318 cars have 8 1/4 diffs?

Transmissions and Rear Ends

  1. MBDale

    MBDale Well-Known Member

    Messages:
    284
    Likes Received:
    72
    Joined:
    Aug 20, 2015
    Location:
    Winnipeg Manitoba Canada
    My '79 R/T has the 318/904 combo. Will it have a 8 1/4 with 3:21 ratio? Or did the only 360 cars have the 3:21 raito? If the 318 cars came with 8 1/4 diffs then I'm going for 3:55 upgrade. Thanks!
     
  2. kkritsilas

    kkritsilas Well-Known Member

    Messages:
    1,754
    Likes Received:
    331
    Joined:
    Jun 21, 2012
    Location:
    Calgary, Alberta, Canada
    My 3 J bodies all have 8 1/4" differentials/rear ends. 2 are 318-4BBL/A999, 1 is a 318-2BBL/A998.
     
  3. efriedrich

    efriedrich Well-Known Member

    Messages:
    725
    Likes Received:
    40
    Joined:
    Mar 9, 2011
    Last edited: Sep 12, 2015
    Captain Caravelle likes this.
  4. MBDale

    MBDale Well-Known Member

    Messages:
    284
    Likes Received:
    72
    Joined:
    Aug 20, 2015
    Location:
    Winnipeg Manitoba Canada
    I'm thinking 70 B-body lines up. 3.55 would be good.
     
  5. Aspen500

    Aspen500 Well-Known Member

    Messages:
    4,301
    Likes Received:
    1247
    Joined:
    Feb 14, 2015
    Location:
    Rib Mountain (Wausau) WI
    I've never seen a V-8 F/M/J body with a 7 1/4", that's not to say there weren't any though. On the other side of it, my dad's S6/3-speed manual '74 Gold Duster has an 8 1/4" not the expected 7 1/4", go figure.
     
  6. MBDale

    MBDale Well-Known Member

    Messages:
    284
    Likes Received:
    72
    Joined:
    Aug 20, 2015
    Location:
    Winnipeg Manitoba Canada
    Ok, I'd be just as happy with a 8 1/4 as with a 8 3/4. I've got to check this thing when I get home. Then I'll just do a ratio change.
     
  7. kkritsilas

    kkritsilas Well-Known Member

    Messages:
    1,754
    Likes Received:
    331
    Joined:
    Jun 21, 2012
    Location:
    Calgary, Alberta, Canada
    You may want to get a Sure Grip in there as well if you are going in there.

    If you do a ratio change you will need to change the speedo gear on the tranmission, or your speedometer and odo will be off.
     
  8. Gator

    Gator Well-Known Member

    Messages:
    301
    Likes Received:
    26
    Joined:
    Aug 12, 2015
    Location:
    south jersey
    Does a 8 1/4 rear have a tag or numbers stamped on it to tell what gear ratio you have with out taking the cover off
     
  9. 79410aspenrt

    79410aspenrt Well-Known Member

    Messages:
    568
    Likes Received:
    113
    Joined:
    Jan 14, 2012
    Location:
    Saskatchewan, Canada
    if you have 3.21 gears now, don't waste your money on 3.55's. there will be no change in performance, just higher rpm's on highway.
     
  10. MiradaMegacab

    MiradaMegacab Well-Known Member

    Messages:
    1,564
    Likes Received:
    393
    Joined:
    Feb 27, 2011
    Location:
    Long Island NY
    To find gear ratio, jack the rear tires up.
    Place trans in neutral.
    If you have a Sure Grip, spin the axle one revolution while counting how many times the driveshaft spins.
    If the drive shaft spins roughly 3 and 1/4 turns, you have 3.21 gears.
    If you have an open differential, only jack up one wheel.
    Spin the axle two revolutions and count how many times the drive shaft turns.
     
    Captain Caravelle likes this.
  11. MBDale

    MBDale Well-Known Member

    Messages:
    284
    Likes Received:
    72
    Joined:
    Aug 20, 2015
    Location:
    Winnipeg Manitoba Canada
    Ahhh. Good call. I'm going to check it out, pull the cover off when I get home. 3.21 be good for the street too.Just add sure-grip.
     
  12. Superpac Ninja

    Superpac Ninja Well-Known Member

    Messages:
    932
    Likes Received:
    69
    Joined:
    Jan 11, 2012
    Location:
    Canada
    alot of 6 cyl only had the 71/4 rear no need for the beefier 81/4 and weight
     
  13. Superpac Ninja

    Superpac Ninja Well-Known Member

    Messages:
    932
    Likes Received:
    69
    Joined:
    Jan 11, 2012
    Location:
    Canada
    a better way to check if you have a sure grip is to just light up the back tires ,ha ha ha you got 1 black strip or 2 ?,,with sure grips you also get some chirping going around sharp corners ive found ,,the inner wheel rotation is slower than the outside tire rotation causing chirping sound
     
  14. Captain Caravelle

    Captain Caravelle Moderator--Mopar Maniac

    Messages:
    10,789
    Likes Received:
    1121
    Joined:
    Jan 29, 2012
    Location:
    Hogtown, Ontario
    Excellent!
     
  15. F body Deconstructor Jim

    F body Deconstructor Jim Well-Known Member

    Messages:
    521
    Likes Received:
    127
    Joined:
    Jun 19, 2011
    Location:
    Phoenix, AZ
    There should be a metal tag on one of the diff cover bolts with the gear ratio stamped into it.
    I've torn down slant six powered F's with 8-1/4 rears and V8 powered F's with 7-1/4's.
    I think all wagons and 4 speeds were 8-1/4's. At least all the ones I've had were. (4 wagons and 5 od 4 speeds)
     
  16. Jnfbodyguy83

    Jnfbodyguy83 Well-Known Member

    Messages:
    357
    Likes Received:
    19
    Joined:
    Feb 29, 2012
    Location:
    Tonwanda, ny
    anyone know the spline number? I was looking to get that mopar sure grip set up for my 8 1/4.
     
  17. 80mirada

    80mirada Well-Known Member

    Messages:
    1,026
    Likes Received:
    271
    Joined:
    Sep 3, 2012
    Location:
    Wisconsin, West Bend depends
    27 spline axles in 8.25/8.375 rears until 96.
     
  18. Jnfbodyguy83

    Jnfbodyguy83 Well-Known Member

    Messages:
    357
    Likes Received:
    19
    Joined:
    Feb 29, 2012
    Location:
    Tonwanda, ny
    Ok. Cool. What made them change it to 29 in 97?
     
  19. 80mirada

    80mirada Well-Known Member

    Messages:
    1,026
    Likes Received:
    271
    Joined:
    Sep 3, 2012
    Location:
    Wisconsin, West Bend depends
    Extra strength for rising power output
     
  20. 8v-of-fury

    8v-of-fury Well-Known Member

    Messages:
    390
    Likes Received:
    27
    Joined:
    May 7, 2014
    Location:
    Orillia, Ontario
    I think my 1988 318 5th has a 7 1/4? I've not verified it yet though..