rear differential: whole axle, gears.. difficulty&cost?

[BerniniCaCO3]

Hey!

Last week I posted a thread asking about fuel economy... since my HO swap I've been getting 12.5mpg, before I got 18mpg... but importantly, in between, my driving habits changed from 80% highway to 20% highway. I checked for exhaust leaks, or rather, had two trusted and vastly more experienced coworkers test for exhaust leaks, and they heard only a very minor one on the passenger rear head, and agreed that it wouldn't cause 12.5mpg-- city driving would.

So!
Right now I've got a 3.08 open differential.
I'm contemplating 3.55 (maybe limited slip? what are the real-world pros/cons?) gears.

Do these parts ever wear out?
If not, then maybe I'll haunt the junkyards for a day or two. I'm too busy/lazy to look much harder.
Hope I find something... do pickup 8.8" rears work on our cars?

Maybe a surer bet, can I just buy the gears?
How involved is rebuilding the rear axle? I've heard that it requires some careful shimming or something to align everything right. I've never taken a differential apart; only done the oil. With a lift and proper tools, can it be done in <12hours? (just don't want to get in over my head with another bigger-than-ever-imagined project!)


thanks!
-Bernard

[1980c10]

I was getting 24mpg with a H.O. And 3.08's. I think you have soemthing else wrong.

[gadget73]

the gears don't wear out really, but the bearings and axle shafts do. Honestly if you're considering this, buy the whole rear, rebuild the thing out of the car, then just swap it as a unit some weekend. That way you won't have to worry much if something goes wrong. I'd probably also consider pulling the control arms from the donor car, and replacing the bushings in them so you can do the rear control arms with the axle in one shot. A full out rear and control arm swap should be very possible in 12 hours time on a lift with the right tools.

[88Vic]

How difficult would it be to install the finished axle without a lift? I'd like mine rebuilt with 3.73's and a locker, which i'll need a professional to rebuild but I could probably install it myself.

[1990LTD]

you need a buddy or two and a few floor jacks.. entire assembly is probably 150lbs+


awkward as fuck when you're underneath the car on the ground!

[1987cp]

There are any number of things that can kill fuel economy. One thing I read about recently was a failure mode for torque converters where they start slipping a bit too much, and generate excess heat while killing fuel economy. I'm pretty sure that's been happening in my P72.

I'm thinking a trolley transmission jack would be just the thing for doing a rearend, possibly with a saddle of some sort attached to hold the pumpkin securely. They certainly work great for doing transmissions safely and almost-easily!

[BerniniCaCO3]

It would be *awesome* if places like autozone opened rent-out lifts, although I suppose the liability would horrify any professional lawyer. Now that I work in the industry I have a lift after hours, but before I had a lift... god it makes things easier.
Heavy as it is, I wonder if I'll be using a transmission jack too, just the taller model for use with lifts. There's a fuel tank jack also. I'd have to look at my options. Two friends with strong arms and amazing endurance is probably not a realistic option.

Good thought. Which rears will fit-- anything panther body at all? Would an 8.8 rear from a town car or pickup work alright (just broadening my options).
Then maybe forget about finding the right gear ratio, just find the cleanest rear, and buy the gears new.
How badly do the axles wear, or can I usually just slide them out and do bearings alone cheaply? New axles I imagine are pricier.
This'll all happen in a few months, probably. I'll be watching craigslist for cheap already-pulled rears however.

What are the symptoms of a failing torque converter?
Fluid is fresh and clean and has stayed that way for the 12,000 miles I've owned and driven the car.

Would an explorer camshaft in an HO, run by an HO computer, run rich?
Unfortunately, given that it seems to drive smoothly with bad fuel economy a singular symptom, I don't know where to start. Or if 12.5mpg in my 5 mile city commutes is just normal. What are your driving habits 1980c10, that you get 24mpg?
Only time I got 24mpg was on a long trip last summer, on the old lopo engine, from pittsburgh to baltimore, literally 100% highway, and possibly downhill (I'd have to look at elevations of pittsburgh and baltimore)

I never changed it, but is the lopo MAP sensor the same as the HO MAP sensor? If not, that could be a problem; or if it needs to be cleaned after 20 years, that could be a problem. Since it's under vacuum though it shouldn't accumulate carbon. I could recheck timing, but I'd swear I've got it at 10BTDC. To make it so, this pushed the distributor a little away from those two marks that are suppose to line up as a starting point; dunno if that's a sign that something's a little off. I could smoke test the vacuum lines, but it does hold 20" at idle, if there were a leak it'd show up at idle vacuum readings? Finally, I was reading a list of "causes of poor fuel economy," and one was too tight valve clearance. When I had the machine shop redo the valve seats, I just trusted them to grind them to the correct height, and never double-checked the valve heights. Didn't have a tool I trusted to measure the heights anyway. Possibility? Not one I'd be eager to check or fix. It also has brand new valve springs, dunno if they need to break in or anything. I can't remember whether i tested their stiffness or not, I don't *think* they were 'performance' springs though. I could also check fuel pressure; just reused the regulator from the HO car I cannibalized.

[86VickyLX]

I'm thinking a trolley transmission jack would be just the thing for doing a rearend, possibly with a saddle of some sort attached to hold the pumpkin securely. They certainly work great for doing transmissions safely and almost-easily!

THIS. I used Scott's trans jack with rachet straps to install my new differential.

[gadget73]

RE: torque converter slippage. While its a possible concern in first and second gear, its not a factor in 3 or 4. The design of the AOD completely bypasses the torque converter in third and fourth gear by driving the trans from a separate input shaft thats attached to the shell of the converter. No clutches to wear out and slip. The AODE and 4R70W and really no other transmission that I know of does not use this design.



12.5 mpg city driving is entirely possibly normal. Run it on the highway sometime and see what you get. I'd wager it will be better. This is where gearing choices relate to how you drive. In town, a 3.08 sucks, and you'll get better results from a 3.55 or 3.73. On the highway though, you will likely be better with a 3.08 or 3.27 since it keeps the RPM down.