8 3/4 vs 8 1/4 weight

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Topless69

Topless69

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I’ve been reading FABO and getting plenty of opinions, but very few actual numbers, so...
Does anybody know the weight difference between a complete 8 3/4 rear with Mopar disc conversion and an 8 1/4 with drums setup?
Both are A body, all stock Mopar parts and big bolt.
 
Topless69 said:
I’ve been reading FABO and getting plenty of opinions, but very few actual numbers, so...
Does anybody know the weight difference between a complete 8 3/4 rear with Mopar disc conversion and an 8 1/4 with drums setup?
Both are A body, all stock Mopar parts and big bolt.
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It sounds like a big splitting hair problem... My first question would be what does it matter? I can't imagine it being that big of a difference... Unless it's some very special race specific application you're needing and even then...
 
j par said:
It sounds like a big splitting hair problem... My first question would be what does it matter? I can't imagine it being that big of a difference... Unless it's some very special race specific application you're needing and even then...
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What I’ve read so far indicates there might be a significant weight difference between the 8.75 and 8.25, and disc or drum may tilt it even further one way or another, so I’m looking for actual numbers.
I’m also trying to figure out if discs or drums are heavier, all else being equal.
 
If you don't need the increased strength and stopping power, why bother with the changeover?
If you DO need the strength increase, then you NEED the strength increase and an extra 100 lbs (and it won't be anywhere near 100, might be 50, I would bet less) won't matter.
You haven't told us why you think you might need an (expensive!) 8 3/4.
Edit: lightweight (drag racing) disc setups (without parking brakes) are a bit lighter than small drums setups, more lighter than the 11"drums. Street disc conversions for the rear with parking brakes are quite a bit heavier than 10" drums.
2nd edit: if you are after light with strength..... 8 3/4 with 9" drums. You have disc's in the front that do 80% of the stopping, right?
 
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Everybody’s gonna think I’m crazy, but since my engine is mostly stock, I’m considering swapping the 8 3/4 for an 8 1/4. IF it saves weight.
OK I’m ready...Bring it
 
Topless69 said:
Everybody’s gonna think I’m crazy, but since my engine is mostly stock, I’m considering swapping the 8 3/4 for an 8 1/4. IF it saves weight.
OK I’m ready...Bring it
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I don't think I would bother, from a weight standpoint. Now, if you were doing it for a substantial ratio change, I could agree with that.
Your 8 3/4 is quite valuable on the market too.
 
33IMP said:
I don't think I would bother, from a weight standpoint. Now, if you were doing it for a substantial ratio change, I could agree with that.
Your 8 3/4 is quite valuable on the market too.
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Yeah, I’m not planning on selling the 8 3/4 and I appreciate your opinion on gear ratios, still looking for actual weights though.
 
Well a little bit of extra weight for drag racing right over the back tires it's probably going to help more than it's going to hurt.. filled 100 lb for 10th of a second so if you have an extra 50 lb that could be a loss of a half a tenth of a second LOL and if you're running a stock engine you're not fighting a half a tenth of a second but then again that half a tenth of a second could be made up with attraction you get with a little extra weight over the back tires... As far as strength my eight and a quarter lasted until I sold it and my well built eight and three quarters lasted about 20 passes till I put in a Dana 60... This was all heavy drag racing and the weight didn't come into any of my calculations...
 
I am guessing perhaps 20 Lbs or so.
Not worth the trouble in my opinion.
It is hard to get gears for it many ratio and
they are darn fragile!!
 
Depends....

What rear discs? Liberty? Imperial?....?

Aluminum center in the 8 3/4?
Spool in the 8 1/4?
Sure grip? No?
GREEN BEARINGS (Oh yeah!)

You get the idea. Worst case, hit a pick n pull, bring a scale
 
@Topless69
I’m in the same predicament with a stock 73 340/4 spd. From what I have gathered is that the 8-1/4 is fine for that application. Plenty of gear choices from Doctor Diff and others to support it.
I think if I were wanting to achieve your goals I wouldn’t look at weight as much as I would rolling resistance. I have read that Dana’s use less HP to spin. I would be trying to figure out where the 8-1/4 is in that regard. What s the pinion location compared to an 8-3/4.
 
Both have 3" axle housings, use the same backing plates, drums, same shock mounts, I think you'd be lucky to save 20 lbs, probably less. IF you really want to save weight, go to the 7.25. I just did a weight by weight comparison of the 7.25 vs 8.75 and the weight is pretty substantial....... so is the strength
 
nI'm in the 20 pounds or less club. I've seen some claims that they weigh the same. But the 8-1/4 is much more mechanically efficient than an 8-3/4. I wish Chrysler had went all AMC with the rear axle in A bodies and used Dana 44s.
 
Garrett Ellison said:
nI'm in the 20 pounds or less club. I've seen some claims that they weigh the same. But the 8-1/4 is much more mechanically efficient than an 8-3/4. I wish Chrysler had went all AMC with the rear axle in A bodies and used Dana 44s.
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Pull one out of a Dakota. Are they 44's or a corporate copy?
 
Dakotas use either 9-1/4, 8-1/4 or (wait for it...), late 7-1/4 in the 4 cylinder variant. There was also a 7-1/4 front center axle in early light duty 4WD. Later 7-1/4s share some parts with either Dana 30 and 35s. This makes the AAM 7-1/4 one of the longest running production axles made, produced in some variant from 1960 to 1993.
 
Garrett Ellison said:
nI'm in the 20 pounds or less club. I've seen some claims that they weigh the same. But the 8-1/4 is much more mechanically efficient than an 8-3/4. I wish Chrysler had went all AMC with the rear axle in A bodies and used Dana 44s.
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what years of AMC ?
 
I don't have exact weight differences but Steve Mags did a side by side comparison of a D60 vs a 8.75 in his Hemi Dart with everything being equal in both rear ends (gear ratio, brakes...) he found that the "extremely heavy" D60 was only about 50 lbs heavier than the 8.75 and actually ran equal 1/4 mile times. Now let's just pretend that your 8.75 weighs 50lbs more than the 8 1/4 (I think the weight difference will be less), I don't think you'll notice anything significant between the two. You would be better off trying to find 50 lbs somewhere else if you want to lose weight
 
I have carried those rears around in the past when I was younger, and if the 8.25 was lighter, I didn't notice it lol. I mean, I'm sure the 8.25 is lighter, but not enough to make a difference to your engine, or to your mpgs. I mean think about it; 20/3500loaded= .0057, or .57%, yes decimal 57percent.

My unused fold-down rear seat was 64 pounds...... so I shelfed it.
One 15 x10 alloy slotmag is 14 pounds. Two alloy slotmags weigh about the same as one steel Rally, size for size.
Gas is about 6pounds per gallon, so a full tank is about 90 pounds.
From the headers back, I think my dual 3" exhaust is 72 pounds; half of which is the Dynomax muffs.

For a streeter, making the rear 20 pounds lighter is, IMO, a waste of time. plus it makes an already nose-heavy car , worse. I like to run the 8.75 under everything; simply for the gear swap-ability.
Just put a 2800convertor in her and your stocker will never care about whatever that weight difference might be.
I'll tell you one thing tho; 295/50-15s on 15 x 10 alloys at 24psi rides real nice like; compared to say; how I remember E70-14s Polyglass GoodYears,@30psi, on steel 5.5 Rallys. 275/50-15s on 8.5s@28 were maybe a lil better.
 
In the original magazine article of the Dana vs 8-3/4, they said that part of the reason a Dana ran the same time as an 8-3/4 with weight penalty was due less to the greater mechanical efficiency of the Dana axle as it was that the 8-3/4 housing actually was flexing enough to cause the rear tires to toe in. There have been some users claim that an 8-1/4 axle upgraded with the LPW girdle and back brace kits with 29 spline positive retention axles will live in a big block a body as well an 8-3/4 will.
 
I’ve read somewhere that they have the same stub out length. I could see that being done because the 8-1/4 was released in 1969 as a B body axle. It wouldn’t surprise me if Chrysler would do that just to avoid having the extra expense of another driveshaft length.
 
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