Are you torn between choosing an NP231 and an NP242 transfer case?
If you’re just curious about differentiating the two, or you’ve held up your decision about choosing from either, then this post is for you.
This article will dig into the nuts and bolts of the NP231 and the NP242. It will highlight the pros and cons features associated with the two models. Furthermore, details on which transfer case will be ideally suited for your terrain demand will be elaborated with facts to help you narrow down your choice.
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NP242 (Selec Trac) is a full-time 4WD earlier owned by AMC, although Chrysler took over ownership. It has a high range (H) in direct drive and a low range (L). There are three modes 2WD, 4WD, and Neutral to which the following factory combination arises: 2WD high, 4WD high, 4WD high locked, neutral, and 4WD low locked. The NP242 features several vehicles, including the XJ Cherokee, MJ Commache, and KJ Liberty.
Contrarily, NP231 (Command Trac) is a part-time 4WD that features two ranges, high and low, with three modes stretching from 2WD, 4WD, and Neutral from which the factory combinations derived include: 2H, 2L, 4H, 4L, and Neutral. AMC also owned the NP242, but Chrysler now holds the rights. The transfer case premiered in several vehicles, including the YJ Wrangler, MJ Cherokee, and the XJ Cherokee. Additionally, aftermarket support allows the NP231 transfer case to be fitted with a 2WD Low position.
The NP231 is responsible for multiplying the torque in 2H, 2L, 4H, and 4L. Moreover, it works in unison with the differential unit to distribute the torque between the front and rear vehicle wheels. Contrarily, the NP242 transfer case is only responsible for multiplying the torque in the high range gears. However, the torque distribution in these vehicles is controlled by the center differential, located between the transfer case and the front differential.
The NP242 has a higher crawl ratio (this is the product of the first gear, final driver, and transfer ratio) when juxtaposed with the NP231. The higher crawl ratio of the NP242 dominates over the NP231 in terms of torque in lower gears. However, the NP231’s advantage over the NP242 massive crawl ratio is its higher speed at low RPM.
NP231 vs. NP242 comparison chart
The primary difference between the NP231 and NP242 is that the NP242 is connected to the center differential unit that distributing torque in front and rear wheels. The central differential locks to divert the torque from 100% 2WD in the rear axle to 50-50% torque between the front and the rear axle. However, for NP231, the transfer case locks the front and rear driveshaft to distribute the torque.
NP231 vs. NP242
Both the NP231 and the NP242 can multiply torque due to planet gear set motion. The gear set is driven by the transmission and is solely responsible for multiplying the torque in the vehicles. However, despite the similarity, NP231 and NP242 differ in many other ways.
For instance, the NP231 has two shifting forks, whereas the NP242 has one. The first shift fork, commonly called shift fork number 2, is responsible for selecting between the low and high gearing for 2WD and 4WD. The extra shifting fork is responsible for selecting between 2WD and 4WD. However, it is unnecessary in an NP242 which remains in 4WD throughout. This shift fork is also tasked with locking the two differentials when in 4WD mode.
Conversely, an NP242 uses a center differential to lock the two propeller shafts together. Inversely to the locking mechanism in the NP231, the locking in the NP242 allows slipping of the driveshafts since it uses pressure plates instead of a physical barrier between the rotating gear sets.
The NP231 has a single ring-planet six-pinion planetary gear set, whereas the NP242 has two sets of the same. The increased number of planetary gear sets allows differential action between the propeller shaft. This factor makes the NP242 ideal for off-road and on-road purposes.
The drivetrain linking the chain connecting the front and rear driveshafts is connected directly in an NP242, unlike in an NP231. This condition ensures that the chain is always engaged. Consequently, the NP242 drive chain will wear faster than in an NP231.
The transfer case input shaft is connected to the transmission on one end and the sun gear on the other. A set of planetary gears meshed between the sun and the transfer case ring gear is rotated by the sun gear. Some transfer cases will have two planetary gear sets and are known as open differentials transfer cases. The extra set of planetary gears allows the shafts to rotate at different speeds.
The ring gear to the transfer case is fixed in the transfer housing, thus preventing it from rotating. Turning of the planet gears multiplies the torque generated by the engine, thus reducing the workload on the engine and transmission, which could cause them to overheat, thereby stalling or become susceptible to damage.
The motion of the planetary gear sets is affected by shift forks mounted on the shift shaft. The shift forks are shifted manually using a shift stick, or the shifting is made possible by a motor initiated by your button pressing action.
One of the shift forks is used to shift between 2WD and 4WD in NP231. This shift fork initiates locking between the front and rear driveshafts, thus forcing them to spin at the same speed. A chain linking the rear and front propeller shafts ensure that the two shafts’ speeds are synchronized. It’s worth noting that this second shift fork is usually missing in a full-time 4WD like the NP242.
On-road driving requires vehicles with high maneuverability of the differential unit. Cars with the NP242 fair remarkably better in these conditions compared to the NP231. Unlike the NP231, where the axles will lock together between the front and rear, the NP242 will allow slippage in the wheels, accommodating the different speeds, especially while cornering. Slippage occurs as the NP242 uses friction plates instead of a locking mechanism like the NP231.
Differential action prevents skidding, which is undesirable in cornering. Additionally, skidding results in higher tire wear rates and, in the worst-case scenario, risks damaging the transfer case.
The NP231 will outperform the NP242 in off-road scenarios. This situation arises since the rear and the front driveshafts are locked together. Therefore, as the wheels rotate at different speeds, the driveshafts will turn at different rates. However, this action is prohibited by the locking mechanism.
Consequently, a twisting force will result in the interface of the two shafts, which is at the transfer case. This force will result in energy being stored and released through the vehicle skidding and skipping action to protect the transfer case from damage. Skipping and skidding allow the cars to escape undesirable traction areas leading to the vehicle overcoming challenges posed in off-road conditions.
Snowy and icy road
The NP242 will perform better in low traction situations such as snowy and icy surfaces. On icy surfaces, the wheels will experience tractional differences. The torque will be distributed by 50% in either axle and 25% in each wheel in ideal conditions. However, in a low traction situation, the torque will follow the path of least resistance.
Whenever a wheel to an NP242 has low traction, it will receive 50% of the torque unlike 100% in an NP231. Since the other wheels still retain some torque, the vehicle can move, whereas the NP231 might be stuck in such a scenario.
The NP242 aftermarket isn’t as enormous as the NP231, which boasts a variety of shifters, yoke options, and gearing. This is because most vehicles are likely to face off-road conditions compared to slick terrain, making them numerous. Moreover, the downsides for the NP231 have vastly contributed to the growth of its aftermarket.
For example, the lightweight aluminum cast housing and ineffective slip-yoke rear axle output have long disadvantaged it, attracting significant support to rectify it. Consequently, there are plenty of slip-yoke eliminator kits that you can choose from and plenty of stronger modified casings to select from.
It is possible to get a preowned NP231 or an NP242 at a modest price, with each ranging between $200 and $350. The cost will vary according to the transfer case’s mileage, the state it is in, and of course, the owner’s set price.
Rebuild kits for the two models will range between $100 and $150. However, the price ranges don’t include oil sumps and planetary gear sets, and their addition would considerably raise your budget. Typically, planetary gear sets might cost about $200, and an oil sump may hike the price of your rebuild kit to almost $500. Stock NP231 and NP242 will cost you upwards of $1,000.
The superiority of the NP242 and NP231 depends on your preference and the road surface you are constantly tackling. If you’re an occasional off-roader, then an NP231 will be suited for you. It will offer you a smooth experience without straining your transfer case. Moreover, there is an enormous aftermarket for you to explore the different options to upgrade your transfer case to grant you that ultimate experience.
Contrarily, an NP242 will be the most reasonable choice if you’re constantly handling slick terrain. People living in snow and icy areas will benefit most from the enormous torque these beasts can generate.
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