Not really an expert on diff's - so just asking this question

Looking to put a 3.54 (or there abouts) diff into my '82 V12 XJS and i'm not sure about the powerloc diff head availability in New Zealand


If another jaguar diff could be found of a similar ratio - that wasn't a powerloc version - would that fit?


Alternatively - can the gears be swapped into the powerloc assembly??


Excuse my ignorance on this

 

Yes, some XJ6 series 2 had non powerlok 3:31 diffs. That would be a great compromise ratio but no powerlok with a v12 is not ideal.


Yes but its not practical. The 3:31 and 3:54 use the same carrier, the 2.88 carrier is unique and has a spacing that wont allow the other ratios to mesh. The solution is to make or find a ring gear spacer, or use a thick gear. I was not able to find any of these items here in America.


Here a few ideas....

Find a diff from a 90's 6 cylinder xjs. That should have a 3:54 and would be low bias power lock and would probably not need a rebuild but that ratio is a bit short for a XJS with a 3 speed auto and the low bias power lock is basically useless with the power of the V12.

Find a 3:31 early XJS or XJ6 series 2 non-powerlock. Then you can retrofit a commonly available 19 Spline Auburn locker carrier. An early diff will certainly need a rebuild, and rebuilding it with the Auburn locker and Dana 44 side bearing will cost way less then using all the OE jag bits. The Auburn locker is also newer and better locker technology. There are a few considerations with the setup but if you have a working 3:31 to start with you can make some precision measurements and have the new diff arrangement setup quickly.

http://www.summitracing.com/parts/aub-5420112

I've done both of these so I know this first hand.

 

Thanks for the comprehensive reply


Really interesting what you were saying on the fitment of the 'low bias' 3.54 diff - and how this doesn't really work in the V12


Seems to be a pretty standard response that this is the one to use from other items I've read. Can the 'low bais' be adjusted I these heads??


Thanks again for your time on this

 

Low bias means it doesn't have much slip limiting effect or the bias is reduced when compared to previous version of powerlok. After a certain amount of torque it basically becomes an open diff. I had a 3:31 power lock and upgraded to a 3:54 powerlock (low bias) because I wanted the outboard brakes and reinforced rear hub carriers that exist on a later rear IRS. First time out of the garage I got wheel spin on acceleration as if there was no slip limiting function. I loved the brakes but traction was a big issue now. I figured the diff needed a rebuild but was slip limiting during a bench test so I did some more research and discovered Jag switched to a low bias powerloc in the mid 90's along with outboard brakes.

See page 3 of this document on the matter. I've had a few of these low bias units and it seems like the bias it so low its basically useless out on the road. Seems like Jag engineers got it wrong and made the bias too low and came up this cackamamy "refinement" excuse to address the owners complaints.
http://www.terrysjag.com/XJS%204.0%2...%20Service.pdf

At this point I explored retrofitting an earier high bias unit and determined it was easier far more economical to replace it with the more modern Auburn un

Also 3:54 is not ideal or a 3 speed auto car. It will be high revving on the highway. My car has a SBC and 700R4 over drive 4 speed transmission so 3:54 was perfect.

No adjustment is possible but anything could be re-engineered given enough time and money. Life is short and funds should always be allocated efficiently so its advisable to drop in a more modern race and trace proven Auburn unit.

 

That's more great information - thank you


So reading between the lines the 'inboard' brake unit with a 3.54 was a higher bias unit - and the outer brake unit was a lower bias setup


I'm also likely to swap to a 4 speed auto or 5 speed manual - so the 3.54 unit is still the one i'll likely go with


The Auburn unit sound ideal however for a rebuild - so i'll track one of those down as well

 

Yes. It sounds like you want the vehicle to be a bit more performance oriented. You may also want to get the out board brakes. The inboards are downright ineffective compared to the outboards. They are left over from the Pre-ABS days and the system is heavily biased toward the fronts so the rear breaks don't so much actual stopping. If you switched to outboard too the stopping performance and pedal feel would be vastly improved. Working ABS is a must to prevent the car from coming around in certain braking situations. BTW, my car is a 76 XJ coupe and the IRS from a 96 XJS convertible was a pure bolt-in except for the brake lines which had to be custom made. Early cars have a single fluid line and a T fitting for the rear brakes. Later cars have 2 individual fluid lines for the rear brakes.




That would be a worthwhile upgrade. Should have been built that way.

They are readily available and are popular in Jeep circles since the Jeep also used a Dana 44.