[music5150]

I'm new to Mopar and don't know if the size says it all or is there a million configurations of 8 3/4 rears. I would hope this means it is Limited slip or Posi, but I have a feeling it is alot more complicated than that.
That being the case how can I tell if it is a posi or limited slip? Mopar Rookie!

[christer]

Maybe it is a good start to determine which case you got. It is either a 741, 742 or a 489 case. Those are the last three digits of the part number. You will find the part number thanks to the pic in this thread.

http://challenger.mpoli.fi/forum/viewto ... 0119#10119

[dave-r]

Originally (1970/71) there were three types of axle used in Challengers.

The low performance cars used a 7.25 inch geared axle fitted with either 2.76:1 or 3.23:1 ratios.

High performance automatics or V8s with manual transmission used a 8.75 inch unit with optional "SureGrip" limited slip gears ranging from 2.76:1 all the way to 3.91:1. Most common were the 3.23 and 3.55:1 gears. The T/A used a 3.55:1 "SureGrip" as standard with an option on a 3.91:1.

Hemi cars fitted with a manual gearbox came with the huge 9.75 inch Dana 60 "SureGrip" axle as standard. Two gear ratios were available: 3.54:1 and 4.10:1. The Dana 60 was an option on the Hemi automatic and 440 powered cars.

From 1972-74 the 7.25 and 9.75 inch rear axles were both dropped. A 3.23:1 ratio 8.75 inch axle was standard on the Rallye with a 3.55:1 "sure grip" limited slip unit optional.

Now if you are wondering if your car has a Suregrip or not then there is a way of testing to see.
On the whole the 340/360 and big block R/T Challengers mostly came with Suregrip. The non-R/T models mostly did not.

Here is how to tell. Jack the rear axle up so that both rear wheels are off the ground with the trans in neutral and the brake off.
Now turn one rear wheel by hand.
If the wheel on the other side turns in the same direction you have a Suregrip limited slip diff.
If the other wheel goes in the opposite direction you have an open diff with no limited slip.

If it does something between those two things you have a damaged sure grip.

For axle gear ratios you need an accurate (not the factory) rpm gauge (tach). You need to tell me what size your rear tyres are and what speed you are doing at 3000rpm or higher. Assuming your speedo gear drive is the correct one I can tell you what gears are in your axle.

If you do have a suregrip you can also count the number of times you have to turn the wheel to get exactly one turn of the driveshaft.

[dave-r]

Well? What did you find?