8 3/4" Questions

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roosduster

Colorado Mopar
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I have a 73 Duster and it has an 8 and 1/4" read end under it, stock except for a Posi unit I had installed. It has the 5 x 4.5" pattern with the wide drum brakes. I really want to replace it with a 8 and 3/4" rear end. Out of what body (A, B, C or E) would I be able to find the rear end that is the easiest to swap into my Duster? Would I be able to keep my same drum brakes? Would I need to by a new parking brake line or will I be able to keep the one I have now? Also is there any differences between 742 and 489 cases? Is one better then the other? I would want to have the 8 and 3/4" rebuilt, what companies would anyone recommend on rebuilding it or that sell axles, ring and pinion gears, etc, for it? I am looking at getting a Detriot TrueTrac Posi Unit, there was a magazine article on it recently where they tried to break it. They were not able to and I will not be doing what they were doing to it. Are their any opinions on that posi unit or any others? Does anyone have any opinion for gear ratios for a car that will mainly be a street car (some highway driving), but that I will race it sometimes? I also plan on getting a 4 speed automatic for my Duster as well.

Thanks for any help you can provide.

Seth
 
Any a-body 8 /34 rear will fit however you will need new axles for the big bolt pattern... what motor you runnin? i'm in the low 13's with my 360 in my cuda, and have had NO problems with my 8 1/4 yet... ( 10 years)
 
DJVCUDA said:
Any a-body 8 /34 rear will fit however you will need new axles for the big bolt pattern... what motor you runnin? i'm in the low 13's with my 360 in my cuda, and have had NO problems with my 8 1/4 yet... ( 10 years)
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The motor i am running is the 360/380 Mopar Crate Engine. Is your 8 and a 1/4" stock? What gear ratio is the stock ratio for a 8 and 1/4"? I don't know what I have and am curious since I would assume they would be pretty HIGH gears except my engine is spinning 3,000RPM at 65MPH. If I kept the the rear end i have now, which is a real possibility, what companies offer ring and pinion sets for the 8 and 1/4" rear end?
 
I have never dealt with them but everybody seems to talk very highly of Randy's Ring and Pinion. Good prices I hear too.

As far as your gear ratio alot depends on how tall your tire is. If your running 3000 at 65 mph it sounds like 3:55 gears (figuring a 27" tall tire) but I never heard of anything newer than 71 coming with gears that low. Most of them came with very tall gears like 2.76's or the like.

If your going with a 4 speed auto overdrive you can run even lower gears for better acceleration. 4:11's would be great. Smokin takeoff and when it shifts into OD your crusin at a decent rpm.

Have you done any measurements to see if the OD unit will clear the trans. tunnel yet. I haven't done one but read on other boards that the OD unit is so big it hits the tunnel so you have to enlarge it some.
 
fishy68 said:
I have never dealt with them but everybody seems to talk very highly of Randy's Ring and Pinion. Good prices I hear too.

As far as your gear ratio alot depends on how tall your tire is. If your running 3000 at 65 mph it sounds like 3:55 gears (figuring a 27" tall tire) but I never heard of anything newer than 71 coming with gears that low. Most of them came with very tall gears like 2.76's or the like.

If your going with a 4 speed auto overdrive you can run even lower gears for better acceleration. 4:11's would be great. Smokin takeoff and when it shifts into OD your crusin at a decent rpm.

Have you done any measurements to see if the OD unit will clear the trans. tunnel yet. I haven't done one but read on other boards that the OD unit is so big it hits the tunnel so you have to enlarge it some.
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My tire is 245/60/15 which makes it 26.57 inches tall, so basically 27 inches. I would be amazed if I had 3.55 gears as well, especially with only a 318 originally. The company I contacted about the 4 speed auto overdrive said I would have to make some adjustments to the transmission tunnel to fit. I have not made any measurements though. I am trying to decide what ratio to get either 3.55, 3.91 or 4.11. I want that great acceleration, but also a decent rpm when cruising.
 
I have a friend with a 67 Cuda with a stout 360 6-pak and an A518 overdrive auto driving a 8.25" with 4.1x gears. Not sure on the tire but they are pretty tall. I believe he gets about 18mpg on the hiway with that set up so it's got to be turning pretty low rpm's when in OD.

Anyway, the tunnel needs to be split down the center from several inches in front of the torsion bar cross member to several inches past the cross member. The cross member needs to be cut too. Everything was opened up a couple of inches and a new piece of cross member was welded in along with etra sheet metal. FWIW, he was able to install a factory console and use the factory shifter. You can't tell when the interior is in that this work was done.

I am running a 360 through a A833OD overdrive manual and a 3.55 geared 8.25 in my 68 Barracuda. I have 235/60-14 tires on the back and at 70mph on the hiway it's truning about 2000 rpm's.

BTW, the 8.25 that I have in my Cuda orignally had 2.94 gears with a sure-grip.
 
dgc333 said:
I have a friend with a 67 Cuda with a stout 360 6-pak and an A518 overdrive auto driving a 8.25" with 4.1x gears. Not sure on the tire but they are pretty tall. I believe he gets about 18mpg on the hiway with that set up so it's got to be turning pretty low rpm's when in OD.

Anyway, the tunnel needs to be split down the center from several inches in front of the torsion bar cross member to several inches past the cross member. The cross member needs to be cut too. Everything was opened up a couple of inches and a new piece of cross member was welded in along with etra sheet metal. FWIW, he was able to install a factory console and use the factory shifter. You can't tell when the interior is in that this work was done.

I am running a 360 through a A833OD overdrive manual and a 3.55 geared 8.25 in my 68 Barracuda. I have 235/60-14 tires on the back and at 70mph on the hiway it's truning about 2000 rpm's.

BTW, the 8.25 that I have in my Cuda orignally had 2.94 gears with a sure-grip.
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Thanks for the information. I get about 15 on the highway with the 360 i have now, the original 904 and rear end, so I think I could installer lower gears and the 4 speed and probably get better gas mileage as well as performance. Do you know how hard was it to split the transmission tunnel? What kind of shop would be able to do this or would you recommend doing this? I do not have the equipment or space to do this.

Thanks
 
My friend did all that work himself. A sawsall is all that is need to to splte the tunnel. I did not see him do it but I am quite sure a BFH will work to move the metal. The hard part would be whether you have a welder and welding skills.

If you brought this to a body shop that is willing to do this kind of work I would plan on paying for 20-30 hours of labor.
 
dgc333 said:
My friend did all that work himself. A sawsall is all that is need to to splte the tunnel. I did not see him do it but I am quite sure a BFH will work to move the metal. The hard part would be whether you have a welder and welding skills.

If you brought this to a body shop that is willing to do this kind of work I would plan on paying for 20-30 hours of labor.
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Thanks for that info, I guess if I have to take it to a body shop it won't happen for awhile. Until I can get the money but this info is good to know.
 
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