Need help - Posi rear end

[Leafsrule16]

First of all, im a huge noob so dont laugh..

Looking to upgrade the rear end in my car to something posi, i was thinking of posting up an add looking for a trac-loc rear end but what do i look for? Any specifics to fit on my 97 grand marquis? Also, what would i need just for a posi rear end? Also, could this be done for less then $300ish?

[1990LTD]

I think a K code rear is what you're after but I could be wrong.

[Leafsrule16]

Need list of what i'd need to buy as well, like, what parts i would need to do the swap.

[91grandmarquis]

Eaton sells an 8.8 limited slip diff for our axles 21 spline? its like 350 bucks though, but its new. Im not sure what to look for in a junk yard.

[pantera77]

Summit sells a brand new complete 28 spline track lok carrier for around 200.

[91grandmarquis]

Summit sells a brand new complete 28 spline track lok carrier for around 200.

Damn I might be interested in that, and you're right it is 28 spline, I am just retarded.

[SVT98t]

Or you can just snag one from the junkyard for $24 like i did.

[86VickyLX]

It depends on what you want to go with. You want gas mileage or do you want to take off like a bat outta hell?

[gadget73]

New one is 200, used one is whatever the junkyard charges, plus 100 for the rebuild kit and your time to do the removal and overhaul. Honestly, for the bother and expense, I'm strongly inclined to just buy a brand new one when I do the Mark VII.


There is no requirement to change rear gears if you install a limited slip, but many do. Its also a good time to replace the bearings in the rear, as well as the axle seals. When you change the carrier, you really should check the gear setup to ensure a proper contact pattern. If you aren't changing gears, chances are very good that you will get a good pattern simply by re-installing the original shims in the original location, but no guarantees. If you're changing the gears, then you may need to play with it.

[merc91]

this stuff is assuming you have the same style 8.8 rear end as boxes and if you have under 3.55 gears i would personally upgrade the ratio while your at it. Your rear end is a tad wider than boxes but i believe the guts and axles also are the same. You can do the posi and gear ratio change as 2 seperate things if you wish.

-i would get a rebuilt genuine ford unit (with new clutches) not a chinese ford racing,... thiers a guy on ebay that does this and a very good job for $100 with 8 clutches set nice and firm.
-might as well get 3.73 gears while your at it, you can find a good set of real oem ford low miles for around $70
-pick up an instal kit with TIMKEN bearings and seals and shims and everything you will need $100.
-i would go with conventional diff fluid and a little bottle of friction modifier for limited slips sold at ford... once you get into synthetics they have a little friction modifier like additives in them and adding the 4 oz bottle will be to much, and not adding enough will be to little so keep it simple and go conventional oil and the additive.

last but definently not least is the install... rear ends are one of few things that really honestly should be done by a good shop that knows what they are doing... $250 or so is about the lowest price you will find for a good quality install.

[1980c10]

My rear is a z code and its also a factory trac-lok. The k code is not the only limited slip diff that came in a panther just the lowest ( highest numerically ) gear ratio from the factory being a 3.55.

Check GoodSamaritan's sig for a list of axle codes

Also most junkyards will sell a complete carrier section for $75-100.

check here http://www.laurelmountainmustangpart...ing-p-851.html for a frpp rebuild kit for cheap

Its where I got mine. I you want to change gears also while your in there that's a whole
different discussion.

[Leafsrule16]

It depends on what you want to go with. You want gas mileage or do you want to take off like a bat outta hell?

Bit of both really, think around 16 mpg in a 97 grand marquis with 3.55's is too much to ask for?

If i get a new rear end, ill probably just look at something brand new actually, i know a shop that could do it for $200.

[johnunit]

16MPG highway or city? Highway you should still be well into the 20s driving reasonably with 3:55s.

[gadget73]

Keep in mind a 97 has a wide ratio transmission. A stock 3.27 with a WR has a lower effective first gear ratio than an AOD with a 3.73. You can get away with a whole lot less gearing on one of those cars for that very reason. 3.55 will act from a gearing perspective like a 4.11 would to an AOD car.

http://www.grandmarq.net/vb/showthre...ght=gear+ratio

has the chart comparing the two. The 97 has a 4R70w, which is what the wide ratio AOD gearset comes from.

[1980c10]

I get 15 mpg highway with 4.30's haha