8.5" rear strength

[UNGN]

400 crank hp or to the tire? Either way, you will be fine with the 8.5". Turbo buick guys run 10s with the completely stock rear and 9s with aftermarket axles, gears, carrier, etc.

GN's are almost all footbrake autos. 8.5's with stronger axles will last for ever under these conditions.

If the pinion is preloaded against the ring gear before force is applied, no worries. When there is a tiny gap between the pinion and ring gear, and the pinion hammers against the ring rear when force is applied, problems will start happening.

 

[lilbowtie]

This is going to be a decision only you can make. I wouldn't invest any money in a 7.5 but you already have a 8.8 that is up for the task. The 8.5 is another piece I wouldn't worry about (especially @ your HP). I put a 9 in mine 17 years ago an haven't had a problem (heavy racing) I have been running stock parts but there are many upgrades - war case, moser, bolt through, gears, splines. It all comes down to $$ and how much HP your going to throw at it but don't over buy.

 

[olds307 and 403]

His torque should be around 500 ft/lbs. Too bad Olds went away from the bolt in axles. The same expert that builds rear ends for a living said 500 ft/lbs was no issue for my 8.5" Type O rear with stock 28 spline axles. It has the superior bolt in axles but a smaller diameter pinion than the Chevy 8.5". It all depends on what he can find locally for 9" parts and a housing for a G body. I see an easy 2 grand putting together a proper 9". Unless you find a used or local housing, used 3rd member and axles, is it runable as is? A new third member with a good posi is close to a grand. Not saying a 9" isn't stronger but stronger never comes cheap.

 

[DRIVEN]

Dollar for dollar the 9 wins. If I were in your position I'd probably sell the 8.5 for the stupid money guys seem to be getting for them and roll that cash into a 9".

 

[pontiacgp]

I have no experience with manual's, but I'd find it hard to believe that the 8.5 won't withstand a 400HP powerplant. For that matter, for a 400HP powerplant I'd leave the 7.5 in there and upgrade it to 28 spline posi from a 4 Gen Camaro and a set of S10 2WD axles if budget was a concern.

the 8.2" I have is a rated to handle up to 500 hp. I agree with leaving the 7.5" in there and upgrading the axles but I would add an Eaton Trutrac

 

[marcar1993]

I do not have a 7.5 in the car now. It has an 8.8 which has been a headache since it went in. I have an open 7.5, 2 8.5s, and an 8.8 plus the one in the car. I want out of the 8.8 since it has given me headaches. I am really leaning towards the S60 for strength, or a quick performance 9". A new rear with a warranty of some kind would be nice. Its just that they are both over $2k. However I do have a ton of money sitting in rears right now. Once there is something good in the car I'd sell all of it except for maybe the 7.5 or one of the 8.5s just to have a spare.

There is NO WAY a 7.5 will live in my car. It 60' under 2 seconds at 4000lbs on street tires walking it out of the hole. Imagine the shock load dumping the clutch on slicks!?
You are right though, I need to make the decision myself. I was just hoping for some real world experience. I also want to know how much more it'll take. Eventually I plan to do another topend and bring the car into at least the 11s. I don't want to do this, again....

 

[L92 OLDS]

I agree with others that best option for high strength with reasonable cost is the Ford 9" but why do you insist on dumping the clutch with slicks? It is not only hard on the rear end but the entire chassis. As noted, the GN crowd has been abusing the 8.5" rears for a long time and you already have 2 of them.

 

[marcar1993]

It isn't particularly that I insist, so much as when I race the car, I launch it hard. I don't want to be in fear that this will be the time it lets go. I do not plan to drive on slicks normally. And normally I don't dump the clutch, there is no problem spinning tires from a roll in first or second gear. I just want to be able to launch it without fear of breaking it.
I am looking on quick performance and it look like a 9" with a detroit locker and nodular iron 3rd member is $2500 or so shipped with drum brakes. An s60 is just about $500 more plus setting up brakes on it. Decisions decisions.

 

[lilbowtie]

I also want to know how much more it'll take. Eventually I plan to do another topend and bring the car into at least the 11s. I don't want to do this, again....


Mines been in the 10's, You can build 9in to hold 1,000 +

 

[64nailhead]

I do not have a 7.5 in the car now. It has an 8.8 which has been a headache since it went in. I have an open 7.5, 2 8.5s, and an 8.8 plus the one in the car. I want out of the 8.8 since it has given me headaches. I am really leaning towards the S60 for strength, or a quick performance 9". A new rear with a warranty of some kind would be nice. Its just that they are both over $2k. However I do have a ton of money sitting in rears right now. Once there is something good in the car I'd sell all of it except for maybe the 7.5 or one of the 8.5s just to have a spare.

There is NO WAY a 7.5 will live in my car. It 60' under 2 seconds at 4000lbs on street tires walking it out of the hole. Imagine the shock load dumping the clutch on slicks!?
You are right though, I need to make the decision myself. I was just hoping for some real world experience. I also want to know how much more it'll take. Eventually I plan to do another topend and bring the car into at least the 11s. I don't want to do this, again....

You and I could get this done on the cheap. You stop up and drop off an 8.5" and $750 for parts. Leave the the other 8.5 for the labor payment and I'll build you an 8.5 that will withstand 700+ crank HP - I'll guarantee it.

I've posted this video before (ignore the dingaling narrater) - this is a home built 8.5 with the parts from Quicks that I linked previously - making close to double of what you have. https://www.youtube.com/channel/UCM3ZoceOSl5dOMSlek4XB5Q?view_as=subscriber

NJ isn't that from far Upstate lol 🙂🙂

All kidding aside - you're worrying way too much about this. An 8.5 properly built will take your HP x 1.5 all day every day.