best 12 bolt posi?

[naphtali5725]

I have a 10 bolt in my 86, and I also have a spare 12 bolt, and would like to have posi for my truck, would like opinions on best option either for the 10 bolt, or the 12

the 10 is 2.73:1
the 12 is 3.07:1
kind regards

[Pyrotechnic]

If all you want to do to either axle is stick a posi and some better gears in it, then the 12 bolt is the way to go because they have beefier axles than the 10 bolt of that year group. I recommend the Eaton clutch type posi units since you can rebuild them.

Now if you want to go to aftermarket axles, that levels the playing field between the two choices. The 10 bolt has a larger pinion shaft than the 12 bolt which gives it somewhat of a strength advantage, but most people don't have enough power or abuse their truck to make the smaller shaft of the truck 12 a weak point.

[naphtali5725]

Originally Posted by Pyrotechnic

If all you want to do to either axle is stick a posi and some better gears in it, then the 12 bolt is the way to go because they have beefier axles than the 10 bolt of that year group. I recommend the Eaton clutch type posi units since you can rebuild them.

Now if you want to go to aftermarket axles, that levels the playing field between the two choices. The 10 bolt has a larger pinion shaft than the 12 bolt which gives it somewhat of a strength advantage, but most people don't have enough power or abuse their truck to make the smaller shaft of the truck 12 a weak point.

thanks for the info!

[ont_guy_38]

Another thing to consider is the 10 bolt 8.5 truck and 8.5 car parts are all interchangeable which can make things easier if on a budget and looking for used parts. Camaro, Nova, Pontiac all had eaton posi. That is what i have in my rear end. Something else to consider is 12 bolts have a few carrier splits where as a 8.5 will use the same carrier from 2.73 up. A 8.5 will take quite a bit of crap. Most trucks do not really have the traction to worry too much about it anyways.

[naphtali5725]

the reason i'm asking about it, I am dropping in an Olds 455 with a crapload of torque, trying to make sure I know how to make one of them as strong as possible...

[Pyrotechnic]

Originally Posted by naphtali5725

the reason i'm asking about it, I am dropping in an Olds 455 with a crapload of torque, trying to make sure I know how to make one of them as strong as possible...

Do you plan to put sticky tires on it and run the 1/4 mile? Are you just messing around smoking the tires on the street?

Axle strength is generally my main concern. With C clip axle retention, which both of these rear ends use, if an axle breaks then it and the wheel/tire leaves the vehicle. Scary stuff.

Also consider gearing. The 3.08 of the 12 bolt with a 28"-29" tall tire would work great with the torque of the 455 and keep the RPM's low for cruising. It'll cut your cost to some if you don't have to buy new gears, assuming what you have is in good shape. With the 30 spline axles of the 12 bolt, this would be a strong combo for the street. That's about the strongest and most cost effective option, using what you have and retaining a mostly stock rear end.

If you can get used posi unit for the 10 bolt, rebuild it with new clutches (a must for any used posi), and get a set of better gears (2.73 sucks) for significantly cheaper than it would cost to install a brand new Eaton posi in the 12 bolt then it's something to consider. You would still be riding on weaker 28 spline axles though, also something to consider when comparing costs between your two options.

When it comes to going all out and building a brute strength rear the 10 bolt would be my choice, but that will take a lot more money and the rear end housing itself is really all that will be used. The rest is all aftermarket. I'll leave that for another post as I'm not sure how you will be using the truck and how much you want to spend.

[naphtali5725]

Pyro,
thanks for the info man, I will not be doing any drag racing whatsoever, and the truck will be a daily driver, so it will probably have BFG radial ta's or equivalent on it. but on the occasion when i want to get down on it hard, i don't want just one wheel burning the tire to a crisp or causing a loss of control of the truck.
the 12 looks to be in excellent shape so i think i'll go with it.

[68Timber]

If you're going to put a posi in the 12-bolt, consider swapping to 3.40 (3.42) gears. They cruise down the highway great and still pull good off the line. They're a good compromise between 3.73's and 3.08's.

[naphtali5725]

Originally Posted by BRUISER

If you're going to put a posi in the 12-bolt, consider swapping to 3.40 (3.42) gears. They cruise down the highway great and still pull good off the line. They're a good compromise between 3.73's and 3.08's.

the cam in the 455 is for 1500-5000 rpm and will require an 1800 stall converter, and it will be making approx 500 ft lbs of torque so i don't think i'll need anything lower than the 3.08 since it will be a highway cruiser etc. unless someone can explain otherwise because i'd like to do it once and done.

kind regards

[Pyrotechnic]

If you were running a small block then I'd say yes, 3.42's are the ideal gear given the stock tire size. I had 3.08's and 3.42's behind the same stock 350 and the difference was like night and day. I'd gladly trade the increase in cruise RPM for the better driving feel with that motor, even if it was all highway driving. The funny part was, the gas mileage was actually WORSE with the 3.08's because the motor just wasn't running in it's sweet spot.

Big blocks in the 450+ cubic inch range are cool because they have so much low end grunt from their sheer size. The 3.08's that would make a small block feel underpowered is no problem for a large motor, it'll just power through them and the driving experience is still nice. You still get to enjoy the low cruise RPM and better gas mileage as well.

Here's some numbers to put on the table to help you decide, with a 29" tire:

3.08 @ 65MPH = 2300 RPM
3.42 @ 65MPH = 2600 RPM

Now that's assuming a perfectly locked converter, so add about 100-200 RPM to account for converter slip. Yes, it will accelerate faster with a numerically higher gear, but in this case I'd reap the rewards of big block torque and go for lower cruising RPM's.

[68Timber]

I had a couple-hundred mile drive home after I bought my 79. It didn't have a tach, but the rpm's seemed pretty low on the hwy but it also pulled pretty dang hard off the line. I didn't know what it had until I had to pull the rear cover to figure out what carrier I had so I could order the correct posi and new gears, so that's when I found out I had factory 3.40 gears and a G80. Had to suggest, they impressed me when I found out what I'd been running.

[naphtali5725]

Pyro,
what do you think the drivability would be like with the stock 10bolt with the 2.73's?
even with the big block would it be sluggish to drive? i'm guessing it would be out of the sweet spot as you say and perhaps mileage would suffer? i'm not going that route, but i'm interested in your opinion since you explained the 3.08/3.42 to where i understood the concept...

btw the 12 bolt came out of a 74 burb... is there anything to watch out for, with that year rear, limitations?

[Pyrotechnic]

Originally Posted by naphtali5725

Pyro,
what do you think the drivability would be like with the stock 10bolt with the 2.73's?
even with the big block would it be sluggish to drive? i'm guessing it would be out of the sweet spot as you say and perhaps mileage would suffer? i'm not going that route, but i'm interested in your opinion since you explained the 3.08/3.42 to where i understood the concept...

btw the 12 bolt came out of a 74 burb... is there anything to watch out for, with that year rear, limitations?

With a 29" tall tire:

2.73 @ 65 MPH is 2100 RPM, add 100-200 RPM for converter slip.

With a 3 speed transmission, you have to find the best balance of cruise RPM, off the line performance, and usefulness of your passing gear.

Overdrive wisdom tells us to keep getting that cruise RPM down for better gas mileage but you have to remember that you have a 3 speed. You want to get the cruise RPM just down low enough to where it will be comfortable during flat road cruising, but you also have to consider ascending hills as well.

For example, take a newer truck with a 350 and a 700R4 and some 3.42 gears. 65 MPH is 1800 RPM. It's saving gas and cruising nicely on flat land. Now it needs to get up a hill. No problem, downshift to third and spin an easy 2600 RPM up the hill to maintain 65 MPH up the hill. No big deal.

See what happened here? The extra gear let it cruise on flat land at a nice low RPM but when it came to needing an extra push, it didn't have to struggle to do it.

With a 3 speed, this is tougher. You want the cruise RPM to be low enough to save gas, but still high enough so that it's in the power band and when a hill comes up, it can get over it without downshifting.

Here's the trouble. Lets say you run your 455 with the 3 speed auto and 2.73's. Driving along at 65 turning 2100 RPM and a hill comes up. I don't know the torque curve of your motor, but lets say that even with application of more throttle it seems to be struggling to get up the hill and maintain speed. So you downshift to 2nd. Now you have to scream up the hill at 3200 RPM just to maintain 65 MPH.

Lets run the same example with your truck with 3.08 gears. 65 MPH at 2300 RPM and you hit a hill. Again, I don't know your motor but lets say that this is about the spot where the torque curve gets nice. You apply more throttle and it chugs up the hill just fine because it was in the "sweet spot."

See what's happening here? In the quest for a low cruise RPM with 2.73's, you end up screaming up hills at 3200 RPM wasting gas. With 3.08's, you spin a little higher RPM all the time, but the motor is in it's sweet spot and it can deal with load increases without breaking a sweat.

Now, this is just purely in example. I don't know the torque curve of your motor, how loose your converter will end up, and I can't say that this is what would actually happen. The differences that I've illustrated between the two gear sets is only to illustrate the importance of getting your cruise RPM in the sweet spot with a 3 speed.

Generally speaking though, if you run a 3 speed auto and your cruise RPM at 65 MPH is in the 2300-2600 RPM range, then you will have balanced performance in all situations. This is speaking for small block and big block V8's in the range of regular production displacements, and even speaks for aftermarket modifications for increased power. For bigger and more powerful motors you can jump to the lower end of that 2300-2600 RPM range, and for something like a stock small block you would want to go right to the higher end of it.

All my theoretical examples aside, the 2.73's would probably work OK, but I feel that the 3.08's would give the best balance in all situations and just feel good all around. Just getting slightly deeper into the rabbit hole for a second, you have to remember that these trucks are as aerodynamic as a brick with a parachute on the back. It seems like above 65MPH is when drag starts getting really ridiculous. It helps to be in the sweet spot of your torque curve, especially if you're in a hurry one day.

Now a Bill Mitchell harcore 632 cubic inch big block...quite the extreme example but I'm sure it could get up that hill without downshifting, even with a 2.50 gear. I don't think any of us can afford that though.

[naphtali5725]

Pyro,
Thanks so much for the detailed explanation... I won't know the torque curve of my motor until it is dynoed after the build which is in the works as we speak, and I have a TH400 that's going behind it, so i'll be better able to tell then.
Thanks again, I really appreciate it, and I'll probably hit you up when I get the info on my motor

Kindest regards

[Pyrotechnic]

Glad I could help, be sure to open a build thread and post pics of that Olds 455 under the hood. I love seeing these trucks with Buick/Olds/Pontiac/Cadillac engine swaps.

[naphtali5725]

Originally Posted by Pyrotechnic

Glad I could help, be sure to open a build thread and post pics of that Olds 455 under the hood. I love seeing these trucks with Buick/Olds/Pontiac/Cadillac engine swaps.

I already have a build thread... check it out
http://67-72chevytrucks.com/vboard/s...d.php?t=515829