Comparative RPM profiles

Our 1972 XJ6 came equipped with a Borg Warner BW12 automatic transmission (three gears with ratios of 2.40, 1.47, and 1.00) and a 3.54:1 Salisbury differential. With stock 15″ tires, the original cruising RPM profile is:

Stock XK6/BW12 Cruising RPM Profile
Speed (MPH) Engine RPM (3rd gear)
55 2488
60 2714
65 2940
70 3167
75 3393
80 3619
110 (max) 5000

The General Motors 4l60E automatic transmission we plan to install has four gears with ratios of 3.06, 1.63, 1.00, and 0.70. If we retain the Salisbury differential and stock 15″ tires, the restomod cruising RPM profile looks like this:

Restomod LS3/4L60E Cruising RPM Profile
Speed (MPH) Engine RPM (4th gear)
55 1741 (2488 in 3rd gear)
60 1900
65 2058
70 2217
75 2375
80 2533
158 (max) 5000

This suggests that highway speeds (65-80 MPH) will be very comfortable for the LS3 engine and reasonably quiet. In contrast, the XK engine strains at speeds over 70 MPH.

Salisbury differential

As the RPM profile above indicates, the 3.54 final drive of the Salisbury differential is optimal for our street application. But the high horsepower of the Chevy LS3 (over 400 HP) suggests the need for a limited slip differential (LSD). Jaguar installed LSDs on the V12 version of the XJ, but not on our model.

Salisbury differential after power washing

Gear ratio tag on the Salisbury differential

There is no way to tell the final gear ratio from an exterior inspection of the case. Jaguar affixed metal tags on the inspection plate with two numbers to indicate the gear ratio. In our case, this is
46 over 13
or 3.5385 (rounded to 3.54 for marketing purposes).

A second tag with BP-L indicates a “Power-Lok” limited slip differential, but unfortunately we don’t have that.

Limited slip upgrade

There are two limited slip enhancement paths: find an old used Powr-Lok assembly or install a new Dana 44 carrier with some modifications for fitment.

Used LSD sources

Another used differential, or the limited slip assembly from one, might be located at a junk yard. However, only some early E-types and the relatively rare XJ12 have the 3.54 ratio with the limited slip (Powr-Lok).

Jaguar Differentials by Model and Year
Year Model Series Track Width Gear Ratios Powr-Lok
1960-1963 E-type I 53.125 3.54 yes
1964-1967 E-type II 53.125 3.54 or 3.31 option
1968-1970 E-type III 53.125 3.31 option
1970-1974 E-type late 56.000 3.31 option
1960-1969 S-type 3.8 55.000 4.54 – 3.31 – 3.77 option
1961-1967 Mark 10 60.500 3.54 – 3.77 yes
1967-1975 Mark 10 60.500 3.54 – 3.77 option
1966-1970 420G 60.500 3.54 – 3.77 option
1969-1973 XJ6 I 61.750 3.54 no
1972-1978 XJ12 II 61.000 3.31 yes
1974-1979 XJ6 II 61.750 3.31 – 3.07 no
1979-1992 XJ12 III 61.000 3.07 yes
1975-1996 XJS all 61.000 2.88 – 3.07 – 3.54 yes
1980-1981 XJ6 III 61.750 3.07 – 2.88 no
1982-1987 XJ6 III 61.750 2.88 no

E-types have escalated in value, and finding a reasonably priced 1960-63 differential in good condition is highly unlikely. A used Powr-Lok 3.54 XJS unit might be located, but hot rod enthusiasts have for years been adapting the Jaguar IRS and snapping up old XJ units making it quite difficult to source good low mileage parts today.

Rebuilt Powr-Lok assemblies are available, however, and we elected to follow that route after a frustrating detour trying to fit a Dana 44 carrier.

Rebuilt differential with Jaguar OEM Powr-Lok carrier

Dana 44 carrier frustration

Since the junk yard alternative is becoming increasingly unrealistic, a new LSD carrier made sense at first. Jaguar no long provides XJ6 differential parts, of course, but the Salisbury differential is nearly identical (but not quite!) to a Dana 44, a popular differential used in hundreds of cars and trucks.

Jaguar first adopted the Dana 44 in the 1961 E-type but there are some “across the pond” differences between the Jaguar Dana 44 and American versions common to Corvettes and other cars. The Jaguar Salisbury version was manufactured by the British division of Dana Corporation and the ring gear uses slightly smaller mounting bolts while the pinion shaft has a different diameter. The Jaguar cast housing is also unique to the Salisbury models and was designed specifically to fit the Jaguar IRS cage. Still, Dana 44 innards can supposedly be transplanted into the Salisbury case while retaining the ring and pinion to preserve the 3.54 ratio.

LSD carriers come in many flavors. The Jaguar differential uses a 19-tooth spline so the correct match is a unit designed for a 3.54 ratio range with a 19 spline count.

Dana 44 Carrier Options for 3.54 ratio and 19 spline
Manufacturer Carrier Type Product Cost
Auburn Gear limited slip High Performance 5420112 $330
Auburn Gear limited slip Pro Series 542085 $430
Yukon Gear & Axle open differential YC D18458 $180
USA Standard Gear mechanical locker SL D44-19 $440
Richmond Gear Powertrax locker 2413-LR $400

Auburn Gear HP LDS carrier

The Auburn Gear High Performance line is the most “street friendly” while Auburn’s Pro Series is more aggressively tuned for track use.

The Yukon open differential is even more aggressive and not suitable for our application.

The mechanical lockers by Richmond and Standard approach traction from a different technical perspective. Lockers are less forgiving for on-road use and can be actually hazardous in wet or ice conditions. In contrast, limited slip carriers work well in all weather conditions; they also have the added benefits of quiet operation (lockers can make a disconcerting “clunk” sound) and better tire life (the unforgiving torque connection embedded in lockers puts more stress on tire treads).

Rebuild bearing part replacements

Our first Salisbury rebuild attempt retained the original case, output flange, and ring & pinion assembly. In addition to the new Dana 44 limited slip carrier, the following bearings were needed (cost information from LocateBallBearings.com):

Salisbury Differential Bearing Parts for Dana 44 Rebuild
Description Part Number Brand Cost
Carrier bearing 25590 Timken $18
Carrier race 25523 Timken $12
Pinion inner bearing Hm89446 Timken $25
Pinion inner race Hm89410 National $8
Pinion outer bearing M88043 Timken $22
Pinion outer race M88010 Timken $8

The rebuild will also need new pinion seals (2ha-019).

However, the experiment didn’t work. The spline shaft diameter was just off enough so that the carrier couldn’t be properly mounted. Plan B went back to using refurbished original Jaguar OEM Powr-Lok components.

Differential installation

Working on the rear suspension restoration

After replacing the original open carrier with Powr-Lok limited slip and adjusting shims and torque specifications, the rebuild differential was bolted to the cage as part of the rear suspension restoration.

Nose of the differential between two rear brake calipers

Differential mounted in the rear suspension cage

It is worth noting that the differential flange is purposely offset from the midline as illustrated by a sight line down the transmission tunnel.

Sight line down transmission tunnel shows differential offset