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HEATHENS AUTO CLUB
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32 Posts
Discussion Starter · #1 ·
I have a '61 chevy short wide that is my daily driver right now but fairly soon will be driven on the street mostly and will see a few runs down the strip several times a year during the summer months. I live in Oregon so that should give me about 4 months a year to play around, so I want it to be streetable and good fun down the strip too.

It has a 12 bolt truck open rear end with 3.08 gears, a TH350 with stage 2 shift kit and 2300 stall. I just bought my boss' old race motor and this is where it gets fun. It's a 350 bored .030 over, flat top 4 relief pistons, forged crank, h-beam rods, 2.02/1.60 valves, 1.6:1 high tensil stamped rockers, Weiand Team G single plane air gap manifold, hei dizzy with MSD module, high output coil, no vacuum advance and the lightest springs available on the centrifigul adv. so it gets full adv. right off idle, I think he said total timing was 30deg. at 3500rpm? 10.5:1 CR and the cam is a summit k1108 (specs below)


RPM Range3,400-7,000
Duration at 050 inch Lift: 244 int./254 exh
Valve Lift with Factory Rocker Arm Ratio: 0.508 int./0.533 exh.
Lobe Separation (degrees): 114
Camshaft Manufacturers Description: Rough idle, excellent high-end horsepower. Needs 10.5:1 and higher compression, 3,500+ stall, and gearing. 350+ c.i.d., bracket race cam.

He says this motor ran 450HP at 6200RPM on the Dyno with some kind of carb style fuel injection system which did not come with the motor so I'm going cabureted. I need to figure out what rear end will best for my application (described above), my low budget and the parts I already have.

I have a corporate 8.5" 10 bolt out of a 79 camaro with 2.41 posi that I was thinking of putting under the truck but I don't think it would hold up with that gear ratio and nobody makes a 3.08 or 3.73 gear set for the series 2 carrier. I know they make a spacer for putting higher (numericaly) gears on 2 series carriers but I've heard some horror storries that I don't want to live through. That being said, I don't know if I should get a series 3 carrier and gear set for the 10 bolt rear end or for the truck 12 bolt, and will the 3.08 12 bolt open rear get me by until I save up the money for the propper rear end solution?

Thanks in advance for all help. You guys are a great wealth of knowlege and I'm somewhat new to the technical aspects of hot rodding though I do have a basic grasp on what will generally make things go faster or slower.
 

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High Performance Rear Ends
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3,782 Posts
I recommend a 4 series Eaton Posi and a 3.73 gear for the truck 12 bolt. It's a stronger rear end than the 8.5 and it already fits your truck. Don't forget to get a good installation kit with the Eaton and gears.
 

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High Performance Rear Ends
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3,782 Posts
Forgot to mention that you will probably need a set of axles for that rear end. I think those had the weak 17 or 19 spline axles and you need to upgrade it to 30 spline axles.
 

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12,338 Posts
What size are the tires on the truck? If you have really tall tires a little more gear.Decide how much high way driving you will be using the truck for and the RPM that you can cruise at with out going crazy. 3.73 is a good place to start your calculations. 62 mph =2700 RPM with 29 inch tall tires,,,62 MPH=3200 RPM with 25 inch tall tires
 

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HEATHENS AUTO CLUB
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32 Posts
Discussion Starter · #5 ·
What size are the tires on the truck? If you have really tall tires a little more gear.Decide how much high way driving you will be using the truck for and the RPM that you can cruise at with out going crazy. 3.73 is a good place to start your calculations. 62 mph =2700 RPM with 29 inch tall tires,,,62 MPH=3200 RPM with 25 inch tall tires
Well, ideally I would like to run a 23000- 2500 stall and 28-1/2" to 29-1/2" by 10" wide cheaters but my cam wants a 3500+ stall so it sounds like I'll have to change my cam to use the stall and tires I want and keep the truck under 3000rpm at 65mph. The truck probably won't see a lot of highway driving but it will see some, mostly street and strip, so I could handle maybe 3000rpm for an hour if I had to. I like the way my truck drives the way it is, as far as highway rpm's but I'd like more power overall and a tad quicker off the line. I'm trying to find out if I will absolutely have to change my rear gears or if more hp is enough to do the trick so that everything else can stay the same (with the exception of adding posi). I'm just trying to make it as mean of a summertime street machine as I can that will also be fun at the track, not trying to break any records.
 

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Differential/Driveline
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1,782 Posts
The 12 bolt usually showed up around 64/65. Maybe you already have a newer 12 bolt?

Might be a good idea to pull a axle and get a spline count and length?

As BGH said the early ones have course spline axles.
 

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HEATHENS AUTO CLUB
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32 Posts
Discussion Starter · #10 ·
Hurry up and get that thing running... There's a couple good cruise Inns here in salem. :)
That's why I'm here asking questions. I'm looking forward to bringing this thing to some events but I don't want to have to worry about breaking stuff and getting stranded. If I take it easy on the pedal, will I get by with my 3.08 12 bolt open rear or would the 8.5" 10 bolt 2.41 posi be a better choice until I can afford a 3 series posi unit for one or the other?
 

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A good way to launch is steeper gears. The 700r4 over drive auto has a lower first gear for better launch.A longer stroke engine can make more torque at lower RPM for a better launch.
You just need to do the math of what you have and what you need to make the performance goals you need.
 

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High Performance Rear Ends
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3,782 Posts
Keep the 12 bolt. The 8.5 has 28 spline axles and the gears will completely suck. If you want to keep the 3.08 gear then you will need a 3 series posi for it, but I don't think you will like the 3.08.
 

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Differential/Driveline
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1,782 Posts
I'd use the 12 bolt if it was me. 342 or 373 ratio with a good posi.

Instead of thinking of a 700 trans.. I'd save up the money and just do your diff with good parts. 308's will be a little doggy but it will get ya by while you save up.
 

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HEATHENS AUTO CLUB
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32 Posts
Discussion Starter · #15 ·
Ok, I've been doing a ton of reading and have come to the conclusion that a '79 8.5" 10 bolt with 28 spline axles is stronger than a '73 8.75" 12 bolt truck with 28 spline axles because the pinion shaft diameter on the 10 bolt is bigger than the 12 bolt. The only other difference I can find between the two besides ring gear diameter and track width is parts availability and interchangability. Parts for the 10 bolt are more interchangable as well as cheaper and easier to find, especially in a posi application. So when I can save up some money and get a 3 series posi carrier and 3.73 gear set for the 10 bolt, I'll adapt it to fit under my truck. I have trailing arm/airbag rear suspension so getting the 10 bolt to mount up will be a breeze. I don't mind the narrower track width since I'm going to run cheater slicks, that means I can get wider tires. In the meantime I just have to hope the 3.08 open rear will hold up. You guys think it will hold up to 400hp or so, or should I go easy on the pedal (which won't be easy)?
 

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High Performance Rear Ends
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3,782 Posts
Ok, I've been doing a ton of reading and have come to the conclusion that a '79 8.5" 10 bolt with 28 spline axles is stronger than a '73 8.75" 12 bolt truck with 28 spline axles because the pinion shaft diameter on the 10 bolt is bigger than the 12 bolt. The only other difference I can find between the two besides ring gear diameter and track width is parts availability and interchangability. Parts for the 10 bolt are more interchangable as well as cheaper and easier to find, especially in a posi application. So when I can save up some money and get a 3 series posi carrier and 3.73 gear set for the 10 bolt, I'll adapt it to fit under my truck. I have trailing arm/airbag rear suspension so getting the 10 bolt to mount up will be a breeze. I don't mind the narrower track width since I'm going to run cheater slicks, that means I can get wider tires. In the meantime I just have to hope the 3.08 open rear will hold up. You guys think it will hold up to 400hp or so, or should I go easy on the pedal (which won't be easy)?

I don't know where you got that information, but it's a bunch of crap. First of all, NO 12 bolt had 28 spline axles. They ALL had 30 spline axles, except the very early truck 12 bolt rear ends that had the coarse splines. Your 12 bolt has 30 spline axles.

Next, the 8.875 ring gear in your 12 bolt is MUCH stronger than the 8.5 inch ring gear in the 10 bolt. The pinion shaft is not a problem either. Both have a 30 spline pinion shaft, the only difference is the truck 12 bolt has a smaller inner pinion bearing. This is not a problem. I have never heard of anyone breaking a truck 12 bolt pinion shaft or having a problem with the inner pinion bearing. 9 inch Ford rear ends have a very small 28 spline pinion shaft, and everybody is always talking about what a great rear end the 9 inch Ford is.

Yes, parts for the 8.5 are usually easier to find, but the truck 8.5 limited slip differentails are all the Governor Lock. This is one of the worst differentals you can use in a high performance application. The only thing that they are known for is how quickly and easily they break.

On to the differential bearings. The truck 12 bolt has larger differential bearings than the 8.5 10 bolt. This means that the differential is going to be held in place a little better in the 12 bolt

All together the 8.5 has nothing better than the 12 bolt except the inner pinion bearing. In all other areas the truck 12 bolt is much better. Your best way to go is to use the 12 bolt that you have and install the correct parts in it to make it do what you want it to do.
 

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HEATHENS AUTO CLUB
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32 Posts
Discussion Starter · #17 ·
This is getting frustrating. So there is no LS diff carrier for 12 bolt truck rear ends that are NOT gov lock? Unless I go aftermarket and pay through the nose. I can't afford to put $700-$1000 into the differential, $300-$400 maybe, that's why I was opting for thr 10 bolt. I paid $200 for the 10 bolt, not knowing I couldn't put better performance ratio gears on a 2 series carrier. I know they make a spacer but I've heard some really nasty tales surrounding them. I drive my truck hard so I need it to hold up. Is it possible to have a 3.73 gear made that will fit the 2 series carrier without a spacer? As many times as I've seen this issue presented, I would think Richmond or Auburn or SOMEBODY would make gear sets for this application. Which ever route I go, it will be a while because I have to save up money. In the meantime, will the 3.08 open 12 bolt get me by? Should I nurse it or do I not have anything to worry about but lack of traction?
 

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High Performance Rear Ends
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3,782 Posts
The 12 bolt is going to hold up fine. It's only going to spin one wheel, but it's not going to break. If you do a lot of one wheel burnouts you will burn up the differential gears and the cross shaft, so I dont recommend it.

The truck 12 bolt did come from the factory with a Eaton Posi in the early '70s, but the case on those was weak and would crack. The new Eaton Posi for the truck 12 bolt is very strong and I highly recommend it. Tom's Differentials probably has the best prices on these, so contact them for the parts you need.

There are no thick gears for the 8.5 10 bolt to allow you to use the 2 series differential. The only option that you have with the 2 series is to use a spacer. I do not recommend that. I have seen the results of using ring gear spacers. Some people get away with it and others don't.

All of the 8.5 truck rear ends that had the limited slip differentail had the Gov Lock. Only the car 8.5 got the Traction Lock differential.
 

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HEATHENS AUTO CLUB
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32 Posts
Discussion Starter · #19 ·
The 12 bolt is going to hold up fine. It's only going to spin one wheel, but it's not going to break. If you do a lot of one wheel burnouts you will burn up the differential gears and the cross shaft, so I dont recommend it.

The truck 12 bolt did come from the factory with a Eaton Posi in the early '70s, but the case on those was weak and would crack. The new Eaton Posi for the truck 12 bolt is very strong and I highly recommend it. Tom's Differentials probably has the best prices on these, so contact them for the parts you need.

There are no thick gears for the 8.5 10 bolt to allow you to use the 2 series differential. The only option that you have with the 2 series is to use a spacer. I do not recommend that. I have seen the results of using ring gear spacers. Some people get away with it and others don't.

All of the 8.5 truck rear ends that had the limited slip differentail had the Gov Lock. Only the car 8.5 got the Traction Lock differential.
You definitely seem to know your stuff so I'll get in touch with Tom's and see what kind of money we're talking about. I'll also have to pay a local shop to set everything up for me too because I hear it can be tricky and is best left to the trained eye. I'm glad to know the way it's set up now will get me by, I won't be too hard on it. I have to get this motor in my truck asap because I don't think the one that's in my truck now is going to last much longer.
 
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