help understanding the rear/gears

vladtepes · May 14, 2010

Have been talking to a lot of people and of course everyone is an expert.. also read a little here and there online.. but need some more explaining..

91 Mustang 5.0 LX Automatic trans AOD

pretty stock at this point (has your typical exhaust, x pipe, headers, etc..) but basically stock... i would like to at some point spray the car with a small shot.. just a daily driver not trying to go insanely fast or anything.. so some people tell me get 4.10s they make a huge difference in acceleration, then others say way too aggressive for a daily driven car and they would drive me nuts.. Now i dont mind something that is a little annoying for the sake of performance, but I also dont want to be cruising down the highway doing 70mph while the car is revved out to 6k rpms..

Also was messing around with a friend of mine who has some factory turbocharged import, and we lined up again just messing around.. when I did a burn out, i noticed that only the pas side really spun and I also noticed some pretty heavy whining from the rear..

what gears are likely in my car stock?
what is the best gear to go with that will provide the most gain in performance and not drive me huts?
what is the likely cause of the whine in the rear?
what do i need to install/fix to get power down to both rear wheels?

I have worked on cars before.. my last car had a motor swapped in from another car and nitrous.. I am not really new to working on cars.. but relatively new to working on Mustangs.. so your patience is appreciated..

thanks in advance..

94GTCoupe · May 14, 2010

to find out the stock gears,there should be a metal tag on the rear diff cover.
it will tell you whats in there.

the "one wheel peel"means your traction lok is shot.
time for a new clutch pack.

the whining,i would do a complete rebuild of the rear end.
new bearings,races,and seals.

for an auto,4.10's are pretty much the consensus for much improved performance.
also,the RPM increase on the highway will NOT be around 6k.
i think the increase will be around 400RPM over what it is now.
i'm not sure about that,someone else may know for sure.

also,with a change in rear gears,you will also need to replace the speedo driven/drive gear for correct speedometer readings.

NIKwoaC · May 14, 2010

vladtepes said:
what gears are likely in my car stock?

Most likely 2.73s, but I believe 3.08s were also an option.

vladtepes said:
what is the best gear to go with that will provide the most gain in performance and not drive me huts?

They say 3.73s or 4.10s for an auto. Something you have to watch out for is running out of gear at the dragstrip, though. An AOD will not shift into top gear at WOT, so if you gear it enough to get to that point, you'll be hosed.

vladtepes said:
what is the likely cause of the whine in the rear?

Your Traction-Lok (limited slip differential) is fried. See below.

vladtepes said:
what do i need to install/fix to get power down to both rear wheels?

You'll need to replace or rebuild your Traction-Lok. Traction-Lok rebuild kits are available from FRPP, and are pretty cheap. Rebuilding the Traction-Lok is not difficult if you take your time and aren't retarded with tools.

vladtepes · May 14, 2010

holy **** thanks for the super fast reply.. that made it pretty easy to understand..

so at some point my car DID have "LSD"?
is there something i can look at to be SURE that it has that before pulling the rear apart (god only knows if someone made any changes prior to me buying the car)
i am not that retarded with tools so should be good there..

I guess 3.73 would be a good compromise in acceleration and still keeping a good top end? i would like to eventually install a shift kit as well.. I don't know if that matters? I don't know how the car will run or what the ultimate goals will be but i definitely do not want to run out of gear.. I mean with good tire, spray, maybe cam, gears, shift kit, etc.. I am hoping the car would be capable of low 13's high 12's I guess that would be a short term goal..

I want the most aggressive gears i can run without running out of gear at the track or being extremely annoyed with highway driving.. since the car will be sprayed, i will not have that power adder during normal day to day driving.. thats why I am concerned with things like good gearing to keep some pep in the car when it is off the bottle..

thanks for the info on the tag
thanks again for the quick info..

94GTCoupe · May 14, 2010

heres an example of what you'll find on the tag.
this is from my 94-98 mustang repair guide,so use only as an example.
i recommend you get yourself a Chilton and/or Haynes manual.

note the "L" with the gear ratio for LSD.

or may be listed on the door tag.
(it is on mine,not sure of the fox body mustangs)

Haus · May 14, 2010

If you cant read the tag on the rear diff. you can look at the driver side door tag. Under AX on the far right bottom it should show 1 of these 3 letters.

M,Z or E

M=2.73
Z=3.08
E=3.27

My 92 came with 3.27

but obviously this only works if the rear gears were never touched by a previous owner if there was one. if any doubt you can also count the time the drive shaft turns to 1 rotation of the rear tire.

vladtepes · May 14, 2010

thanks for all the help after reading a little more i think i understand how to rebuild the trac lok.. and what i need to consider for the gears.. may just play it safe and do 3.73 i imagine that is a decent improvement over factory.. the bearing/whinning issue on the other hand i am still a little unsure of.. i only really hear that whine when the car is burning out, not under normal driving.. does that have any bearing on the problem? what bearings am i replacing?

thanks again..

NIKwoaC · May 14, 2010

Get a total ring and pinion installation kit from Ford Racing. Should have everything you need, including axle bearings and seals, which you should change while you have the rear end apart anyway. It will also have the carrier (LSD) bearings and shims, and the pinion shims, bearing, and seal.

Do you homework before you try to change the gears yourself. Yanking the traction-lok and rebuilding it is one thing, installing gears is something completely else. It takes quite a bit more effort, knowledge, and specialty tools.

vladtepes · May 14, 2010

NikwoaC said:
Get a total ring and pinion installation kit from Ford Racing. Should have everything you need, including axle bearings and seals, which you should change while you have the rear end apart anyway. It will also have the carrier (LSD) bearings and shims, and the pinion shims, bearing, and seal.

Do you homework before you try to change the gears yourself. Yanking the traction-lok and rebuilding it is one thing, installing gears is something completely else. It takes quite a bit more effort, knowledge, and specialty tools.

I have friends who have spent tons of time wrenching on rwd domestics.. they are familiar with a rear and the workings of gears.. the issue is these people are not mustang specific guys.. I have spent quite a bit of time messing with cars as well.. but rears, gears, and such are totally foreign to me beings as how the car that i previously built up was fwd...

with that said is this something between a few of us we could hammer out.. or is simply being familiar with those parts not enough, and Mustang specific knowledge is required..

I prefer to turn my own wrenches unless I absolutely can not.. then I prefer to do it under the guidance of a friend.. obviously as a last possible resort i would pay someone to do it.. but i would like to avoid the last at all costs..

NIKwoaC · May 14, 2010

It's definitely do-able, but you need to find some good reference instructions, either online or in a book/manual. You'll also need a magnetic base dial indicator, dial calipers (or a micrometer), a good torque wrench and access to a press.

bynummustang · May 14, 2010

If someone has been in the rear (gigidy) before, just jack up the rear, tie a plumb line to the yoke on the diff, and rotate the rear wheels 1 FULL revolution. Now count how many times the line wrapped around.

Just under 3 = 2.73
Just over 3 = 3.08
About 3.5 = 3.55

And so on.

Just make sure you take the plumb line off before you start driving again.

NIKwoaC · May 14, 2010

bynummustang said:
If someone has been in the rear (gigidy) before, just jack up the rear, tie a plumb line to the yoke on the diff, and rotate the rear wheels 1 FULL revolution. Now count how many times the line wrapped around.

Just under 3 = 2.73
Just over 3 = 3.08
About 3.5 = 3.55

And so on.

Just make sure you take the plumb line off before you start driving again.

That is actually really good advice. I've never heard that method before.

I've heard of counting the turns, but never wrapping a string around it. Cool.

vladtepes · May 14, 2010

NikwoaC said:
It's definitely do-able, but you need to find some good reference instructions, either online or in a book/manual. You'll also need a magnetic base dial indicator, dial calipers (or a micrometer), a good torque wrench and access to a press.

thanks man!

bynummustang said:
If someone has been in the rear (gigidy) before, just jack up the rear, tie a plumb line to the yoke on the diff, and rotate the rear wheels 1 FULL revolution. Now count how many times the line wrapped around.

Just under 3 = 2.73
Just over 3 = 3.08
About 3.5 = 3.55

And so on.

again thanks for the info!

bynummustang said:
Just make sure you take the plumb line off before you start driving again.

LOL

here is where i am at..

I am changing the gears.. based on what im learning here 3.73 would be the likely pick...

i will rebuild the LSD...

i am in the air on the entire rear rebuild.. was reading about it and im not sure if i can handle it or not... if the rear is only whining when im burning out is it possible that the culprit is the LSD and not the bearings?

thanks again..

94GTCoupe · May 14, 2010

i strongly suggest a complete rebuild.
your doing the gears,so the diff has to come out anyways.
a good 2 arm puller,a press race/seal driver,and slide hammer are all you will need for all bearings

vladtepes · May 14, 2010

davis3 said:
i strongly suggest a complete rebuild.
your doing the gears,so the diff has to come out anyways.
a good 2 arm puller,a press race/seal driver,and slide hammer are all you will need for all bearings

makes sense.. after reading more and more it looks like doing the gears myself might be a little over my head i also lack the tools...

oh well..

thanks again for all the info..

vladtepes · May 14, 2010

since changing the gears may be cost prohibitive at the moment.. is there any case that part of my problem is low or nonexistent oil... should i rebuild the LSD and add fresh oil in the mean time? or just wait?

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