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8.5" Rear Instead?

This is a discussion on 8.5" Rear Instead? within the Drivetrain forums, part of the LSx Technical Help Section category; I see a lot of stuff regarding the Ford 8.8" but why doesn't anyone ever swap in a GM 8.5"?...

  1. #1
    Member slade8200's Avatar
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    Question 8.5" Rear Instead?

    I see a lot of stuff regarding the Ford 8.8" but why doesn't anyone ever swap in a GM 8.5"?

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    Moderator Firebirdjones's Avatar
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    The 8.5's are capable rearends with the same 1.625 pinion diameter as the 8.875 12 bolt and 8.8" ford. Only big difference is the slightly smaller ring gear diameter. The other difference is that the older 8.5's were 28 spline axles, however GM did switch that around 1988-90 to the 30 spline axles since they were still using this rear in the trucks at that time.

    Makes this rear pretty strong for it's size, and very capable when built right. I've seen them in 3400lbs race cars going down into the 8's at 160 mph.

    The only big issue you'll find is trying to adapt a torque arm to it. Guys are adapting torque arms to the 8.8 so it could be done to the 8.5 I suppose.

    In the end if you really want to fab something rather than just buy a drop in housing, it's much easier to find and modify the 9" housing as Currie makes a bolt on torque arm bracket that makes it very simple.

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    Member slade8200's Avatar
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    But why not use the smallest and lightest differential that won't break? Wouldn't that be an 8.5"? You're saying the Ford 8.8" is proven stronger?

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    Moderator Firebirdjones's Avatar
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    Quote Originally Posted by slade8200 View Post
    But why not use the smallest and lightest differential that won't break? Wouldn't that be an 8.5"? You're saying the Ford 8.8" is proven stronger?
    Are you worried about HP losses and weight?

    Yes in stock form the 8.8 would be the stronger of the 2, especially if you get into high ratio ring and pinions where the pinion diameter shrinks and makes less contact with the ring gear. The larger the ring gear diameter the better that situation becomes.

    The rest of the 8.5/8.8 rearend differences are marginal at best.

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    Member slade8200's Avatar
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    I'm not actually planning on doing this, or at least I hope I'm not. I was just curious why I didn't see more GM truck rear ends used.

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    Moderator Firebirdjones's Avatar
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    It's a good question, either rearend would take the same amount of fab work to get it in a 4th gen, and they are both very close in strength, plus either rear is still readily found out there.

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    Junior Member 99ss405's Avatar
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    8.5 rear ends aren't cheap .. they came in GN's ... if I were u id step up to a 9 inch like I did ... just haven't installed it in the ls1 car yet ! works perfect in my lil 89 TTA

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    We have a pile of old school 10 bolts in our cars... no issues with any of them.

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    Moderator Firebirdjones's Avatar
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    The 8.5's have been in production since 1971 in everything from X-bodies, to F-bodies, to A-bodies etc... And became standard issue in all 1/2 ton GM trucks starting about 1980 up to this day. You don't necessarily need to find one from a GN. Those gained in popularity simply because that G-body platform was the last of the full frame rear wheel drive cars and they made great racecar platforms, so guys with malibu's were seeking them. Drove the prices up......for a while.

    Any 8.5 is going to need ALOT of fab work to get it in a 4th gen F-body, so starting with any application would do the job (some more work than others)

    Honestly if buying a drop in unit isn't something you want to do, then fabbing a 9" is much easier than doing a conventional 8.5-8.8. All the brackets, including the troublesome torque arm mount, are made for the 9 inch (Currie sells them) so you aren't trying to weld something on an iron center section of a 8.5-8.8, which can be a real challenge.
    In the end though, you are going to spend nearly as much fabbing a 9" as you would just plunking the coin down for a ready to go unit.....give or take $500.
    Been there done that years ago before these drop in units were on the market.
    Last edited by Firebirdjones; 02-19-2015 at 04:43 AM.

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