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Author Topic: Rear axle swap  (Read 1499 times)

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Offline Billman

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Rear axle swap
« on: February 11, 2013, 01:17:14 PM »
Has anyone tried a 7.5 Mustang rear end with disc brakes yet or am I nuts for considering it?
1983 SX4
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1984 wagon DD
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Offline eaglefreek

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Re: Rear axle swap
« Reply #1 on: February 11, 2013, 10:14:22 PM »
Nuts? Not at all. The Ford 7.5 is just as strong if not a little stronger than the AMC 15. You will loose the 2 piece axle and gain rear disc brakes. The brackets will need to be removed and leaf brackets welded on. The Ford axle is probably a little wider than the AMC, but not by much. You will also have to make sure you can match the gears to the front axle.
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Offline TheWraith

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Re: Rear axle swap
« Reply #2 on: February 12, 2013, 07:30:24 AM »
Don't mean to disagree...but, yes, nuts.

The Ford 8.8 is not just a little stronger, but a LOT stronger then the 7.5.  The junkyard is full of 8.8 axles w/disc brakes.  And the 8.8 has tons of support.  It is a popular swap for the Cherokee which has similar dimensions as the Eagle.  The 8.8 is something like 1.5" narrow then the d35 it often replaces, or 0.75" per side.  For not much money the 8.8 is a very reasonable option, and very strong to boot.

If you want a cheap 8.8 and don't mind drum brakes, you can piece one together from a couple Ford Rangers.  You'll have to mix and match axles from two donor trucks, but you'll have a strong axle with a good spline count on the shafts.

There is a reason Ford replaced the small 7.5 with the much stronger 8.8.  Strength.  And as I mentioned, the 8.8 has lots of aftermarket support from a wide selection of ratios to traction aids/lockers (to include factory junkyard pieces).  Rear disc are a plus IMO.

Offline eaglefreek

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Re: Rear axle swap
« Reply #3 on: February 12, 2013, 12:14:00 PM »
An 8.8 is a great axle but overkill for a stock Eagle that will mostly see road duty. With the price of fuel the extra weight and more mass to rotate will cost MPG.
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Offline Billman

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Re: Rear axle swap
« Reply #4 on: February 12, 2013, 07:11:53 PM »
Thanks for the input, guess I should have noted that the Eagle will probably never see any off-road but will likely see simple to and from driving. The reason I asked is my neighbor is parting out a Mustang not sure on the year, he will get me the id tag soon. Its possible that it is a 8.8 but not sure yet.
1983 SX4
1983 wagon almost DD
1984 wagon DD
1984 wagon

Offline eaglefreek

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Re: Rear axle swap
« Reply #5 on: February 12, 2013, 08:01:12 PM »
What year? I'm assuming 94 or newer. V6 or V8? My 84 Mustang SVO had a 7.5 and it had more HP and torque than an Eagle. I abused it as a teenager and did finally break it, but you can only dump the clutch at over 5000 rpm so many times before breaking something. A buddy of mine would drag race is 84 Mustang GT with the 7.5 and would consistently run high 13's with the stock 7.5 without issues.
"I know he'd be a poorer man if he never saw an eagle fly,
Rocky mountain high"  John Denver
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Offline SpreadEagle

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Re: Rear axle swap
« Reply #6 on: February 24, 2013, 03:48:18 PM »
I've been looking at this as a solution to some issues on my eagle lately and I am wondering what a reasonable price is to get a complete axle? any opinions? and what year and model ford 8.8 or Chrysler 8.25 is a good pick?
I have a 99 Explorer from end to end here close for about $100 but I don't know the ratio, on the axle or my car.

Mine is an '81 Kammback with the 4.2  manual 4 transmission. Which from what I read means it has 2.73 gears? Currently running over sized tires. I think tire size is 215/75r15
« Last Edit: February 24, 2013, 03:51:41 PM by SpreadEagle »

Offline TheWraith

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Re: Rear axle swap
« Reply #7 on: February 25, 2013, 07:48:54 AM »
After some reading I do not think the typical Mutang/Ranger rear axles are wide enough for the Eagle.  Are they?  Has anybody measured them?  If they are not wide enough, the Exploder 8.8 might be your best Ford choice.  The factory late '80s Cherokee Dana 44 is still the best choice I'm aware of as it is almost 100% direct bolt-in.

To measure the additional fuel cost of an 8.8 over a 7.5 would be splitting hairs.  I doubt one could accurately measure the difference in anyway other then in a lab with a dyno.  Fuel pumps are notoriously inaccurate to the point that the extra fuel to drive an 8.8 would not be measurable.  The only way I can think of to get a rough idea in the typical garage would be to measure the force needed to rotate the axle with a beam torque wrench.  Even then the measurement would be in lb/in, not lb/ft.  Car Craft did an article sometime within the past two years where they measured the (drag) difference between a GM 12-bolt, D60, and the Ford 9 inch using this method.  The point was to see how much hp it took to drive the various and most common axles used for high hp cars.  The big difference was not the weight of the components so much as the difference in pinion angle in comparison to the ring grear.  The 7.5 and 8.8 are close enough that if setup properly should be very close in terms of the power needed to drive them.  I predict one would have to do a lot, a LOT of driving for a long, long time to realize any difference in fuel expense between the two axles all else being equal (ratio, driving habits, car, engine, load, etc...).

In terms of on or off road driving, if you are going to go through the expense and labor of swapping to a different rear axle the 8.8 is still the smart choice over the 7.5 IMO.  The 8.8 has a lot of aftermarket support where the 7.5 does not.  Gears, bearings, brakes, weld-on brackets, axles, etc...all readily available for low cost.  Other then a free or near free axle from a neighbor, the junkyard is full of the 8.8 axle so finding one for cheap is easy enough.  If you are fortunate enough to have a pull-a-part-type salvage yard near by a rear axle will be cheap.  Find a 1995 through 2001 Ford Exploder.  Lastly, the additional strength is not just a little, but a LOT.  All things being equal (cost, labor/time to complete the swap, parts, etc...) getting the extra strength is just smart.  Even if you don't intend to go off-roading in time you might go to a taller or wider tire, pull a trailer, or load up the car and go camping for a week.  Might as well have the extra strength, no reason not to.

Matching the front axle gear ratio is key.  You'll have to look around and see what options are available for your factory front axle.  Highest I could find for the 8.8 is a 3.08 gear.  If you can find a front axle out of a four cylinder Eagle you can change to a deeper gear and possible find one that will match up with an 8.8 offering.  There are a lot of variations in the Eagle front axles/housings, so I'm not going to go into that here.  It is documented here on the AMC Eagle Nest.  Finding a tall gear for the 8.8 is difficult as the Exploder is heavy and needs a deeper gear.  Add in the aftermarket is supporting drag racing and off-road applications and they make them deeper, not steeper.  Finding a matching gear ratio between an Eagle front axle and an 8.8 might be the only sticking point.

Personally I'm looking for the 4 cylinder Eagle front axle housing and the late '80s Cherokee rear d44.  If you take the time to research and settle on a set of matching front axle and 8.8 rear axle gear ratios, please post up your findings.

Offline Billman

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Re: Rear axle swap
« Reply #8 on: February 26, 2013, 06:26:37 PM »
Thank you guys for all the input. After reading through all the answers I told the neighbor to let it go and I'll be looking for a Cherokee that has a good running 4.0 drivetrain so that I can go full tilt one time and have all the pluses taken care of without multiple returns to the garage for add-ons. I'm so busy with other projects and broken cars that patience is the only thing I can put into it right now. Bummer! :'(
1983 SX4
1983 wagon almost DD
1984 wagon DD
1984 wagon

Offline carnuck

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Re: Rear axle swap
« Reply #9 on: March 01, 2013, 02:13:08 AM »
Double check what your ratio is. That setup may have 3.08 (that's what I got my non-vacuum front axle from) 8.25 and M15/D35s in that ratio are common (almost all 4.0/5 speeds) Early Comanches' ('86/87) with Metric Tonne package could have a Model 20 with 1 piece axles too. (YJ guys love those!)
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'82 Eagle Wagon (Brown Betty) 4.2L/727/NP229 and soon 8.25 rear axle and non-vacuum 3.08 front to match. 235/75/15 tires. It's down to minor body work (someone creamed my driver's door but I have another in good shape) almost rust free and interior work (seats are worn and carpet torn). $2000 but about to go up due to more work being done.

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Offline Billman

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Re: Rear axle swap
« Reply #10 on: March 28, 2013, 02:19:40 PM »
I am still toying with the notion of pulling all the stuff from my 94 ZJ and  going way overboard with it.
Only time will tell.
1983 SX4
1983 wagon almost DD
1984 wagon DD
1984 wagon

 

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