Just how durable are GM 7.5 rear ends? - Hot Rod Forum : Hotrodders Bulletin Board
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Old 01-20-2009, 04:14 PM
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Just how durable are GM 7.5 rear ends?

I have an 85 Olds Cutlass Supreme that I've made major engine upgrades to, of course I haven't been able to fully realize those upgrades because I still have the stock gears of the Cutlass installed...

I haven't had much luck at all finding an 8.5 for a decent price, so now I'm looking at the option of just upgrading the 7.5 that is currently on the vehicle. I've run the idea by some of the people in my car enthusiast circle, and most tell me that the 7.5 isn't a great rear end but no one has been able to give me a clear idea of what kind of durability I can expect from this rear end once it's been upgraded with new gears and a locking differential...

I'm building a street/strip car, that's going to be on the street a lot more than it will be on the strip...

Can anyone give me a reasonable estimate of durability?

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Old 01-20-2009, 04:34 PM
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It depends on how much traction you can get. That's what destroys them. Sticky tires, slicks. If the gear, carrier, axles & rear cover are upgraded they will hold some torque. But then the 8.5" doesn't look so expensive. How much torque????? Difficult to say.
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Old 01-20-2009, 04:41 PM
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Quote:
Originally Posted by SSedan64
It depends on how much traction you can get. That's what destroys them. Sticky tires, slicks. If the gear, carrier, axles & rear cover are upgraded they will hold some torque. But then the 8.5" doesn't look so expensive. How much torque????? Difficult to say.
I figured that torque and tires fed into the equation...

The 8.5's I've seen were going for $1,200. The upgrades I'm looking to install cost about $600...

I'm probably going to stick with street tires, since like I said it's a street/strip set up, but it's going to see a lot more street than strip...

I'm not sure of my torque rating yet as I haven't been able to have the vehicle dynoed...
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Old 01-20-2009, 04:42 PM
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I don't know if this helps, but judging from the parts I used to upgrade this 350 block, I should be generating around 400 hp...
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Old 01-20-2009, 05:01 PM
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to 355

I bought a 8.5 from a junkyard out of a 69 chevelle for 200. paid about 250 more for a richmond gear and install kit. then about 450 for a aubrun pro series limited slip unit. to top it off i put a summit support cover on it. this sits behind 500hp and has proven to hold up very well on street and strip. if you go this route you could do like i did and buy a part whenever you have some spare change. I'd love to have a ford 9 inch, but for now this daily driver does good. Me personally, I would take my time and put something in that you won't have to go back and fix. hope this thought helps. good luck.
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Old 01-20-2009, 05:08 PM
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look into nine inch stuff, its surprisingly affordable, when you look at the cost/benefit of it all. housing on ebay, third member from the swap meet or junk yard, custom axles would be the biggest outlay.

I've got a nine inch in my 84 montecarlo, I might have 400 or 500 dollars into it for a used center section, gears, and a full rebuild kit, but i did the work myself and got the housing for free.
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Old 01-20-2009, 05:38 PM
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Thanks for the input, guys...

I doubt that I'll be able to go the Ford route, but I plan on fiddling with this car for years so that might be something for me to look at down the road...

For now I just don't want this 7.5 to leave me holding the bag in the near future. From what I can gather, what I'm doing is not ideal, but isn't a total liability, so as long as this upgrade lasts me a couple of years, I'll feel like it was worth it...
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Old 01-20-2009, 06:00 PM
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Why put money in rear end that you are going to replace in a couple years? Build it once with a 9 in ford and forget. Why go threw headache twice?
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Old 01-20-2009, 06:09 PM
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Quote:
Originally Posted by heathj1
Why put money in rear end that you are going to replace in a couple years? Build it once with a 9 in ford and forget. Why go threw headache twice?
Honestly, I'm in somewhat of a time crunch...

I'd like to be able to take the time out to find an 8.5, but time is luxury at this point...
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Old 01-20-2009, 06:37 PM
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The downfall of the GM7.5 is the flimsy stock carrier. It is so weak that when loaded the ring and pinion push themselves apart until the teeth pop off. The aftermarket carriers are a great improvement. Also consider TACKING the tubes to the housing. And consider using Richmond gears. They tend to be noisy especially in the GM7.5, but they are cut with a different helix which really does make them stronger in situations where any strength increase is beneficial.
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Old 01-20-2009, 06:48 PM
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At a minimum put a differential girdle on the 7.5. About a $150 bucks and will help the carrier stay in place. I had a 7.625" (the same thing) in a T-Bucket with huge tires on the back and beat the snot out of it. Yeah, it's a light car but the traction with the big tires will still put some strain on it. When the posi clutches needed replacing, I installed an Eaton TrueTrack instead. When I had the rear apart for the Eaton installation, there were zero signs of excessive stress anywhere - good tooth pattern, backlash had not changed, bearings and races were good. Good luck to you.
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Old 01-20-2009, 09:57 PM
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Great information, guys...

Many thanks to all that replied...
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Old 01-20-2009, 11:29 PM
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I ran a 7.5 with richmond gears behind a 383 in a malibu wagon - no issues. Ran 99 mph 3900# with driver.
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Old 01-20-2009, 11:41 PM
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Quote:
Originally Posted by ca434sbc
I ran a 7.5 with richmond gears behind a 383 in a malibu wagon - no issues. Ran 99 mph 3900# with driver.
Sweet...

That's exactly what I want to hear, I won't even be running my vehicle that hard, so this is good news...

Thanks for replying...
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Old 01-21-2009, 12:01 AM
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Yep, just don't put sticky tires on them or it will show its TEETH.
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