7 3/4" and 8 1/4" rear weaknesses? (and other ?'s)
The 7.25" rear is junk , even behind a 170 Slant Six / 904 combo , with an old lady driving .
Both cars I've driven with that stupid waste of metal were :
1. ) 1972 Swinger 318/904 . I bought it when it was 17 years old ; and , its prior (original) owner kept-up with all services (showed me receipts and everything) . I had the car for less than a year and that 7.25" growled , howled and protested .
I swapped it for an 8.25" junk yard rear (3.21 gear w/ Sure Grip) from a '74 Duster . I did nothing except change its fluid and install it . Over 10 years and engine upgrades , that 8.25" held up perfectly (again , with servicing it every other year).
2.) 1967 Dart GT 225/904 . This was a $500 special my friend bought in 1996 . The car wasn't perfect , but it was presentable and its miles were low and original (35,000 if I recall) . It was used as a "lead-in" car for a classic car lot .
The 7 mile home drive after purchasing it revealed a grumbling , growling , howling 7.25" .
Next day , a trip to the pick-a-part found us an 8.25" out of a '75 or '76 Dart sedan . Ratio was 2.94 , open .
We also ganked its driveshaft (no rust , dents , nor anything obviously wrong with it).
Took 'er home , filled 'er up with 75w-90 synthetic (he's not as vigilant about servicing his vehicles as I am) , spanked its on its butt and drove away .
I'd say that the 8.25" is a good diff which got a "bad" name when it replaced the coveted 8 3/4 in '73 'A' Bodies (though the 8 3/4 survived through '74 in 'B' , 'C' and 'E' cars and '76 in trucks ).
Its advantages are :
- Lighter weight
- Less "reciprocating" weight = *potential* better fuel economy
- Parts are on-the-shelf at auto parts stores
- 5x4.5" bolt circle wheel pattern
- Dozens of 'em in the 'yards
- Can handle decent amounts of torque and h.p.
( e.g. , a warmed 225 , 318 or mild 360 )
The 9.25" is an oft-overlooked alternative . They're prevalent in '74 & later 'B' , 'C' , 'M' and 'R' bodies , and numerous pickups and vans
( albeit , with a larger 5x5" bolt circle or larger in trucks and vans; custom or redrilled axles needed).
The advantages of the 9.25" are similar to the 8.25" .