Well the I finally got my rear for the car.

[rebelgtp]

Tonight the new rear end for the Cutlass was dropped off. I was going to have to go pick it up this weekend but the guy was coming to town for a basketball game anyway.

Its a GM 12 bolt unit from a '68 Buick Skylark, non posi unit and not 100% sure on what gear ratio it is and I'm having a hard time finding the code on the axle tube (thats where it should be right?). The codes I'm finding on the axle tube are LE then a 11 U/K. On the pumpkin is 3917124N. Granted whatever it is I will more than likely gut it and swap out for all new gear and a posi.

 

[username]

Spin the hub one rev and denote the revs on the input. Ta-DA the gear ratio.

 

[axisg]

hmm just wondering. Why did you buy that rear ? Do you have an A body to put it in? It won't fit in a gbody without some work.

 

[rebelgtp]

It is a 60 inch wide 12 bolt axle, it will fit with conversion arms and changing the back spacing on the rear wheels or adding a bit of fender roll for good measure. Oh possibly need to relocate the spring perches I have read kind of mixed info on that subject.

Found the thread where I talked this conversion over with Shotgun, he runs a '68 12 bolt rear in his car.

viewtopic.php?f=14&t=14947

 

[DRIVEN]

They typically had taller gear ratios due to the higher torque (in general) made by BOP engines. My dad's El Camino has a 2.56 posi from a '65 Lemans. I know that the BOP 10 bolts used a Ford style axle retainer instead of c-clips. Are the 12 bolts the same way?

 

[rebelgtp]

Not sure if they normally are though I figured out (I think) that this rear is a 2.73.

That should offer a better top end and mileage but take away from outright acceleration correct? Gearing always screws with my head.

The car will have a built 355 and a 4 spd transmission.

 

[DRIVEN]

Pretty much. Depends on which 4-speed you use. I ran 2.79s with 26" tires on my '35 Ford. It had a strong 283 and a Saginaw with 3.50 first gear (Vega?) but had a pretty big jump between 3-4. It was great off the line and got better than 20mpg but not really the optimal combo. If you run a T-10 or Muncie with that gear, get the widest ratio you can find. Shorter tires will help too. My El Camino was a 327, M20, 3.08 10 bolt. It was fine as long as I wasn't towing or hauling a heavy load. After I swapped in the 350 it got a little better. The horsepower was probably similar but the torque was noticeably better. If the engine you build is pretty torquey you won't hate it as much.

 

[rebelgtp]

Yeah that is kinda what I figured and the mileage benefit would be nice. Obviously if I was going to change the gearing it would be optimal to do it now before it goes in the car.

The transmission is a Saginaw I believe from a Camaro, I will pick it up tomorrow so I can run the numbers.

 

[rebelgtp]

Just found this http://www.f-body.org/gears/ nice little calculator for "theoretical" speeds based off tire size, transmission gearing and rear gearing. That has helped me kind of wrap my head around the gearing and how it effects things by changing the gears around.