New Rear Diff, need recommendations...

[7milesout]

Guys - I have a perfectly fine open diff at the moment, but will be swapping it out with a Detroit / Eaton TrueTrac. I wanted to get your advice and recommendations on a couple things.

• Should I try to do this diff replacement myself?

I'm pretty decent with mechanical work. Pulling a diff is not something I've done before, but most everything I do, I've not done before. But I figure it out, take my time and usually do a good job. But on the diff, I've heard stories of guys not shimming them right (or something like that) and it screws the diff up in short order. If it's straight forward enough, I think I would like to do it myself, simply to learn. However, I have a guy I trust that does really good work himself. He is a mechanic and has a commercial shop, with a lift and all the right tools. He says he will do it for $150 labor. He also said he would let me help him on a Saturday (so I can learn). I'm thinking that's the way to go. Advice?​

• What gear ratio should I go back with?

The car has a 3 speed (727), 14" wheels/tires, and currently 3.55 gears. It is not loving the interstate. 3,000 rpm at 60 mph. I'm thinking about going back with the same 3.55 gears as well. Because my goal is to be able to cross the quarter mile in 13.99 seconds. With 3.55 gears, I would be trapping about 96 - 97 mph and turning about 4,800 - 4,850 rpm to meet that goal. That sounds like perfect rpms. With a 3.23, it would be turning about 4,400 rpm at 96 - 97 mph. I put priority on the 13.99 second quarter mile E.T., because ... I'm not planning on taking the Scamp on vacation! It won't see interstate runs very often. Not only that, but in the future, I may replace the 727 with a 4 speed. And then, it would be good to go for both the strip and the interstate. But I wanted to know your opinions. Do you guys think I could still hit a 13.99 with 3.23 gears?
​

7milesout

 

[HTMLmopars]

Me and my buddy spent about 4 days swapping gears on his '66 C-10. It took about a dozen trips to Randy's Ring and Pinion (fortunately their distribution center is only 15 min away), and four big guys to torque the crush sleeve, but we got it done and it hasn't failed after a summer of driving. Given that we're 19, and have only been wrenching for a few years, you can probably do it.

 

[72bluNblu]

727 to 4 speed isn't going to change much if anything for the cruising speed. I mean, you lose the TC and maybe a little drivetrain loss but the 4th gear ratio is still 1.00, just like 3rd in the 727. You might pick up a couple miles per hour, but that's about it. You must have some pretty short tires in the back? Mine are 26" and I still run 65mph or so at 3k. And 3k isn't all that bad really. I usually blast the freeway closer to 3,500 and about 75mph, around here less than 70 gets you run over most of the time. I wouldn't want to do that for 6 hours, but I do it for 20-30 minutes during my commute. Depends on your engine too, with the cam I run I wouldn't want to cruise at less than 2,500 anyway. I liked 3.23's better on the freeway for sure, but you probably wouldn't love them at the track.

As far as the diff swap, I think taking it to the guy that will let you help and learn for $150 is the best money spent. You can do it yourself for sure, but if you don't have all the right tools, and you haven't done it before, $150 for the right tools and piece of mind seems like the way to go.

 

[7milesout]

A 4 speed transmission (at least in this case) would include a 0.67 OD. So yes, the rpm would drop. 3rd gear would remain 1:1 as it is now.

 

[72bluNblu]

A 4 speed transmission (at least in this case) would include a 0.67 OD. So yes, the rpm would drop. 3rd gear would remain 1:1 as it is now.

That’s a 4 speed overdrive transmission then. The standard 833 is like every other non-overdrive 4 speed out there, 1.00 in 4th.