I tore apart a set of LSD differentials I had laying around this weekend. It appears that I have two gear ratios, since the ring gears are significantly different. One has much larger teeth on it. I am assuming that there is a GSL and a GSLSE ratio.

My question is, which is better for a 12A car? A 3.933 or a 4.076?

Correct me if I am wrong, but the 4.076 will have a faster acceleration but a lower top end.

I plan to keep the engine a 12A in this car but may put a streetport into the car in the next few years.

 

4.0, it really would not make much of a difference.

 

agreed. i've run both rears with 12As and 13Bs.

 

Tooth size is not a direct indicator of gear ratio.

There were fine and coarse tooth gearsets used during the 12A 1st gen's production run,well before the GSL-SE came out and offered the lower 4.07 ratio in 84.
The only sure way to confirm the gear ratio is to count the teeth on the ring and the pinion.Divide those numbers and youll have your ratios.The fine teeth are quieter and actually stronger since there are more teeth meshing at any given time,which provides more contact area to transmit torque and spread the load.

That said,there is only a small noticable difference between the two.The 4.07 is a better choice if your gonna run wheels/tires that are bigger than stock,as it will help get you back down to the factory final ratio.
If your gonna retain the 13" wheels and are planning to drop tire profile for handling,then the 4.07 might get you too low, and youll be constantly upshifting at lower speeds and wearing things out excessively.

 

I thought by no explaination except my own assumption that the larger teeth held up better.

When I used to beat the crap out my car, rev her to 8K and dump the clutch, after getting the drag radials real sticky from a nice burnout (track or street) It seemed I busted up SE rearends quicker....

 

Okay... well I have two diffs sitting apart, and it now is just a matter of choosing the better of the two. I was going to leave tires/rims alone, so the quiet/stronger small gears seem like the way to go.

 

Its all about surface area. The more that is contact the more tq can be transmitted. Think pushing a broken 7 into a garage by yourself with on hand, kinda difficult, thats because you are transmitting all of your tq through one point of contact thus risk denting your body work. Now put 2 hands, the load is transmitted through 2 points of contact and is more evenly distributed.... you get the point.

 

Thats probably because the LSD gave up the ghost.