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Discussion Starter #1
I am confused about the "power" rating on an 8" rear. I have heard they are good for either 300 or 350 horse power. Shouldn't rear ends be rated by torque? If so, what is the torque limit for an 8"?

I read an article about tranmissions that said you can run much more torque thru them than their advertised rating because Ford used a "severe duty cyle" to rate them. Wouldn't they rate rear ends the same way?

I know a guy that drag races a '56 T Bird that has a modified 352 and the original 8" rear. He's been racing it since '60 and says all he's changed are the limited slip clutches (twice) and the gear ratio (4.10).

I hope my new 302 puts out about 400 hp and 375 ft/lbs of torque. I have a Tremmec 3550, an 8" limited slip with 4.10 gears.The car weighs 2,600lbs. Do I really need a nine inch? They are soooo heavy.
 
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I will vouch for the 9" being a heavy sucker. I think the pumpkin alone wieghs what my entire 6 cyl rearend weighed. I stepped up to a 9" with 3.50 gears from the 7.25" integral with 2.80's and my time was exactly the same in the quarter! Anyone want to swap me for a freshly built 8"???

John

[color:blue]'68 Coupe
250/C4, 221 Argentine Head
Breakaway Converter, 9", 3.50's/T-lock</font color=blue>
 

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It just depends on how much abuse you're going to give the car. If you replace the axles, etc and reinforce the housing, you should not have any problem with routine street driving (with a little play on the weekends). But if its use is primarily to drag, then it's proably time to step up to a 9", or Curry built unit or something similiar.

Jeff
Member: Flatheads ain't so bad association
Never trust an over 40 Chevy owner association (They have to cheat to win)
 

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I've heard the same story that 8" rears are OK up to 300-350 hp. What I think may be an important point is what sort of use this is for - on the street with occasional "fun" an 8" may be fine with 400 hp, but continued track use may be a different story.
Here's what I did for my 8" to beef it up a bit: I added a set of 66 non-tapered axles to replace the 65s, and added a 3.25 4 spider t-lock in a stronger 67 case. For my use I think all will prove to be overkill.
Good luck! Dickson

1965 GT fastback / "Fastbasketcase" (by Midlife)
There's a fine line between "hobby" and "mental illness" - Dave Barry
 

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Hi, that 8" rear will take more than you guys are giving it credit for!!! a friend of mine has a 1939 chevy sedan with a blown 454 in it and a stock 8" 3.50 posi out of a 78 Granada and it holds together for him so I wouldn't be afraid of that rear unless you were going to thrash it all day long everytime you drove it. John

Opal frost pearl metallic 67 coupe http://www.jps.net/binay/webdocs/febef75b01.JPG
also have a 95 opal frost pearl G/T BOSS edition convertible belongs to swmbo
 
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My '67 coupe that is used for open track duty has an 8" rear with stock axles and a center section that I had built by Currie Enterprises with 3.80:1 gears and a 4 spider limited slip diff. The rear end holds up just fine, and I've never had an ounce of trouble from it. And I'd guess I'm running about 400hp through it. (not sure about that # yet, need some dyno time to be sure). Unless you're a hard core drag racer doing wheel stands at every launch, I'd guess the 8" would be fine. Lots of folks get stuck on the "bigger is better" idea without ever having actually had a problem with an 8' rear. Good luck!

Red '67 Coupe / 289 bored-stroked to 306 / toploader 4spd / 8" Currie Ent. 3.80:1 rear / lowered front A arms / 620 lb springs / etc..
Used for drive-in nights and open track days!
 

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Your combo with an automatic with no trans brake would likely be OK for street use with an 8"...I just wouldn't run the risk with your combo racing with that rear...it's so close to it's design limit that a failure in service could make for a very bad day...

It's the shock loading that will get yours, not the mean torque....remember that flywheel is storing energy..and then multiply that figure by the gear reduction in the trans in the lower gears and it becomes pretty formidable...

Take a look at the rear pinion support bearing on your rear....especially the webbing near the ring gear...that's what will likely break...

Your friend is likely using an older casting, which IMO were made from better materials as well as being a bit beefier....it would be interesting to weigh the housings...

My opinion is to start budgeting for a 9"....

Your combo will be making nearly as much HP as my race car engine and putting it through a clutch...I wouldn't consider running an 8" in my car even though I use an automatic...
I use a modified 9" and 31 spline axles...

Good luck!

Pat
http://www.jps.net/binay/webdocs/strtmstng002_sml.JPG
 

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Oh man, this argument will never be settled. I know tons of people who've gotten great service from 8s - using them in all kinds of applications. However, last year when I was looking for a better differential I called both Currie and Randys and told them I was running an estimated 358hp and 406lbs of torque. Both unequivocally said "9-inch". Maybe it's the torque figure that concered them. I'd have preferred the lighter weight axle for handling.

Glenn Morgan: 66 GT V-Burgundy Fastback 351w+toploader+9 in. TracLoc. Started out as a rusted-out Chicago-area crusher. After sacrificing a solid 66 coupe for its sheetmetal sub-assemblies, I have one solid (and expensive) work in progress!
 

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ONE THING TO KEEP IN MIND WITH ALL THE GUYS WHO ARE BEATING AN 8 INCH INTO THE GROUND WITH ALL THAT HORSE POWER. IF THE WHEELS ARE SPINNING THE IT WILL NEVER BREAK THE DIFFERENTIAL. BUT WHEN THEY DON'T! SOMETHING HAS TO GIVE. I'V SEEN 3200 lbs CARS DISTROY A GOOD 9 INCH WHEN THEY HOOK THE TIRES UP. THEN THEY GO TO A NODUALR CASE AND THEY ARE GOOD TO GO. EVERYTHING HAS A BREAKING POINT, AND THE 8 INCH AS GOOD AS IT IS HAS ONE THAT COMES A LITTLE QUICKER THAN THE 9 INCH. I HAVE BEEN REBUILDING THE 8 AND 9 INCH DIFFS FOR A LONG TIME. AND HAVE SEEM THINGS THAT WOULD BLOW YOUR MINDS AND WALLETS. TRUST ME WHEN I SAY "IT WILL BLOW". THE REAL QUESTION IS WHEN? DENVER
 
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