Hello all I was wondering could someone tell me what the gear ratio's were for a posi that came in a 1972 electra, or any of the full size cars? Also I would like to know does anyone make a disc brake conversion for these rear ends, and if so for this particular rearend would be losing or gaining stopping/exceleration power. I would think I would be gaining stopping power with the conversion, but I would like it to know has someone tried this, and if it would be a option that should or shouldn't be done. I just feel I would like to have disc brakes all the way around on my 72 limited, but I would like it to have the posi also. Please feel free to give any input or insight on this. Thanks all:beers2:
The axle in your car should be a 9 3/8. Available ratios: 2.93, 3.23, 3.42 2.56 and 2.72´s were found in some 9 3/8 Olds rears. I never saw a disc brake conversion kit for this. Why do you think that you need this. You won´t gain stopping power. I don´t know how well your drums work. But I can get my rear wheels to lock at nearly every speed. And a wheel can´t do more than stop. The biggest advantage of disc brakes is that they have less fading when your driving hard or going down a long mountain road or something like that. Maybe you can use the conversion kit made for full size chevy´s.
You will know the gear ratio when you took off the rear cover. Maybe there were different tags on the covers when the car was new. But in most cases they had been removed. I don´t think there is another way to tell the ratio without looking inside.
You can sometimes get an approximation by marking the drive shaft and tire. Turn the tire one revolution and count how many revolutions the drive shaft makes. You can also approximate it by putting a tachometer on the engine and seeing what the RPM is at 60 MPH. There should also be a 2 or 3 letter code on one of the axle tubes.
If the car is not for racing or severe driving...no need to swap to rear discs....but if you are determined to do it....visit www.scarebird.com and he may have something for you.
You can sometimes get an approximation by marking the drive shaft and tire. Turn the tire one revolution and count how many revolutions the drive shaft makes. You can also approximate it by putting a tachometer on the engine and seeing what the RPM is at 60 MPH. There should also be a 2 or 3 letter code on one of the axle tubes. i think that's a bit confusing Larry. it also appears to conflate the pegleg and posi diff methods. first, block the front tires so the car doesn't move around. 2nd, put the trans in Neutral. if you have an open rear end, you need to jack up one rear tire and divide the driveshaft rotations by two for one rotation of the tire. if you have a posi rear end you need to jack up both rear tires ( jack under the pumpkin and put stands under the rocker panels ) and the driveshaft rotation count IS your ratio. you can rotate the tire extra turns to get a more accurate number for either type of rear.
Re: 72 electra posi and discs If you're on a budget and mechanically inclined, you can also do the conversion using salvaged Cadillac parts. On my '71 I used '78 Cad Seville rear disc caliper mounting brackets and rotors. The brackets require the their original mounting holes be welded shut and new mounting holes to be drilled to match the axle flange. I then used calipers that are identical to the original fronts, so have the same pads all the way around. These calipers are common and inexpensive, also being used on 70's GM 1/2-ton pickup fronts. Disadvantage is that I have no p-brake. I used the master cylinder and prop valve from a '71-'76 Cad Eldo that had rear discs. Direct bolt on.
Years ago I had a 70 convertible Electra, the stock 2:78 rear died and I found a 68 Riviera GS in the junk yard, all Rivi GS'S came with a 3.42 posi rear, I spent 2 weekends busting out the rusty rear end to get it, got it home and found out a previous owner had swapped in 3.07 gears that were standard in the older Rivieras, DANG IT! Went ahead and installed the 3.07 posi, it really helped acceleration but, 2 weeks later a guy stops me outside a parts store and asks me if I need any parts as he has been collecting parts for big Buicks for 10 years+, I mention, no I just replaced the rear, he says, Oh, I have a couple 3.42 posi's and one a 3.92 rear, he doesn't really want the 3.92 cuz it revs the motor too high, just what I was looking for! But too late, I didn't have enough to buy that rear now that I had spent all my money already on the 3.07 swap. I hate when stuff comes my way a day late and a dollar short! Good luck on your endeavors, your stock drums are massive already, I don't think there is much to be gained by a disc swap. if you are really intent on spending money, check out Currie and see if they can make you up a custom 9" with discs, not that there is anything wrong with the Buick 9 3/8, just that there are more options available with a more common rear.
