8 3/4 third member case types?

[wildrob72cuda]

Help me out
I was under the understanding that 489 and 742 were the only ones that were equiped with the suregrips. I have seen advertizements for aftermerket lockers that will fit the 741 as well. {i may have the 741 and 742 confused}.... Is this correct or do they have them confused as well? Thanks

[DartGT66]

the sure grips are interchangeable between all tyhe different 8 3/4" cases (there are more numbers than just those three, but those are the main styles).

[wildrob72cuda]

What about gears, are they allso interchangeable or do they have to stay with the right case.{ as for as i can tell they all look the same } The reason i ask is that I currently have a 742 with a suregrip and 4.11s The car went a 12.20et @ 112 mph and crossed the line on the rev limiter @ 6400. This is not where i wanted to be. I have a extra 741 third member with 3.20s and a open dif.and want to try the 3.20s in the 742 wth the suregrip to see how much rpms will drop before ordering a new set of gears. I just dont know enouph about 8 3/4s , will this swap- work?

[DartGT66]

No, the gears are not interchangeable between the three different styles of cases.

[dewme5]

if you want to try the other gears, you are going to have to keep the 3.23's in the 741 case, and put the sure-grip in. Which means breaking down your 742 case.

on the other hand. you can just do the math.
6400/4.1=1561*3.23=5042. with just the mph being the same, you would drop just over 1500 rpm. Not to mention a lot more streetable. Howerver, 3.23's are not track gears. if you had the torque to run 135 mph, but you couldn't spin the motor past 6200 for some reason, then you could look into 3.23's.

if you give an overall idea of what you want the car to do. mph, speed thru the traps, rev limit, tire size, and how much track time vs street time. There are a few people on here with enough experience to find just the right size of gears for you without all the trouble of just trying some out.

[littlecampbell]

autohobbydigest.com good info on the 8 3/4.

[b-1ken]

As DEWME says, it's easier to do the simple math. If you really wanted to try the gears anyway, just run the 741 as is. You really don't HAVE to run the Sure-Grip. I ran a standard differential rear in my low 11 sec '67 'Cuda many years ago and it worked O.K. until I blew the diff. gears. This was a 440 with a 9" converter. The rear gear was a 4.56.