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Original submission by T:

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Differential Information

A Differential in an old Holden consists of a Pinion Gear Driving a Crown Wheel. The Crown Wheel rotates a Planetary Gearset. The Planetary Gearset then drives the Axles which are connected to the Rear Wheels. The Planetary Gearset allows the Differential to distribute torque to the wheel that is the easiest to turn. This allows a car to turn a corner without having the rear wheels lockup. The Planetary Gearset is inherently non-linear. That means that the torque is distributed to the free spinning wheel disproportionately. If you jack up one wheel, that wheel will spin twice as fast as it would spin if both the axles were rotating together. * Diff Trouble

Differential Operation:

This is how the gearset behaves when both axles are rotating at the same speed. The view is looking from the front of the car towards the rear with the vehicle travelling forward. Image by T. DiffEqRot.gif  
Diff Pinion Gear. Photo by Crocky. Click to Enlargen.


if the car is original and you can guarantee it has the original diff you could go off the body ID plate under the bonnet.
However if someones replaced the diff at some stage it may have a different ratio.Look for the plate under the bonnet on the cowl with ENG TRANS RRAXLE down the bottom. compare the code next to RRAXLE to these:GM9 = 4.44:1
GU4 = 3.08:1
GU5 = 3.23:1 Salisbury Borg Warner 78
GU7 = 2.78:1
GU8 = 3.90:1
GV2 = 3.36:1 (V8 HQ's Only)
GV3 = 2.78:1 Salisbury (V8 HQ's Only)
GV4 = 3.36:1
GV7 = 3.55:1
G70 = 2.60:1
G95  = 3.36 Banjo 

Grey Motor 3.89 And Red Motor 3.90 Diffs:

FX to EJ had a 3.89 rear axle ratio standard. These had a 35 tooth crown wheel & a 9 tooth pinion gear. Optional for EH onwards was a low 3.90 diff ratio which had a 39 tooth crown wheel & a 10 tooth pinion. A different diff centre altogether. The 3.90 was a very rare option & usually only seen in 149/161 manual commercials & some taxis. Dr Terry

Differential Components:



Excellent Youtube Demonstration



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