replacing the diff in an XP

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O.K, I want to replace my diff...the original is in it still, and while it was great for thr trusty 200 super pursuit, the motor has now got extractors, is about to get elect igntion, a Weber 32/36 carb, and possibly a lumpy cam.
So it will growl along nicely, but I want a diff that gives much better (higher) cruise speed without pulling a hugh number of revs...for those long trips we all do over here in west oz.

I have been advised to get one from an ea-ef falcon, get it shortened etc...but has anyone had this done or can anyone suggest an alternative?
I think a 9' diff would be overkill...I really don't want to roar off from the lights in a cloud of smoke...I just want a higher cruise speed from the old chap.

1965 4 door falcon XP, Kurrajong green!
 
My suggestions (based on learning the hard way):

First we need to know if it's 4 or five stud wheels. You will either use Ford or Skyline axles, then. 5 stud makes life easier.

Get a housing from an XW/XY/XA/XB/XC/XD. Buy a 2.92 gearset. Buy an XR pinion flange (U-bolt type). Look for an old-style locking centre in good nick (4 pinion type). The newer ones can't be shimmed to correct for wear. You'll need new wheel bearings and pinion bearings; maybe carrier bearings to be safe.

Shorten the housing to less than the original XP width by ¼" each side. This gives more room for modern tyres and wheels. You might consider welding on some swaybar brackets. The short axle off the donor diff becomes the new long axle. The old "long axle" is resplined to be the shortie. If using 4-stud, you will need to possibly weld/redrill your old backing plates to fit the later bearing retainer flange, and check the spacing out from the flange of the Skyline axle, so the drum matches the backing plate.

5-stud is easier; the Falcon offsets are pretty right and you're only fooling with one axle. Plus it's easy to buy new drums, wheel cylinders, shoes, etc... I fitted extra long studs for peace of mind, too.

Either one should recycle the XP brake hose union and lines if possible.

For the lower shock plates, buy XR-XC commercial units. They are wider than needed, but fit readily unmodded. At least something is easy!

This all sounds hard, and is a few bob worth of parts, but consider it compared to importing a used 8 inch diff (similar total cost):

•One gives you a used part, no locking centre, no easy choice of ratios, no warranty on work.

•One gives you the LSD, warranted new work, new clearances and bearings, new brakes, plus a smidge extra room for the tyres.

:D There you have it - one biased opinion!

Adam.
 
there is an easier way... chrysler centura diffs had 2.92gears if they were out of a 245ci centura or 3.23 from a 265 centura. cut the upper and lower control arms off and all brackets, weld the new leaf perches onto the chrysler diff. xp and centura diffs are 56" wide, the flange from a the chrysler can be used or any car that uses a 2031 uni joint, and get the the flange welded to your tail shaft. the xp handbrake cables can be reused. the stud pattern is ford pattern, but uses holden wheel nuts (7/16).
the only problem you might have is if the car is very low, (I mean low) the centre of the centura diff is offset and will rub the tunnel.
with a 2.92 centre and top gear (1:1) the `p will cruise at about 2750rpm.I had a 2.92 set in mine but replaced them with an xd lsd and vl commodore 3.45 gears, loves the fuel, but its not my everyday car either.
if you want to know any more just ask or look at clubs web site under "Greg cars" www.cruizers.netfirms.com
 
Well, I started out that way, and can say a few things against it.

I wasn't happy with the wheel nuts - they got oversized to Ford spec, then overlength as mentioned.

Wanted to run the 10 inch Ford brakes (safety); welded up and redrilled the backing plates. Now the Centura bearing retainers don't fit perfectly. To maintain the correct positioning of drum to backing plate, I had the axles' bearing shoulder remachined 6.2mm outboard. Brand new drums, slaves, pads, resurfaced wear spots on backing plates, reconditioned drum hardware. Painted either with POR-15/ChassisCoat, 2 pack satin black or High temp VHT black depending on parts. The centre casting was smoothed and filled.

Spring saddles - bought NOS BTR ones; had them narrowed and redrilled. NOS swaybar brackets. Idiots at diff place couldn't copy XP diff setup next to them. Warped the housing over ¼", too.

If you don't mind using Centura brakes (which these days requires special-order oversize shoes in most cases) then the Centura diff may work. But after a conversation with the diff place who sorted out most of the mess the first mob made, I came to appreciate their point. You need to do a bit here, a bit there, and getting precious about the starting point for all this is not worth it. They used a Falcon or Commode diff for most conversions, running from that point as the specifics required.

I'm about over the 2K mark with a Centura diff and next time would just do exactly what I suggested in my first post. Or go IRS. :twisted:

Cheers, Adam.
 
Firstly thanx for the replies to my original question...
I forgot to mention that it had 4 stud wheels...
I've made some calls to diff centres since posting this, and been advised to go for an EA/EF housing.
1 company has quoted $1750 for a custom job...cut down to size, new internals, lsd and all fitted...seems like a good deal to me.
I am interested to know what ratio's I should go for in the diff tho', and bear in mind that I really do want a diff that gives the car really long legs for highway speed...at a comfortable level of revs...
 
That sounds like a 2.92 centre for long distance cruising. If the place will give you 12 months unlimited (or 100 thousand) km warranty on the unit, and it is with new larger brakes, it's a good price.

The Centura ideas have pushed their diff prices beyond what they're really worth as a "core". Drum to drum is often $180.

Adam.
 
I have an 8" in my XM, but it is still an unknown as the car hasn't been driven yet. It was $450 or so a few years back, and if the centre needs to be rebuilt I will be up to the same sort of figure ($1800-$2000) as to do a BW78 swap. If I was doing it again I would be shortening an XY housing to take 2 short axles.
 
again thanx to everyone....
being as my knowledge of diff's is limited to the fact its the thing @ the back of the car (actually not really that bad, but don't know a lot about them), can someone explain a little as to ratio's...what is good for what type of driving...there has been a couple of mentions in regards to the 2.92 ratio for highway driving...
is a bigger ratio better, and what is the max you should go for?
remember, I am interested in turning the XP into a good cruiser, with long legs for the highway, not for roaring off from the lights in a cloud of rubber smoke!

:roll:
 
The diff ratio, and we will use 3.00:1 as an example, indicates the amount of times the driveshaft will turn to get the wheels to turn 1 complete revolution. ie, for every 3 driveshaft rotations the wheels rotate once.
If you have a 4.00:1 diff ratio the driveshaft will rotate 4 times for the wheels to do 1 rotation.
Therefore to get low RPM at highway speed you need a low number diff ratio like a 2.92 or 2.77.
BUT....tyre size has a lot to do with the overall MPH vs RPM equation.
For example going from 13" wheels to 14" wheels could easily make as much difference as going from 3.25 diff gears to 3.00.
The next bit that comes to mind is how many RPM do you want to be doing at your normal highway cruising speed?
If you go to a very tall ratio like 2.53 or 2.77 with tall tyres your car may struggle to hold speed any time there is a small hill.
You mentioned the possibility of a lumpy cam, which will raise the HP of the engine but will also raise the RPM that the HP is made, so that will need to be kept in mind as well.
Now for my suggestion, I would be aiming for about 3000 RPM at 75MPH/120KPH. If you are running 215-65 x 14 tyres, which are about 25" tall, a 3.00:1 ratio will give about 74mph at 3000 RPM.
check this for some easy to use calculators
http://www.bgsoflex.com/auto.html
play around with some figures and see what you come up with.
 
Thanx to every1 who posted replies...and special thanx to Fingers for a great explaination of ratio's etc.
I think I've had things cleared up and can now make a good informed decision.
 
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