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Old 06-16-2004, 03:53   #1 (permalink)
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Diff Changes

I am interested to know a bit more about a diff change on my XF EFI Auto.

I know that the XF has tall gearing, (the XF was released when there was an oil crisis, and fuel was expensive, and hence taller gearing was given to these cars to preserve fuel) I know the potential of these cars/engines, producing 121kw and massive low down torque (hence being a long stroke engine) but how far can you go with the gearing, 4:11 diff is probably out of the question, as I do highway travel 90-100km/h and dont want the long stroke engine to rev too high. Is 3:45 the highest you want to go with a diff change, especially doing a bit of highway traveling?

When you change your diff, do you have to change the speedo thing too, as it will be giving a different speed?

And being auto transmission, does anything else need to be changed?

How much can I expect to pay for a diff?, is it worth getting second hand, or should I go brand new?

Thanks for any input.
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Old 06-16-2004, 04:41   #2 (permalink)
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Re: Diff Changes

lots of questions there man!

ok...because your car is auto i think that you may have a few dramas getting really short diff gears and no auto mods.

AFAIK pre-electronics autos work directly off oil pressure that is set by a governer, that is directly attached to the output shaft, and if you make the output shaft spin faster for a given road speed (by putting in short gears) i think you would find your auto would change up gears too early, even under full throttle.

the borg warner 35 boxes (which i think yours is?) have a cable that runs to the throttle (at least on the carby models) which depending on its adjustment can make the shifts higher and harder, or lower and softer - you could maybe adjust this cable to compensate a little for the diff gears, but i think driveability in normal conditions and gearbox life would suffer.

you do need to change the little gear in the transucer on the box, but some people reckon there is a little screw you can adjust in the top of the XD instrument panels - though i assume you have a digital dash being an EFi car?

this is all theroetical however, and would depend entirely on the diff gears you choose - the more different from stock the more potential for probs you have though.

find out what ratio your diff is stock, and what rpms you are doing @ 100km/h, then decide what is the most rpm you would want to put up with at this speed, from that we can work out what the lowest ratio diff you can have is, IE: (guessing)

2.77 stock, and say...1600rpm@100

and we will assume you want to rev it at 2200 on the freeway.

2.77/1600*2200 = 3.808

so you would get either a 3.7 or a 3.9 diff, depending on if you wanted to go a little under or over your taget figure. HOWEVER to fit anything above a 2.77 to a 2.77 diff center will require a spacer to be machined up and longer anticlockwise threaded bolts of sufficient strength to be found - WBT56 is not keen on this idea, and i have to say i would rather not do it either, but i have a 3.9 diff built using this method and so far its coped with my thrashing the guts out of it at willowbank, archerfield and QLD raceway, along with general irresponsible driving on the street with 200hp at the wheels, no worries.

if i was you (and had a 2.77 center) i would source a 2.92 or shorter LSD center from a wreckers or something, and get someone to rebuild it with the gears you want in it, either sourcing the gears you want from another wreckers or whatever, or getting the shop to supply gears. i think they are about $270 for lapped gears second hand, but im not sure at all on that point. maybe check ebay and see if there is anything there you can use - Aussie R31 skylines are usefull for their 3.7 (manual) or 3.9 (auto) diff gears - if you can get them at the right price, and find them!

dont bother changing your gears if you arent going to get an LSD - i would rather a 3.08LSD than a 3.9 open diff!!!!
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Old 06-17-2004, 00:43   #3 (permalink)
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Re: Diff Changes

Thanks for that, big help!

So basically is it worth all that hassle? I thought a diff change would be simple; just change over the diff and change the speedo to suit the diff, but it seems it not so simple. So I ask the question is it worth it?

What if I changed the diff, but I didn't go too far from the default specs my current diff has, would this be worth it, or wouldn't there be much change?
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Old 06-17-2004, 00:48   #4 (permalink)
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Re: Diff Changes

Actually on that, I realise you said, I will have some probs without doing some auto mods, by this I am guessing you mean a stall converter? If so, then thats cool, I was going to put a stally in eventually anyways, but If I need a stally to do the diff change then I can get that done with the diff too....or do you mean other auto mods?

you obviously seem to know heaps about diffs, you work in a diff workshop or something?
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Old 06-19-2004, 04:17   #5 (permalink)
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Re: Diff Changes

Rollin you there man?
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Old 06-19-2004, 04:59   #6 (permalink)
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Re: Diff Changes

I would not want to go shorter then a 3.45 on a 3 speed auto.

My XF has a 3.45 and sits around 2800 @ 100kmh.

You will need a different speedo sender but XG ute senders are a bolt on AFAIK nothing else needs to be changed. You won't find a new diff, best to locate an EF XR6 diff as i believe they are coil sprung, 3.45 ratio LSD's.
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Old 06-20-2004, 02:25   #7 (permalink)
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Re: Diff Changes

Ok thanks xdc351, but where will I get the speedo sender unit from? The same car that the diff comes with? What final drive gear ratio did the EB XR6's come with? Because I know they are LSD, so if I grabbed one of those and chucked that on it would be good, but im not sure of how the final gear ratio is, if its too low or too high?

Your XF has a 3:45 and sits on 2800 @ 100km/h is this good for an XF engine, being a long stroke and all?

One last one, you say your XF gets a mid 15 second quarter in your signature, what did you do to achieve that? Im assuming its an auto, thats damn good for an auto to get 15.63, is that an EFI engine or carby?

Thanks for you help.
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Old 06-20-2004, 18:20   #8 (permalink)
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Re: Diff Changes

Quote:
Originally Posted by fOrD mAn Xf
Ok thanks xdc351, but where will I get the speedo sender unit from? The same car that the diff comes with?
All you need is the plastic drive gear, that should fit your standard XF speedo sender. Either the donor car, Ford or your local wreckers should be able to get you one.

Quote:
Originally Posted by fOrD mAn Xf
What final drive gear ratio did the EB XR6's come with? Because I know they are LSD, so if I grabbed one of those and chucked that on it would be good, but im not sure of how the final gear ratio is, if its too low or too high?
Not sure of EB XR6, quite possibly could have a 3.45 but i'm not sure. Best to check either the tag on the diff or try asking in the e-series forums.

Quote:
Originally Posted by fOrD mAn Xf
Your XF has a 3:45 and sits on 2800 @ 100km/h is this good for an XF engine, being a long stroke and all?
Never had a problem, not even driving interstate @110kmh which is over 3000rpm. If your not comfortable with RPM that high then a 3.23 will still give you a marginal improvement over the 2.77 stock ratio. As a bonus 3.23 ratios came out in 3.3L powered XE's (and possibly XF's) so they are much easier to find and rarely thrashed!

Quote:
Originally Posted by fOrD mAn Xf
One last one, you say your XF gets a mid 15 second quarter in your signature, what did you do to achieve that? Im assuming its an auto, thats damn good for an auto to get 15.63, is that an EFI engine or carby?
Its an auto station wagon, so it has a few very surprised holden scalps under its belt :) The engine has a stock, rebuilt and balanced bottom end, 10.5:1 comp, dynotec stage II cam, oversize valves and a head cleanup, reline 4bbl manifold and a 465 holley. The auto has a 2500rpm stallie and being a station wagon I need to run a leaf sprung diff.... in my case an XG ute diff lives under the back. In a sedan with a bigger cam i've no doubt this combo would be in the 14's.... in a corty it'd be eating v8's for breakfast!
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Old 06-21-2004, 01:34   #9 (permalink)
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Re: Diff Changes

Quote:
Its an auto station wagon, so it has a few very surprised holden scalps under its belt :) The engine has a stock, rebuilt and balanced bottom end, 10.5:1 comp, dynotec stage II cam, oversize valves and a head cleanup, reline 4bbl manifold and a 465 holley. The auto has a 2500rpm stallie and being a station wagon I need to run a leaf sprung diff.... in my case an XG ute diff lives under the back. In a sedan with a bigger cam i've no doubt this combo would be in the 14's.... in a corty it'd be eating v8's for breakfast!
It amazes me more so that its a 3 speed auto, but I do see you have done a bit of work to get this, was it expensive to get all that done to the engine? What final drive ratio are you using in the diff?

I plan on adding extractors + sports exhaust, diff change, preferably 3.45 final drive gear ratio, a stally, and then finally a stage 3 crow cam.....that would easily get me into low 15's being an auto and all, and thats only cause its EFI XF, so I would benefit from the 121kw + power.....
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Old 06-21-2004, 01:37   #10 (permalink)
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Re: Diff Changes

Also on that, seem as though I haven't heard from Rollin, how come Rollin made out it was so difficult to change the diff in my car, i mean it doesnt seem that difficult, did you have to change anything from stock to get the auto transmission to work ok? is a stally compulsory if you get a diff change in an auto?
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