Using 8 3/4 with /6

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Greenmachine225

Greenmachine225

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Hi,
I'm new to the forum, I have a 71 Plymouth Duster with a slant and I have a question on something I'm looking to doing to the car. Currently I have 7 1/4 rear end and I want to swap it with an 8 3/4 but I'm not sure what exactly I would need or have to replace when I go to do this. Also what would be the best gear ratio for daily driving that's not a slug off the line, I was thinking 3.55 but I'm not quite sure. Im still only 16 and have the car for just about 2 years now so any information would help.
 
Nice looking Duster and welcome to the forum! You need the complete assembly from drum to drum and the shock plates (although some have elongated the holes in the 7 1/4 plates). I would install new U Bolts. For safety's sake, go through the brakes and think about installing new hard lines/hose available from several different vendors. I will be running a slant in my 65 wagon with 3.23 gears in my 8.75; others will probably chime in with some additional info.
 
You will be looking for an A body housing and axles. They less common these days so many people buy B body or C body diffs and get them cut them down at an engineering shop. Axles can be shortened or bought at the correct length new.

You will need to get your driveshaft shortened. You could also consider getting a whole new bigger one if you are going to make more hp in the future.

New shock plates. New U bolts.
 
I think a 3.55 is too tall for a daily driver. I have a 3.23 in mine and think it is the max I would go being I drive 25 miles one way on the freeway.
 
AJ/FormS said:
Ok I'll ask; WHY? I cannot think of one good reason to do this if you have to lay out the money. Nice car,
Click to expand...
The main reason is because I would like to buy a rear suspension kit but I can only use it with the 8 3/4, plus the slant makes a decent amount of power but with the tiny rear end it just can't put the power down.
 
Greenmachine225 said:
The main reason is because I would like to buy a rear suspension kit but I can only use it with the 8 3/4, plus the slant makes a decent amount of power but with the tiny rear end it just can't put the power down.
Click to expand...

Ah-so, the plot thickens
What kind of suspension kit and what are you hoping it will do for you?
 
It takes more power to turn the 8 3/4 than it does the 7 1/4 stay with the 7 1/4. A heavy duty suspension option was available with the slant six 7 1/4 rear package.
 
If it is for a rear sway bar mount (Hotchkis) then simply get U bolts to fit (2.5" I think) and make a new bracket that mounts to the axle housing. Make sure that you an change it up later. Just use the 3" mount when you get a 8.75 down the track. Most suspension improvements will happen without the rear swaybar.
 
Keeping the same rollers? I had 3.55's & an 8 3/4 in My '72 swinger, w/235/60HR14 tires(about 25.2" dia.), & the correct speedo pinion my slanty topped out at 118mph. That
was w/a stock torque converter. Your tires look taller, and your intake system capable of more top end than my 2bbl setup.
 
Montana A Body said:
How much more power does an 8 3/4 use compared to the 7 1/4 or 8 1/4?
Click to expand...
That I cannot tell you, I read an article I believe on the B body site comparing the power the Dana 60 absorbed compared to the 8 3/4. Look at the size and weight differience between the Dana 60 the 8 3/4 and the 7 3/4.
 
66 coronet said:
It takes more power to turn the 8 3/4 than it does the 7 1/4 stay with the 7 1/4.
Click to expand...
Poor & pathetic notation. This statement is misleading but true. Read more below.

Montana A Body said:
How much more power does an 8 3/4 use compared to the 7 1/4 or 8 1/4?
Click to expand...

(Insignificant, amounts that total squat.)

66 coronet said:
That I cannot tell you, I read an article I believe on the B body site comparing the power the Dana 60 absorbed compared to the 8 3/4. Look at the size and weight differience between the Dana 60 the 8 3/4 and the 7 3/4.
Click to expand...

My favorite words above, "I can not tell you."
An insignificant amount.

You would NEVER EVER know the difference when driving it.
You will NEVER EVER know the difference in your wallet thickness when paying for gas.
The drag strip will show the difference more so in driver error.
The dyno is the only place where you can (sadly and pathetically) point the finger and claim (like a brat teenager) that your right.

GREENMACHINE

The 8-3/4 rear can be swapped in easy.
You will need to find an "A" body 8-3/4. The expense of cutting down another rear is crazy and stupid to do unless your doing a custom or drag racing.

The rear will also need the normal 3 inch U bolts and shock plates. The 7-1/4 uses a odd smaller 2-1/2.

The big expense after the rear end purchase is shortening the driveshaft to fit.
(Or fixing the worn out brakes.)
The 8-3/4's nose, where the driveshaft bolts to is slightly more forward or longer. The driveshaft needs to be cut down to fit.

As mentioned before, a 8-1/4 is MUCH cheaper and still very strong. But not available with the small wheel pattern.

Excellent gas mileage? Look for a 2.76 ratio, about the most common ratio there is. A 3.23 & 3.55 offer somewhat of street performance. But they drive up the RPM's and will consume more fuel.

I did indeed notice your engine. Can you post up some specs on the engine? I would like to specifically know about your cam specs. The duration @ .050 would be a BIG/HUGE help in suggesting a rear gear ratio.

I also get being 16. You have a lack of funds to do things much less do them twice! Frack that!

(Edited, added a ",".
 
Last edited:
rumblefish360 said:
Poor & pathetic notation. This statement is misleading but true. Read more below.



(Insignificant amounts that total squat.)



My favorite words above, "I can not tell you."
An insignificant amount.

You would NEVER EVER know the difference when driving it.
You will NEVER EVER know the difference in your wallet thickness when paying for gas.
The drag strip will show the difference more so in driver error.
The dyno is the only place where you can (sadly and pathetically) point the finger and claim (like a brat teenager) that your right.

GREENMACHINE

The 8-3/4 rear can be swapped in easy.
You will need to find an "A" body 8-3/4. The expense of cutting down another rear is crazy and stupid to do unless your doing a custom or drag racing.

The rear will also need the normal 3 inch U bolts and shock plates. The 7-1/4 uses a odd smaller 2-1/2.

The big expense after the rear end purchase is shortening the driveshaft to fit.
(Or fixing the worn out brakes.)
The 8-3/4's nose, where the driveshaft bolts to is slightly more forward or longer. The driveshaft needs to be cut down to fit.

As mentioned before, a 8-1/4 is MUCH cheaper and still very strong. But not available with the small wheel pattern.

Excellent gas mileage? Look for a 2.76 ratio, about the most common ratio there is. A 3.23 & 3.55 offer somewhat of street performance. But they drive up the RPM's and will consume more fuel.

I did indeed notice your engine. Can you post up some specs on the engine? I would like to specifically know about your cam specs. The duration @ .050 would be a BIG/HUGE help in suggesting a rear gear ratio.

I also get being 16. You have a lack of funds to do things much less do them twice! Frack that!
Click to expand...
Well cuttlefish if they
rumblefish360 said:
Poor & pathetic notation. This statement is misleading but true. Read more below.



(Insignificant amounts that total squat.)



My favorite words above, "I can not tell you."
An insignificant amount.

You would NEVER EVER know the difference when driving it.
You will NEVER EVER know the difference in your wallet thickness when paying for gas.
The drag strip will show the difference more so in driver error.
The dyno is the only place where you can (sadly and pathetically) point the finger and claim (like a brat teenager) that your right.

GREENMACHINE

The 8-3/4 rear can be swapped in easy.
You will need to find an "A" body 8-3/4. The expense of cutting down another rear is crazy and stupid to do unless your doing a custom or drag racing.

The rear will also need the normal 3 inch U bolts and shock plates. The 7-1/4 uses a odd smaller 2-1/2.

The big expense after the rear end purchase is shortening the driveshaft to fit.
(Or fixing the worn out brakes.)
The 8-3/4's nose, where the driveshaft bolts to is slightly more forward or longer. The driveshaft needs to be cut down to fit.

As mentioned before, a 8-1/4 is MUCH cheaper and still very strong. But not available with the small wheel pattern.

Excellent gas mileage? Look for a 2.76 ratio, about the most common ratio there is. A 3.23 & 3.55 offer somewhat of street performance. But they drive up the RPM's and will consume more fuel.

I did indeed notice your engine. Can you post up some specs on the engine? I would like to specifically know about your cam specs. The duration @ .050 would be a BIG/HUGE help in suggesting a rear gear ratio.

I also get being 16. You have a lack of funds to do things much less do them twice! Frack that!
Click to expand...
Well rumblefish if they are your favorite words I ll try and repeat them more often just for you
 
The Slant is a very temperamental little critter. It WILL see the difference in weight and rotating mass going to an 8.75.
Automatic trans? No Stick? Not spraying 150 horse at the hit? KEEP the 7.25!
3.23 and 3:55 were popular gear sets in early cars, as were Sure Grip units. A little hunting will get you hooked up with a killer 7.25 for less than an ABody 8.75 housing will cost. If you're on a real budget, weld up the spiders and have at it.
Spider gears aside, that rear is MUCH stronger than people give it credit for.
 
Greenmachine225 said:
The main reason is because I would like to buy a rear suspension kit but I can only use it with the 8 3/4, plus the slant makes a decent amount of power but with the tiny rear end it just can't put the power down.
Click to expand...

what do you mean "it just cant put the power down" ?
you may only need a gooder set of rear gear, or different tires

there are a few things you can do to enhance traction, and indeed, dialing in the suspension is one of those

but if your slanty does not make more power then the 71.4 can handle (and by handle i mean, turn tires without killing itself) i think your chasing your tail
 
whitepunkonnitro said:
The Slant is a very temperamental little critter. It WILL see the difference in weight and rotating mass going to an 8.75.
Click to expand...
LMAO!!!!
Automatic trans? No Stick? Not spraying 150 horse at the hit? KEEP the 7.25!
3.23 and 3:55 were popular gear sets in early cars, as were Sure Grip units. A little hunting will get you hooked up with a killer 7.25 for less than an ABody 8.75 housing will cost. If you're on a real budget, weld up the spiders and have at it.
Spider gears aside, that rear is MUCH stronger than people give it credit for.
Click to expand...

I do like his headers and 4bbl. though!
 
Put a 65-70 B-body housing in the car. I have a 68 model one in my Duster and we put a 65 model one in my friend's Demon. Track width is a bit wider, yes, but its a good front to rear match, especially with the large bolt disks.
 
Greenmachine225 said:
The main reason is because I would like to buy a rear suspension kit but I can only use it with the 8 3/4, plus the slant makes a decent amount of power but with the tiny rear end it just can't put the power down.
Click to expand...
OK, too much missing info to help You effectively, "a rear suspension kit" tells us nothing that we can use other than the kit apparently has 3" axle tube clamps instead of 2.5".
Putting the power down means having enough traction, IS that the issue? If so SureGrip anything will fix that for most slantys w/decent rubber, and I'm guessing it's an open
diff. at present? Which bolt pattern do you have,4" or 4.5", and do you plan on keeping it that way? Keeping the same rollers & what are they(size/type)? As far as slow out of
the hole goes, a better torque converter is the cure, not gears.....assuming an auto. I tried to make out kickdown rod or dipstick, don't see a clutch rod/bellcrank either, so I'm
going w/auto. Anyways, welcome to FABO! That is a awesome looking greenmachine indeed!
 
rumblefish360 said:
Poor & pathetic notation. This statement is misleading but true. Read more below.



(Insignificant, amounts that total squat.)



My favorite words above, "I can not tell you."
An insignificant amount.

You would NEVER EVER know the difference when driving it.
You will NEVER EVER know the difference in your wallet thickness when paying for gas.
The drag strip will show the difference more so in driver error.
The dyno is the only place where you can (sadly and pathetically) point the finger and claim (like a brat teenager) that your right.

GREENMACHINE

The 8-3/4 rear can be swapped in easy.
You will need to find an "A" body 8-3/4. The expense of cutting down another rear is crazy and stupid to do unless your doing a custom or drag racing.

The rear will also need the normal 3 inch U bolts and shock plates. The 7-1/4 uses a odd smaller 2-1/2.

The big expense after the rear end purchase is shortening the driveshaft to fit.
(Or fixing the worn out brakes.)
The 8-3/4's nose, where the driveshaft bolts to is slightly more forward or longer. The driveshaft needs to be cut down to fit.

As mentioned before, a 8-1/4 is MUCH cheaper and still very strong. But not available with the small wheel pattern.

Excellent gas mileage? Look for a 2.76 ratio, about the most common ratio there is. A 3.23 & 3.55 offer somewhat of street performance. But they drive up the RPM's and will consume more fuel.

I did indeed notice your engine. Can you post up some specs on the engine? I would like to specifically know about your cam specs. The duration @ .050 would be a BIG/HUGE help in suggesting a rear gear ratio.

I also get being 16. You have a lack of funds to do things much less do them twice! Frack that!

(Edited, added a ",".
Click to expand...
Unfortunately I didn't build this myself and I have limited information on the engine specs but I do have this link, Mopar 225 Cam & Kit 268 Mechanical this is the set up that the guy told me is in it, it was converted to big bolt pattern, and I believe what I said, "it can't put the power down", isnt the right thing to say which is what some people are confused by it but what I mean is that if I give it full throttle from a stop it is obnoxiously slow and think this is due to the rear end which is why I am looking to swap it. Please correct me if I'm wrong on any of this because I'm still learning.
 
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