2 part 742 Question re Ratio & Suregrip

[Divenut]

Hi Folks,

So I was looking at my rearend today (my car's not my fat a$$) and have a couple questions that I am hoping FABO could help with.

727 TF/manual valvebody
742
Open Diff
28" tires

1. The PO said he thought the ratio was 3:23. But while driving the car it seems like the RPMs are high when cruising. Tachs not connected so I am going purely on gut/experience. I jacked it up, marked the drive shaft and tire. Getting a hair short of 1 full driveshaft revolution for every 4 tire rotations. Would that indicate something more in the line of a 3:92 ish ratio?

2. Being and open spool, traction is non-existent when hitting it in first. And breaks loose when shifting into 2nd if I wind it up. That said, short of picking up a second chunk (suregrip equipped) what is involved it setting up the chunk with a Sure-grip or aftermarket limited slip/posi? Also (guess it is really now a 3 part question :dontknow is this something the average shade tree mechanic can do, i.e. bolt on some parts and go, or would one suggest gettiing "professional" help. I am fairly handy with the wrenches, but know one can screw up a diff if not done correctly.

Sorry for the laundry list of questions guys & girls. I'm heading back into the garage to sort out my butchered wiring...another story LOL. Look forward to your advise, suggestions and tips.

Pat

And Happy St Pat's Day.

 

[cudamark]

It should be the other way around....one turn of the wheel and just short of 4 revolutions of the driveshaft for a 3.91 ratio and a sure grip. If you're just jacking up one side on a non-sure grip, count the driveshaft revolutions and double it. You can add a sure grip unit to your existing gears but if you don't like that ratio, why not get a complete sure grip 3rd member with the gears you want? It's not a real difficult job to install a suregrip to your gears but it does have to be done right. I think I remember there being a few threads on this subject so you might want to run a search.

 

[Divenut]

It should be the other way around....one turn of the wheel and just short of 4 revolutions of the driveshaft for a 3.91 ratio and a sure grip. If you're just jacking up one side on a non-sure grip, count the driveshaft revolutions and double it. You can add a sure grip unit to your existing gears but if you don't like that ratio, why not get a complete sure grip 3rd member with the gears you want? It's not a real difficult job to install a suregrip to your gears but it does have to be done right. I think I remember there being a few threads on this subject so you might want to run a search.

Thanks for the reply cudamark. I had both wheels off the ground (being an open diff they spun in opposite directions.), and triple checked. Got 1 turn on the driveshaft for every 4 turns of the wheel. :scratch:

The ratio seems OK for me, just trying to determine exactly what the ratio is. Will ultimately come down to $$$ and difficulty level when tackling the Suregrip, i.e. swap used Suregrip 3rd member vs buying and installing an aftermark "posi" unit.

Have a good one,
Pat

 

[cudamark]

If you don't have a sure grip, you'll need to lock one axle from turning and do your count again otherwise it won't be accurate.

 

[Divenut]

If you don't have a sure grip, you'll need to lock one axle from turning and do your count again otherwise it won't be accurate.

You rock cudamark. Jacked up 1 side, driveshaft turned 3 1/4 times for 2 (Another source claims that was the way to do it) revolutions of the wheel. So guessing it is a 3:23 as the PO thought.

Thanks again,
Pat