Rear end swap help!

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cinimod2

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So i bought a dart sport that was converted over to a 318 with 904 tourqe flite trans from a /6. I was wondering if i need to change the rear end or gears in it? I think its a 7.25 rear end. If i want better performance than should i swap out the 7.25 for a 8.75 rear end? Thanks Dom
 
Since the 8-3/4 weighs quite a bit more it'd actually be slower than with a 7-1/4, given the same gear ratio. But it'll hold together much better than the 7-1/4. If it were me I'd look for a 8-1/4 to put in it. They are plenty strong to stand up to a 318 and much cheaper than an 8-3/4 and some lighter. Don't forget to change from a 7-1/4 to either the 8-1/4 or 8-3/4 you'll have to also change the driveshaft and shock plates.
 
fishy68 said:
Since the 8-3/4 weighs quite a bit more it'd actually be slower than with a 7-1/4, given the same gear ratio. But it'll hold together much better than the 7-1/4. If it were me I'd look for a 8-1/4 to put in it. They are plenty strong to stand up to a 318 and much cheaper than an 8-3/4 and some lighter. Don't forget to change from a 7-1/4 to either the 8-1/4 or 8-3/4 you'll have to also change the driveshaft and shock plates.
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EXCELLENT advice, I think....
 
What kind of gears do think i should go with considering i want performance but will probably doing highway driving too. I looked at 3:91 gears but people say highway sucks becuase the engines roaing at : at 3000 rpms. Any help would be appreciate thanks dom. P.s. sorry im on a phone trying to type this
 
Tire size will play a part in this decision. 3.55's are usually considered the most gear you'd want for freeway driving but if you have real short tires, you may be better off with 3.23's.
 
Now can I use any 8.25 rear end or does it have to be one specifically from 60s and 70s? Tanks Dom
 
You can use any rearend that you can make fit under the car but only ones used on A-body's will bolt right in. They were used in B and E body's also but they are too wide.

To answer your question of what gears you should run you need to post your tire size because as Mark mentioned tire size greatly affects what rpm the engine spins. I've seen cars with 3:23 gears spin more rpm at 60 mph than cars with 3:73 gears cause the 3:73 geared car had much taller tires.
 
Im not sure what the tire size is id have to check tomorrow. But i do know that they are stock tire size for a dart sport if that helps at all.
 
Stock tires are usually pretty short so a 3:21 gear should do fine for you. Get the size tomorrow and post it and that'll give us an exact idea.
 
cinimod2 said:
They are p215/70r14 tires
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those are pretty short tires (25.85" tall) so a 3.21 gear will do good and still be decent down the hwy. At 60 you'll be buzzing approx. 2600 rpm. If you want a little more performance you could step up to a 3.55 gear. With a 3.55 gear at 60 mph you'd be buzzing approx. 2900 rpm. That's getting up there pretty high for hwy. cruising but livable. Do you have a tach in the car now? If so you could take it for a spin and kick it up to 2900 rpm and see if you would like cruising at that rpm. If it has real tall gears right not you may need to run it in 2nd gear for the test or you may be running 90 before you hit 2900 rpm... LOL
 
cinimod2 said:
So i bought a dart sport that was converted over to a 318 with 904 tourqe flite trans from a /6. I was wondering if i need to change the rear end or gears in it? I think its a 7.25 rear end. If i want better performance than should i swap out the 7.25 for a 8.75 rear end? Thanks Dom
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is your car small 4 or large 4 1/2 bolt pattern wheels. some 6 cyl. 73 cars were small bolt pattern.
 
Oldmanmopar said:
is your car small 4 or large 4 1/2 bolt pattern wheels. some 6 cyl. 73 cars were small bolt pattern.
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Actually, to set the record straight, all slant 6 cars were sbp except for the '73 to '76 a bodies that had power brakes, and then they had automatically the bbp disc brakes and the 7 1/4'' bbp rear ends minimum. Some did come with the bbp 8 1/4'' rear diffs and pre '73s some slant six cars came factory with 8 3/4'' sbp. It all depended on availability at the time the cars were built as well as the options on it such as trailer towing etc. They are not common, but they are out there.
All 8 3/4'' a- body rear diffs were sbp regardless of the year.
 
One other thing to consider is u-joint size. Some a-bodies used 7260 joints; others used 7290's. But if you do wind up with different size joint, a conversion joint is available at NAPA or any local driveshaft shop.
 
I run 3.55's with the same tire you have and I turn around 2800 at 55 MPH. I dropped an early B body 8 3/4 in my Dart Sport, it wasn't all that much work. Had to move the perches in 1/2" on each side, and get different U bolts and shock plates.
 
Badsport said:
I run 3.55's with the same tire you have and I turn around 2800 at 55 MPH. I dropped an early B body 8 3/4 in my Dart Sport, it wasn't all that much work. Had to move the perches in 1/2" on each side, and get different U bolts and shock plates.
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Me too; a piece of cake.... lots cheaper than buying an overpriced A-Body rear....:blob:
 
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