistance. Combine this with some sort of a "lift kit" for the additional necessary tire clearance and this means more of the underside of the vehicle is exposed, which results in what is commonly referred to as "wind resistance." These factors all contribute to making the engine work harder for a given vehicle speed, so to compensate for it, a lower axle gear ratio is recommended.

Another reason for a change in axle ratio is that the "computer" in late-model vehicles reads engine rpm versus vehicle speed versus wheel speed and analyzes all this information to set certain parameters as to how the engine runs, the transmission shifts, and the braking system works. Generally speaking, changing to a tire size within less than 10 to 15 percent of the original OEM size does not have any effect on the system.

If you're happy with the fuel mileage gain and the way the truck performs, then I don't see any problem with the present combination.

Question: I am building a Jeep YJ with a spring-over lift with a BDS 3 1/2-inch lift kit and a 2-inch shackle lift. I will only be driving this vehicle on the sand dunes in Michigan and it will never see the road again. I have Chevy 3/4-ton axles, so my question is, would it be possible to avoid crossover steering by fabricating a mount on the outside of the frame and using a steering box from a Chevy truck with a dropped pitman arm and drag link? As I said, I will not drive it on the road so there is less wear on the steering.
T Boulter
via fourwheeler.com

Answer: Well, there are a couple of issues here that concern me. First, why so high up in the air? The spring-over will give you about 5 inches; add on the 3.5 inches of spring lift and the 2 inches of shackle lift, and wow! That puts the Jeep way up in the air. For sand running, I like my Jeeps as low as possible while still maintaining enough clearance for the tires to fully articulate.

Secondly, why go to the trouble of trying to figure out how to mount a steering box outside of the frame? I would think that it would be best to keep the original Jeep box, use a dropped pitman arm, and go to crossover steering. It will work so much better than what you had planned. Yes, with the Jeep sitting as high as you plan, you will have a bit of angle on the drag link, but most likely you can live with a bit of bumpsteer when out in the dunes.

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