HP30 Information/Upgrades
 

So you want to know more about your XJ front differential?

Or you want to keep and upgrade your XJ Dana 30 front axle?

But you’re not sure what to do, what type of locker? Selectable, Lunch box style, Detroit Locker, ARB, Limited Slip?, Etc..

What Gears and ratio?

Selectable Hubs?

I hope this information helps!!

Let's start with some basic terms

High Pinion (or HP) refers to an axle that the pinion enters the axle above the axle tube centerline. These axles use a Reverse Spiral cut gears and derive alot of additional strength from that Reverse Spiral cut of the gears. They also give more ground clearance because of their higher pinion design.

Low Pinion (or LP) refers to an axle that the pinion enters the axle below the tube center line, the ring and pinion gears in these axles actually run with the weakest side of the gears propelling the vehicle forward




There are 3 front axles found in XJ's (all 3 are Dana 30 or “D30” differentials in either the HP or LP design)

Credit should be given to Crash for much of this information

All of them use an outer Unit Bearing style hub assembly (the bearings are built into the hub and the hub and bearings are replaced as a unit) The hub is always turning the outer axle shaft that is bolted thru the Unit Bearing and the shaft is what keeps the Unit Bearing from separating and falling apart. (Useful to know if you have u-joint or shaft failure on the trail, you cannot just remove the entire shaft and drive the vehicle with nothing bolted thru the center of the unit bearing, it will fall apart)

1984 to 1992 XJ's came equipped with a HP30 vacuum disconnect front axle that has an intermediate shaft on the passenger side that physically disconnects from the outer shafts, saving on gas and wear and tear on the drive train. The vacuum shift is shifted by the transfer case.

1993 to 1999 XJ's came equipped with a HP30 front axle (non-disco) that has one piece inner axles and rotates the differential and front drive shaft whenever the vehicle is moving.

2000 and 2001 have a low pinion D30 front axle (sometimes referred to as the TJ axle)


Strengths and weaknesses

Axle Joints:

Some 1984 to 1986 XJ's came equipped with CV (Constant Velocity) axle shafts. These are higher maintenance, hard to find, and most people swap them out for U-Joint shafts.

1986 to 1994 XJ's came equipped with the small (260 series) u-joint, EXCEPT All ABS and NP242 equipped XJ's came with the 297 size joint.

1995-2001 had 297 sized (larger and stronger) u-joints. 297 sized joints are the strongest of the three!

Housings:
The earlier disco housing has a two part tube on the right side with a separate cast piece pressed in the middle for the disco assembly, it is generally considered a slightly weaker housing then the non-disco housing and is much more difficult to install a truss onto. All disco housings have smaller 260 series u-joints and a three piece right axle shaft. (However it is possible to install a 2 piece axle shaft from a non-disco housing if the carrier is removed and a special seal is installed for the right side.)

The non-disco housing is the preferred housing stock, or as an upgrade to the disco housing and the Low Pinion housing. It is the easiest to install a truss system onto and very strong for it’s size. Many people have run 35’s with this housing by upgrading to Alloy axle shafts.

The Low Pinion housing is arguably the weakest of the three, and more prone to ring and pinion failure IMHO.


Upgrades:

Gears:

Ratios from 3.07 to 4.88 are available for all three of these axles. There is a carrier break at ratio 3.73, so any gear replacement on 3.73 or numerically higher gears, requires a different carrier to be replaced with the gears. All High Pinion axles require Reverse Spiral cut gears and are not interchangeable with standard cut gears. However low pinion and high pinion axles do use the same carriers, so they are interchangeable.

Selectable hubs: (AKA lock out hubs)

There are several kits available that replace the unit bearing etc. with parts and hubs that can be locked or unlocked at the hub itself. These kits are fairly expensive but do have advantages in fuel economy and give more options for locker installation.

Lockers: (not to be confused with selectable or lock out hubs)

A locker refers to a device installed inside the housing that locks both front wheels together when power is applied; this eliminates spider gears in stock open carriers, which only allow power to be applied to one front wheel when power is applied.

There are many different styles of lockers available for these axles.

Lunchbox Style: (one of the cheaper options)

A lunchbox style locker is installed inside the stock carrier, it replaces the spider gears with gears that lock together when applied but still can ratchet or disengage when the vehicle is cornering. These style lockers are very noticeable and noisy when driving on the road and can have some adverse effects to drivability, they are generally not recommended on an HP30 with no selectable hubs in place.

Full Locker: (non- selectable)
A full locker like a Detroit Locker for instance, replaces the entire carrier with a new unit that is a stronger assembly than a lunchbox locker, and does the same job of locking the two wheels together when power is applied, and ratchets when the vehicle is cornering. They are generally a more durable and less noisy locker, but they also are generally not recommended on an HP30 with no selectable hubs in place.

Selectable Locker:
A selectable locker can be disengaged when normal driving is desired and then engaged for off road driving and have the same fully locked characteristics of a lunchbox or full locker. They are more expensive than the lunchbox or Detroit lockers, but a much better safer choice for on road drivability. An ARB or an OX for instance, are totally unlocked when disengaged and perform the same as stock open carriers, but when engaged are fully locked. The ARB uses an airline and compressor to engage the locker, an OX uses a cable to engage the locker and also comes with a very strong differential cover

There are also selectable lockers that use electrical power to engage them, such as the Auburn and the Eaton

Limited slip:
Another option for these axles is to install a Limited slip style carrier, like the Detroit Tru-Trac Limited Slip, this style uses a special gear style that helps apply power to both wheels when needed, but it is not locked, if one wheel is not in contact with the ground, all power will transfer to that wheel and loose traction, it is effective in mud and racing applications, but has limited effectiveness in rock crawling if one front wheel leaves the ground. They are considered to be mostly transparent for on road driving.

There are also clutch style limited slip differentials from the factory and aftermarket, but I don’t believe any are available for Dana 30’s. So I won’t go into those.

Spools:
These replace the stock carrier with a spool that locks both front wheels together at all times and has no provision for ratcheting or disengaging on corners. Generally considered for use in trail only vehicles.

Trusses:
A truss like the TNT truss for instance, is a metal structure that attaches across the housing and is welded to the axle tubes on both sides. It is an excellent upgrade and adds a lot of strength to the housing to prevent bending the housing, it also adds strength to the control arm and track bar mounts and also helps limit gear deflection that occurs under high torque situations. A skilled welder should install a truss and take care not to warp the housing from excessive weld heat.


Quick reference:

HP Dana 30 – 27 spline inner, Carrier break at 3.55/3.73, Gears available in 3.07, 3.54, 3.73, 4.10, 4.56, 4.88

LP Dana 30 – 27 spline inner, carrier break at 3.55/3.73, Gears available in 3.07, 3.54, 3.73, 4.10, 4.27, 4.56, 4.88, 5.38

Stock ratios:
As a general rule - the axle ratios are as follows:
4.0L - Manual Transmission - 3.07
4.0L - Auto Transmission - 3.55
2.5L - Manual Transmission - 4.11
2.5L - Auto Transmission - 4.56
BUT! Towing packages and other un-explained variations came with different weird gearing options - so be sure to check!

Tire size to gear ratio:
As a general rule for preferred performance & fuel economy:
31’s to 32’s = 4.10 ratio
33’s to 34’s = 4.56 ratio
35’s to 36’s = 4.88 ratio