ACTIVE AUTOMOTIVE UPDATES & BLOG

Diff Gear Swap Tech

One of the best ways to improve the performance of your car on the street or the race track is a Diff Gear Swap – installing a set of lower ratio diff gears.

In LS-powered Commodores with more radical upgraded camshafts and high stall torque converters, the next logical step to make the most of these modifications is to run 3.7:1 or 3.9:1 diff gears instead of the often used 3.07-3.25:1 standard ratios.

Understanding the benefits of a Diff Gear Swap can significantly impact your car’s performance, allowing for better acceleration on the time slip and through the seat of your pants.

In this article we show the key steps to shaping your tall ratio for a ring gear and pinion set that will increase your mechanical advantage and help the engine to move the mass of your vehicle more easily.

The standard diff centre is removed from the vehicle.  On the left is the centre and the standard crown wheel (ring gear) on the right is the standard pinion gear.  Behind are the two stub axles with the chopper wheels on each for the ABS system.
On the left is the standard 3.07:1 ratio ring and pinion gears.  On the right are the brand new 3.7:1 ring and pinion gears.
Notice how much smaller the low ratio 3.7 pinion gear on the right is compared to the taller 3.07 pinion.  This is because the lower ratio pinion must rotate approximately 20% more to provide the extra mechanical advantage through gearing.
A new bearing is pressed on to the pinion shaft.
The new pinion has its new bearing mounted and the new ring gear has been pressed into place on the existing centre carrier.
The new ring gear is bolted to the centre carrier.  After applying thread lock and an initial tighten, the bolts are tightened to the correct torque setting with a torque wrench.
The diff housing remains in the car and has been thoroughly cleaned prior to reassembly.  Note that the driveshaft has been removed and the exhaust lowered for clearance.
The new pinion is fitted.  Shims and torque on the pinion nut are used to set the preload on the pinion bearing.  The torsional torque on the pinion is critical and is measured with a strain gauge.  This ensures quiet operation and long service life.
Fresh bearings are greased and fitted to the carrier, which is then inserted into the housing.
The diff centre sits in place, located by a carrier adjuster on each side that also houses a bearing and axle seal.
The adjusters allow the crown wheel to be moved in or out of mesh with the pinion gear to give the correct lash.  This lash is critical to quiet operation and maximum load bearing capacity on each tooth and is checked with a dial indicator.
Marking compound is used to mark several teeth.  The diff centre is then rotated in both directions under light load to check the mesh pattern between the ring gear teeth and the pinion teeth.  The pattern visible in the centre of the yellow marker is perfect, showing a wide pattern across the tooth that is also centred in both directions.  This mesh pattern is critical to the life and correct operation of the diff for years to come.
The diff is now adjusted correctly and is ready to have the rear cover housing bolted back into place and be filled with LSD additive and oil.  The stub axles have been inserted, ready to accept the CV joints mounted to the ends of the half shafts.  The 3.7:1 ratio swap is now complete.

If you require a diff ratio swap, then contact us at Active Automotive. We can also supply and fit LSD diff centres at the same time to replace your open centre or worn factory LSD centre.

9456-1111.

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