How strong are 8 1/4s

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Richie

Richie

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Is and 8 1/4 rearend strong enough for a 340 with about 320 hp. Not for racing but for poopin around the street with.
 
i run my 8.25 with slick and run 12.56. Had it inthe car since i put it on the road 14 years ago.
 
If you still have you 7 1/4 in it, do a massive burnout with can count how many seconds it last doing it. Mine last 15-16 before it stopped working.
 
I'm glad I swapped up to an 8 3/4 on my build. I had an 8 1/4 out of an F body in mine and when I pulled the cover to take the c clips out so I could pull the axles for the brake setup I was amazed at how small everything looked. The axles were a joke and probably not even an inch thick at the smallest section. The ring gear looked tiny as well, but I don't know if there was any difference in the setups from the different years or body styles. I pulled one out of a 69 Charger that I didn't think looked as small, but for driving around the street I wouldn't worry about it. I intend to race mine and do some big engine upgrades in the future, so I'm planning ahead. There is definitely more of a difference than just a 1/2 inch of ring gear diameter between the two though, lol.
 
My 73 Dart Sport 340,4 speed came factory with a 8 1/4 sure-grip with 3.23,s.I now have a 8 3/4 for the added HP.Anything over 350HP you,d be pushing your luck.I had an axel snap on me,wasn,t much fun.
 
Map63Vette said:
I'm glad I swapped up to an 8 3/4 on my build. I had an 8 1/4 out of an F body in mine and when I pulled the cover to take the c clips out so I could pull the axles for the brake setup I was amazed at how small everything looked. The axles were a joke and probably not even an inch thick at the smallest section. The ring gear looked tiny as well, but I don't know if there was any difference in the setups from the different years or body styles. I pulled one out of a 69 Charger that I didn't think looked as small, but for driving around the street I wouldn't worry about it. I intend to race mine and do some big engine upgrades in the future, so I'm planning ahead. There is definitely more of a difference than just a 1/2 inch of ring gear diameter between the two though, lol.
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Sounds like you had a 7-1/4 not a 8-1/4.

The 8-1/4 is a lot tougher than most people give it credit for. I had a 93 Dakota that weighed 3780 race weight and I raced it for several yrs. with the stock 8-1/4. It ran mid 14's so it wasn't real fast but it honked for a heavy truck. Sold it with over 90,000 miles on it with the stock 8-1/4 going strong. Last I heard the guy had 130,000 on it and it was still going.
 
fishy68 said:
Sounds like you had a 7-1/4 not a 8-1/4.
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I kinda wondered the same thing, but from what I've found F bodies (what the rear end came from) only had an 8.25 and it all measures out to match the chart. I'm pretty sure the car came with a 7.25 originally since it was a /6 auto from the factory, so I wouldn't see why they would swap sideways so to speak. I wouldn't be surprised anymore though with the stupid crap I've found out about my car. They didn't just swap the rear end, they pulled the whole rear suspension with it, so I have the rubber spring isolator boxes on the leafs and the wrong shocks with a post mount on one end and the bushing on the other.
 
Nope F-bodies with \6's and 318's came stock with a 7.25. The 8.25 came in the 360 powered cars and was an option in the others.
 
Huh, well either way I also realized that it had 3" axle tubes since I reused the U bolts and shock plates for the time being, so that may have been another reason why I was pretty sure it was the 8.25. I should take some pictures next time I yank it apart. The gears and axles are just wimpy looking compared to what I remember seeing in our 8.25 Dakota and 69 Charger I took apart. Wonder if something changed between bodies or something.
 
The later 7.25's had 3" axle tubes that necked down to 2.5" where they went into the housing.
 
I used to race a 66 cuda and one night in final round street class i went to line reved up green light and snap no go. Built custom 8.75 for racing no more problems ever! Wish I still had it for my current 65 so I could do some racing, need to get one bigger rear so no snap at track.....318 w little over 300 hp mid 14's so be careful with small rear....
 
dgc333 said:
The later 7.25's had 3" axle tubes that necked down to 2.5" where they went into the housing.
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I just found that info today as well. To reassure myself I'm not crazy I went and pulled the casting numbers and it shows being a 8.25. I'm going to grab some pictures for fun to see what sort of crazy contraption I pulled out of the car, lol. I know better than to ask what the previous owners were thinking anymore...
 
Well, just for fun I grabbed some pictures. The axle is a tapered one, which might account for why it looks so pathetic, but it measures out a pretty much one inch at the narrowest part before the splines. It's a 2.94 gear too, which might account for something, but I just thought the ring gear looked really thin for an 8.25. Guess I had the new 8.75 sitting around too long for me to look at and compare, lol.

IMAG0034.jpg


IMAG0035.jpg
 
Map63Vette said:
Well, just for fun I grabbed some pictures. The axle is a tapered one, which might account for why it looks so pathetic, but it measures out a pretty much one inch at the narrowest part before the splines. It's a 2.94 gear too, which might account for something, but I just thought the ring gear looked really thin for an 8.25. Guess I had the new 8.75 sitting around too long for me to look at and compare, lol.
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The thickness of the ring gear is a function of the ratio, as the ratio gets numerically smaller (3.9 towards 2.9) the ring gear gets thinner to provide room for the larger diameter pinion (has more teeth). The same situation occurs with an 8.75 or even a Dana 60.
 
An 8-1/4 stock should handle a 400 HP engine without concern. Upgraded parts should be a real treat for much more power. Drag race cars are much more abusesive and if the car is still heavy, the rear would fail under all the stress of power and weight.

Back to the streetabilty of the rear........ 300 no problem.
 
dgc333 said:
The thickness of the ring gear is a function of the ratio, as the ratio gets numerically smaller (3.9 towards 2.9) the ring gear gets thinner to provide room for the larger diameter pinion (has more teeth). The same situation occurs with an 8.75 or even a Dana 60.
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Wondered if that had something to do with it. Any notes on the little axles? I was pretty sure I remember other 8.25's being bigger, but I don't think they were tapered either.
 
I think so but not for prolonged abuse I had mine for about 2 years before I shattered the spider gears and whatever the big bolt in the middle is with about 270hp with 2.94 maybe I just had a lemon???
 
1970Duster said:
If you still have you 7 1/4 in it, do a massive burnout with can count how many seconds it last doing it. Mine last 15-16 before it stopped working.
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:-D I have to tell you my "back in the day" story about my first 7 1/4 blow up.

I am 17, in 67 and took my "almost new" 64 Barracuda, 273-2 4 speed to a drag strip, 75 miles away, in Ark City, Kansas.

Well, it turned real ugly, real quick. I was broke, and broke down, it busted the case.
Got a couple friends to come after me with a tow rope, and it would only tow, after I took the carrier out of the rear end. They were no longer friends after that, as it was one crazy ride. The axles were flopping around inside the housing.
It was one heck of a mess, as I had just moved out of my parents house.

I ended up finding a brand spanking new wrecked 67, in the salvage yard, and changed it over to the 8 3/4. It was $100 at the time, and that was big money. Not sure where I scraped it up.

Now I have probably converted over another half dozen since.
 
An 8.25 rear with a sure-grip is a fairly strong unit. At a range of 300-350hp(pushing it)- you will be OK.

If you have to have an 8.75 rear, get one from a b-body and move the spring perches. 8.75's are stupid expensive to build and sure grips cost a mint!!

To be honest, a 7.25 sure-grip is quite strong. Just don't expect it to hold up to you're 408 stroker lololol...
 
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